Java – Language Fundamentals

Q 01: Give a few reasons for using Java? LF DC
A 01: Java is a fun language. Let’s look at some of the reasons:
􀂃 Built-in support for multi-threading, socket communication, and memory management (automatic garbage
􀂃 Object Oriented (OO).
􀂃 Better portability than other languages across operating systems.
􀂃 Supports Web based applications (Applet, Servlet, and JSP), distributed applications (sockets, RMI. EJB etc)
and network protocols (HTTP, JRMP etc) with the help of extensive standardised APIs (Application Program
Q 02: What is the main difference between the Java platform and the other software platforms? LF
A 02: Java platform is a software-only platform, which runs on top of other hardware-based platforms like UNIX, NT etc.
The Java platform has 2 components:
􀂃 Java Virtual Machine (JVM) – ‘JVM’ is a software that can be ported onto various hardware platforms. Byte
codes are the machine language of the JVM.
􀂃 Java Application Programming Interface (Java API) -
Q 03: What is the difference between C++ and Java? LF
A 03: Both C++ and Java use similar syntax and are Object Oriented, but:
􀂃 Java does not support pointers. Pointers are inherently tricky to use and troublesome.
􀂃 Java does not support multiple inheritances because it causes more problems than it solves. Instead Java
supports multiple interface inheritance, which allows an object to inherit many method signatures from
different interfaces with the condition that the inheriting object must implement those inherited methods. The
multiple interface inheritance also allows an object to behave polymorphically on those methods. [Refer Q 8
and Q 10 in Java section.]
􀂃 Java does not support destructors but rather adds a finalize() method. Finalize methods are invoked by the
garbage collector prior to reclaiming the memory occupied by the object, which has the finalize() method. This
means you do not know when the objects are going to be finalized. Avoid using finalize() method to
release non-memory resources like file handles, sockets, database connections etc because Java has only
a finite number of these resources and you do not know when the garbage collection is going to kick in to
release these resources through the finalize() method.
􀂃 Java does not include structures or unions because the traditional data structures are implemented as an
object oriented framework (Java collection framework – Refer Q14, Q15 in Java section).
􀂃 All the code in Java program is encapsulated within classes therefore Java does not have global variables or
􀂃 C++ requires explicit memory management, while Java includes automatic garbage collection. [Refer Q32 in
Java section].
Q 04: Explain Java class loaders? Explain dynamic class loading? LF
A 04: Class loaders are hierarchical. Classes are introduced into the JVM as they are referenced by name in a class that
is already running in the JVM. So how is the very first class loaded? The very first class is specially loaded with
the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the
classes, which are already loaded and running. A class loader creates a namespace. All JVMs include at least one
class loader that is embedded within the JVM called the primordial (or bootstrap) class loader. Now let’s look at
non-primordial class loaders. The JVM has hooks in it to allow user defined class loaders to be used in place of
primordial class loader. Let us look at the class loaders created by the JVM.
CLASS LOADER reloadable? Explanation
No Loads JDK internal classes, java.* packages. (as defined in the sun.boot.class.path
system property, typically loads rt.jar and i18n.jar)
Extensions No Loads jar files from JDK extensions directory (as defined in the java.ext.dirs system
property – usually lib/ext directory of the JRE)
System No Loads classes from system classpath (as defined by the java.class.path property, which
is set by the CLASSPATH environment variable or –classpath or –cp command line
(rt.jar, i18.jar)
JVM class loaders
Classes loaded by Bootstrap class loader have no visibility into classes
loaded by its descendants (ie Extensions and Systems class loaders).
The classes loaded by system class loader have visibility into classes loaded
by its parents (ie Extensions and Bootstrap class loaders).
If there were any sibling class loaders they cannot see classes loaded by
each other. They can only see the classes loaded by their parent class
loader. For example Sibling1 class loader cannot see classes loaded by
Sibling2 class loader
Both Sibling1 and Sibling2 class loaders have visibilty into classes loaded
by their parent class loaders (eg: System, Extensions, and Bootstrap)
Class loaders are hierarchical and use a delegation model when loading a class. Class loaders request their
parent to load the class first before attempting to load it themselves. When a class loader loads a class, the child
class loaders in the hierarchy will never reload the class again. Hence uniqueness is maintained. Classes loaded
by a child class loader have visibility into classes loaded by its parents up the hierarchy but the reverse is not true
as explained in the above diagram.
Important: Two objects loaded by different class loaders are never equal even if they carry the same values, which mean a
class is uniquely identified in the context of the associated class loader. This applies to singletons too, where each class
loader will have its own singleton. [Refer Q45 in Java section for singleton design pattern]
Explain static vs. dynamic class loading?
Static class loading Dynamic class loading
Classes are statically loaded with Java’s
“new” operator.
class MyClass {
public static void main(String args[]) {
Car c = new Car();
Dynamic loading is a technique for programmatically invoking the functions of a
class loader at run time. Let us look at how to load classes dynamically.
Class.forName (String className); //static method which returns a Class
The above static method returns the class object associated with the class
name. The string className can be supplied dynamically at run time. Unlike the
static loading, the dynamic loading will decide whether to load the class Car or
the class Jeep at runtime based on a properties file and/or other runtime
conditions. Once the class is dynamically loaded the following method returns an
instance of the loaded class. It’s just like creating a class object with no
class.newInstance (); //A non-static method, which creates an instance of a
class (i.e. creates an object).
Jeep myJeep = null ;
//myClassName should be read from a properties file or Constants interface.
//stay away from hard coding values in your program. CO
String myClassName = "" ;
Class vehicleClass = Class.forName(myClassName) ;
myJeep = (Jeep) vehicleClass.newInstance();
A NoClassDefFoundException is
thrown if a class is referenced with
Java’s “new” operator (i.e. static loading)
but the runtime system cannot find the
referenced class.
A ClassNotFoundException is thrown when an application tries to load in a
class through its string name using the following methods but no definition for the
class with the specified name could be found:
􀂃 The forName(..) method in class - Class.
􀂃 The findSystemClass(..) method in class - ClassLoader.
􀂃 The loadClass(..) method in class - ClassLoader.
What are “static initializers” or “static blocks with no function names”? When a class is loaded, all blocks
that are declared static and don’t have function name (i.e. static initializers) are executed even before the
constructors are executed. As the name suggests they are typically used to initialize static fields. CO
public class StaticInitilaizer {
public static final int A = 5;
public static final int B;
//Static initializer block, which is executed only once when the class is loaded.
static {
if(A == 5)
B = 10;
B = 5;
public StaticInitilaizer(){} // constructor is called only after static initializer block
The following code gives an Output of A=5, B=10.
public class Test {
System.out.println("A =" + StaticInitilaizer.A + ", B =" + StaticInitilaizer.B);
Q 05: What are the advantages of Object Oriented Programming Languages (OOPL)? DC
A 05: The Object Oriented Programming Languages directly represent the real life objects like Car, Jeep, Account,
Customer etc. The features of the OO programming languages like polymorphism, inheritance and
encapsulation make it powerful. [Tip: remember pie which, stands for Polymorphism, Inheritance and
Encapsulation are the 3 pillars of OOPL]
Q 06: How does the Object Oriented approach improve software development? DC
A 06: The key benefits are:
􀂃 Re-use of previous work: using implementation inheritance and object composition.
􀂃 Real mapping to the problem domain: Objects map to real world and represent vehicles, customers,
products etc: with encapsulation.
􀂃 Modular Architecture: Objects, systems, frameworks etc are the building blocks of larger systems.
The increased quality and reduced development time are the by-products of the key benefits discussed above.
If 90% of the new application consists of proven existing components then only the remaining 10% of the code
have to be tested from scratch.
Q 07: How do you express an ‘is a’ relationship and a ‘has a’ relationship or explain inheritance and composition? What
is the difference between composition and aggregation? DC
A 07: The ‘is a’ relationship is expressed with inheritance and ‘has a’ relationship is expressed with composition. Both
inheritance and composition allow you to place sub-objects inside your new class. Two of the main techniques for
code reuse are class inheritance and object composition.
Inheritance [ is a ] Vs Composition [ has a ]
class Building{
class House extends Building{
is a [House is a Building]
class House {
Bathroom room = new Bathroom() ;
public void getTotMirrors(){
is a has a [House has a Bathroom]
has a
Inheritance is uni-directional. For example House is a Building. But Building is not a House. Inheritance uses
extends key word. Composition: is used when House has a Bathroom. It is incorrect to say House is a
Bathroom. Composition simply means using instance variables that refer to other objects. The class House will
have an instance variable, which refers to a Bathroom object.
Which one to use? The guide is that inheritance should be only used when subclass ‘is a’ superclass.
􀂃 Don’t use inheritance just to get code reuse. If there is no ‘is a’ relationship then use composition for code
reuse. Overuse of implementation inheritance (uses the “extends” key word) can break all the subclasses, if
the superclass is modified.
􀂃 Do not use inheritance just to get polymorphism. If there is no ‘is a’ relationship and all you want is
polymorphism then use interface inheritance with composition, which gives you code reuse (Refer Q8 in
Java section for interface inheritance).
What is the difference between aggregation and composition?
Aggregation Composition
Aggregation is an association in which one class
belongs to a collection. This is a part of a whole
relationship where a part can exist without a whole.
For example a line item is a whole and product is a
part. If a line item is deleted then corresponding
product need not be deleted. So aggregation has a
weaker relationship.
Composition is an association in which one class belongs to a
collection. This is a part of a whole relationship where a part
cannot exist without a whole. If a whole is deleted then all parts are
deleted. For example An order is a whole and line items are parts.
If an order deleted then all corresponding line items for that order
should be deleted. So composition has a stronger relationship.
Q 08: What do you mean by polymorphism, inheritance, encapsulation, and dynamic binding? DC
A 08: Polymorphism – means the ability of a single variable of a given type to be used to reference objects of different
types, and automatically call the method that is specific to the type of object the variable references. In a nutshell,
polymorphism is a bottom-up method call. The benefit of polymorphism is that it is very easy to add new classes
of derived objects without breaking the calling code (i.e. getTotArea() in the sample code shown below) that
uses the polymorphic classes or interfaces. When you send a message to an object even though you don’t know
what specific type it is, and the right thing happens, that’s called polymorphism. The process used by objectoriented
programming languages to implement polymorphism is called dynamic binding. Let us look at some
sample code to demonstrate polymorphism: CO
+area() : double
+area() : double
+area() : double
+area() : double
//client or calling code
double dim = 5.0; //ie 5 meters radius or width
List listShapes = new ArrayList(20);
Shape s = new Circle();
listShapes.add(s); //add circle
s = new Square();
listShapes.add(s); //add square
getTotArea (listShapes,dim); //returns 78.5+25.0=103.5
//Later on, if you decide to add a half circle then define
//a HalfCircle class, which extends Circle and then provide an
//area(). method but your called method getTotArea(...) remains
s = new HalfCircle();
listShapes.add(s); //add HalfCircle
getTotArea (listShapes,dim); //returns 78.5+25.0+39.25=142.75
/** called method: method which adds up areas of various
** shapes supplied to it.
public double getTotArea(List listShapes, double dim){
Iterator it = listShapes.iterator();
double totalArea = 0.0;
//loop through different shapes
while(it.hasNext()) {
Shape s = (Shape);
totalArea += s.area(dim); //polymorphic method call
return totalArea ;
Sample code:
For example: given a base
class/interface Shape,
polymorphism allows the
programmer to define
different area(double
dim1) methods for any
number of derived classes
such as Circle, Square etc.
No matter what shape an
object is, applying the area
method to it will return the
right results.
Later on HalfCicle can be
added without breaking
your called code i.e.
method getTotalArea(...)
Depending on what the
shape is, appropriate
area(double dim) method
gets called and calculated.
Circle 􀃆 area is 78.5sqm
Square 􀃆 area is 25sqm
HalfCircle 􀃆 area is 39.25
+area() : double
+area() : double
+area() : double
+area() : double
Inheritance – is the inclusion of behaviour (i.e. methods) and state (i.e. variables) of a base class in a derived
class so that they are accessible in that derived class. The key benefit of Inheritance is that it provides the formal
mechanism for code reuse. Any shared piece of business logic can be moved from the derived class into the
base class as part of refactoring process to improve maintainability of your code by avoiding code duplication. The
existing class is called the superclass and the derived class is called the subclass. Inheritance can also be
defined as the process whereby one object acquires characteristics from one or more other objects the same way
children acquire characteristics from their parents.
There are two types of inheritances:
1. Implementation inheritance (aka class inheritance): You can extend an applications’ functionality by reusing
functionality in the parent class by inheriting all or some of the operations already implemented. In Java, you can
only inherit from one superclass. Implementation inheritance promotes reusability but improper use of class
inheritance can cause programming nightmares by breaking encapsulation and making future changes a problem.
With implementation inheritance, the subclass becomes tightly coupled with the superclass. This will make the
design fragile because if you want to change the superclass, you must know all the details of the subclasses to
avoid breaking them. So when using implementation inheritance, make sure that the subclasses depend only
on the behaviour of the superclass, not on the actual implementation. For example in the above diagram the
subclasses should only be concerned about the behaviour known as area() but not how it is implemented.
2. Interface inheritance (aka type inheritance): This is also known as subtyping. Interfaces provide a mechanism
for specifying a relationship between otherwise unrelated classes, typically by specifying a set of common
methods each implementing class must contain. Interface inheritance promotes the design concept of program to
interfaces not to implementations. This also reduces the coupling or implementation dependencies between
systems. In Java, you can implement any number of interfaces. This is more flexible than implementation
inheritance because it won’t lock you into specific implementations which make subclasses difficult to maintain. So
care should be taken not to break the implementing classes by modifying the interfaces.
Which one to use? Prefer interface inheritance to implementation inheritance because it promotes the design
concept of coding to an interface and reduces coupling. Interface inheritance can achieve code reuse with the
help of object composition. If you look at Gang of Four (GoF) design patterns, you can see that it favours
interface inheritance to implementation inheritance. CO
Implementation inheritance Interface inheritance
Let’s assume that savings account and term deposit
account have a similar behaviour in terms of depositing
and withdrawing money, so we will get the super class to
implement this behaviour and get the subclasses to reuse
this behaviour. But saving account and term deposit
account have specific behaviour in calculating the interest.
public abstract class Account {
public void deposit(double amount) {
//deposit logic
public void withdraw(double amount) {
//withdraw logic
public abstract double calculateInterest(double amount);
public class SavingsAccount extends Account {
public double calculateInterest(double amount) {
//calculate interest for SavingsAccount
public class TermDepositAccount extends Account {
public double calculateInterest(double amount) {
//calculate interest for TermDeposit
The calling code can be defined as follows for illustration
purpose only:
public class Test {
public static void main(String[] args) {
Account acc1 = new SavingsAccount();
Account acc2 = new TermDepositAccount();
Let’s look at an interface inheritance code sample, which makes
use of composition for reusability. In the following example the
methods deposit(…) and withdraw(…) share the same piece of code
in AccountHelper class. The method calculateInterest(…) has its
specific implementation in its own class.
public interface Account {
public abstract void deposit(double amount);
public abstract void withdraw(double amount);
public abstract int getAccountType();
public interface SavingsAccount extends Account{
public abstract double calculateInterest(double amount);
public interface TermDepositAccount extends Account{
public abstract double calculateInterest(double amount);
The classes SavingsAccountImpl, TermDepositAccountImpl
should implement the methods declared in its interfaces. The class
AccountHelper implements the methods deposit(…) and
public class SavingsAccountImpl implements SavingsAccount{
private int accountType = 1;
//helper class which promotes code reuse through composition
AccountHelper helper = new AccountHelper();
public void deposit(double amount) {
helper.deposit(amount, getAccountType());
public void withdraw(double amount) {
helper.withdraw(amount, getAccountType());
public double calculateInterest(double amount) {
//calculate interest for SavingsAccount
public int getAccountType(){
return accountType;
public class TermDepositAccountImpl implements
TermDepositAccount {
private int accountType = 2;
//helper class which promotes code reuse through composition
AccountHelper helper = new AccountHelper();
public void deposit(double amount) {
helper.deposit(amount, getAccountType());
public void withdraw(double amount) {
helper.withdraw(amount, getAccountType());
public double calculateInterest(double amount) {
//calculate interest for TermDeposit
public int getAccountType() {
return accountType;
The calling code can be defined as follows for illustration purpose
public class Test {
public static void main(String[] args) {
Account acc1 = new SavingsAccountImpl();
Account acc2 = new TermDepositAccountImpl();
if (acc1.getAccountType() == 1) {
((SavingsAccount) acc1).calculateInterest(500.00);
if (acc2.getAccountType() == 2) {
((TermDepositAccount) acc2).calculateInterest(500.00);
Encapsulation – refers to keeping all the related members (variables and methods) together in an object.
Specifying members as private can hide the variables and methods. Objects should hide their inner workings from
the outside view. Good encapsulation improves code modularity by preventing objects interacting with
each other in an unexpected way, which in turn makes future development and refactoring efforts easy.
Being able to encapsulate members of a class is important for security and integrity. We can protect variables
from unacceptable values. The sample code below describes how encapsulation can be used to protect the
MyMarks object from having negative values. Any modification to member variable “vmarks” can only be carried
out through the setter method setMarks(int mark). This prevents the object “MyMarks” from having any negative
values by throwing an exception. CO
setName (String name)
String getName()
int getMarks()
setMarks(int mark)
private int vmarks;
private String name;
variables are
so that they
can only be
accessed via
Class MyMarks {
private int vmarks = 0;
private String name;
public void setMarks(int mark)
throws MarkException {
if(mark > 0)
this.vmarks = mark;
else {
throw new MarkException("No negative
public int getMarks(){
return vmarks;
//getters and setters for attribute name goes here.
Sample code
Q 09: What is design by contract? Explain the assertion construct? DC
A 09: Design by contract specifies the obligations of a calling-method and called-method to each other. Design by
contract is a valuable technique, which should be used to build well-defined interfaces. The strength of this
programming methodology is that it gets the programmer to think clearly about what a function does, what pre
and post conditions it must adhere to and also it provides documentation for the caller. Java uses the assert
statement to implement pre- and post-conditions. Java’s exceptions handling also support design by contract
especially checked exceptions (Refer Q34 in Java section for checked exceptions). In design by contract in
addition to specifying programming code to carrying out intended operations of a method the programmer also
1. Preconditions – This is the part of the contract the calling-method must agree to. Preconditions specify the
conditions that must be true before a called method can execute. Preconditions involve the system state and the
arguments passed into the method at the time of its invocation. If a precondition fails then there is a bug in the
calling-method or calling software component.
On public methods On non-public methods
Preconditions on public methods are enforced by explicit checks
that throw particular, specified exceptions. You should not use
assertion to check the parameters of the public methods but
can use for the non-public methods. Assert is inappropriate
because the method guarantees that it will always enforce the
argument checks. It must check its arguments whether or not
assertions are enabled. Further, assert construct does not throw
an exception of a specified type. It can throw only an
public void setRate(int rate) {
if(rate <= 0 || rate > MAX_RATE){
throw new IllegalArgumentException(“Invalid rate 􀃆 ” + rate);
You can use assertion to check the parameters of the
non-public methods.
private void setCalculatedRate(int rate) {
assert (rate > 0 && rate < MAX_RATE) : rate;
//calculate the rate and set it.
Assertions can be disabled, so programs must not
assume that assert construct will be always executed:
//Wrong: if assertion is disabled, CarpenterJob never
//Get removed
assert jobsAd.remove(PilotJob);
boolean pilotJobRemoved = jobsAd.remove(PilotJob);
assert pilotJobRemoved;
2. Postconditions – This is the part of the contract the called-method agrees to. What must be true after a
method completes successfully. Postconditions can be used with assertions in both public and non-public
methods. The postconditions involve the old system state, the new system state, the method arguments and the
method’s return value. If a postcondition fails then there is a bug in the called-method or called software
public double calcRate(int rate) {
if(rate <= 0 || rate > MAX_RATE){
throw new IllegalArgumentException(“Invalid rate !!! ”);
//logic to calculate the rate and set it goes here
assert this.evaluate(result) < 0 : this; //this 􀃆 message sent to AssertionError on failure
return result;
3. Class invariants - what must be true about each instance of a class? A class invariant as an internal invariant
that can specify the relationships among multiple attributes, and should be true before and after any method
completes. If an invariant fails then there could be a bug in either calling-method or called-method. There is
no particular mechanism for checking invariants but it is convenient to combine all the expressions required for
checking invariants into a single internal method that can be called by assertions. For example if you have a class,
which deals with negative integers then you define the isNegative() convenient internal method:
class NegativeInteger {
Integer value = new Integer (-1); //invariant
public NegativeInteger(Integer int) {
//constructor logic goes here
assert isNegative();
//rest of the public and non-public methods goes here. public methods should call assert isNegative(); prior to its return
//convenient internal method for checking invariants. Returns true if the integer value is negative
private boolean isNegative(){
return value.intValue() < 0 ;
The isNegative() method should be true before and after any method completes, each public method and
constructor should contain the following assert statement immediately prior to its return.
assert isNegative();
Explain the assertion construct? The assertion statements have two forms as shown below:
assert Expression1;
assert Expression1 : Expression2;
􀂃 Expression1 􀃆 is a boolean expression. If the Expression1 evaluates to false, it throws an AssertionError without any
detailed message.
􀂃 Expression2 􀃆 if the Expression1 evaluates to false throws an AssertionError with using the value of the Expression2 as
the errors’ detailed message.
Note: If you are using assertions (available from JDK1.4 onwards), you should supply the JVM argument to
enable it by package name or class name.
Java -ea[:packagename...|:classname] or Java -enableassertions[:packagename...|:classname]
Java –ea:Account
Q 10: What is the difference between an abstract class and an interface and when should you use them? LF DP DC
A 10: In design, you want the base class to present only an interface for its derived classes. This means, you don’t want
anyone to actually instantiate an object of the base class. You only want to upcast to it (implicit upcasting, which
gives you polymorphic behaviour), so that its interface can be used. This is accomplished by making that class
abstract using the abstract keyword. If anyone tries to make an object of an abstract class, the compiler prevents
The interface keyword takes this concept of an abstract class a step further by preventing any method or function
implementation at all. You can only declare a method or function but not provide the implementation. The class,
which is implementing the interface, should provide the actual implementation. The interface is a very useful and
commonly used aspect in OO design, as it provides the separation of interface and implementation and
enables you to:
􀂃 Capture similarities among unrelated classes without artificially forcing a class relationship.
􀂃 Declare methods that one or more classes are expected to implement.
􀂃 Reveal an object's programming interface without revealing its actual implementation.
􀂃 Model multiple interface inheritance in Java, which provides some of the benefits of full on multiple
inheritances, a feature that some object-oriented languages support that allow a class to have more than one
Circle Square
Diamond problem & use of interface
No multiple inheritance in JAVA
Circle Square CircleOnSquare
Multiple interface inheritance in JAVA
Abstract class Interface
Have executable methods and abstract methods. Have no implementation code. All methods are abstract.
Can only subclass one abstract class.
A class can implement any number of interfaces.
Can have instance variables, constructors and any
visibility: public, private, protected, none (aka package).
Cannot have instance variables, constructors and can have
only public and none (aka package) visibility.
When to use an abstract class?: In case where you want to use implementation inheritance then it is usually
provided by an abstract base class. Abstract classes are excellent candidates inside of application frameworks.
Abstract classes let you define some default behaviour and force subclasses to provide any specific behaviour.
Care should be taken not to overuse implementation inheritance as discussed in Q8 in Java section.
When to use an interface?: For polymorphic interface inheritance, where the client wants to only deal with a type
and does not care about the actual implementation use interfaces. If you need to change your design frequently,
you should prefer using interface to abstract. CO Coding to an interface reduces coupling and interface
inheritance can achieve code reuse with the help of object composition. Another justification for using interfaces
is that they solve the ‘diamond problem’ of traditional multiple inheritance as shown in the figure. Java does not
support multiple inheritances. Java only supports multiple interface inheritance. Interface will solve all the
ambiguities caused by this ‘diamond problem’.
Design pattern: Strategy design pattern lets you swap new algorithms and processes into your program without
altering the objects that use them. Strategy design pattern: Refer Q11 in How would you go about… section.
Q 11: Why there are some interfaces with no defined methods (i.e. marker interfaces) in Java? LF
A 11: The interfaces with no defined methods act like markers. They just tell the compiler that the objects of the classes
implementing the interfaces with no defined methods need to be treated differently. Example Serializable (Refer
Q19 in Java section), Cloneable etc
Q 12: When is a method said to be overloaded and when is a method said to be overridden? LF CO
A 12:
Method Overloading Method Overriding
Overloading deals with multiple methods in the same class
with the same name but different method signatures.
class MyClass {
public void getInvestAmount(int rate) {…}
public void getInvestAmount(int rate, long principal)
{ … }
Both the above methods have the same method names
but different method signatures, which mean the methods
are overloaded.
Overriding deals with two methods, one in the parent class and
the other one in the child class and has the same name and
class BaseClass{
public void getInvestAmount(int rate) {…}
class MyClass extends BaseClass {
public void getInvestAmount(int rate) { …}
Both the above methods have the same method names and
the signatures but the method in the subclass MyClass
overrides the method in the superclass BaseClass.
Overloading lets you define the same operation in
different ways for different data.
Overriding lets you define the same operation in different
ways for different object types.
Q 13: What is the main difference between an ArrayList and a Vector? What is the main difference between Hashmap
and Hashtable? LF DC PI CI
A 13:
Vector / Hashtable ArrayList / Hashmap
Original classes before the introduction of Collections
API. Vector & Hashtable are synchronized. Any
method that touches their contents is thread-safe.
So if you don’t need a thread safe collection, use the ArrayList or
Hashmap. Why pay the price of synchronization unnecessarily at
the expense of performance degradation.
So which is better? As a general rule, prefer ArrayList/Hashmap to Vector/Hashtable. If your application is a
multithreaded application and at least one of the threads either adds or deletes an entry into the collection
then use new Java collection API‘s external synchronization facility as shown below to temporarily synchronize
your collections as needed: CO
Map myMap = Collections.synchronizedMap (myMap);
List myList = Collections.synchronizedList (myList);
Java arrays are even faster than using an ArrayList/Vector and perhaps therefore may be preferable.
ArrayList/Vector internally uses an array with some convenient methods like add(..), remove(…) etc.
Q 14: Explain the Java Collection framework? LF DP
A 14: The key interfaces used by the collection framework are List, Set and Map. The List and Set extends the
Collection interface. Should not confuse the Collection interface with the Collections class which is a utility class.
A Set is a collection with unique elements and prevents duplication within the collection. HashSet and TreeSet
are implementations of a Set interface. A List is a collection with an ordered sequence of elements and may
contain duplicates. ArrayList, LinkedList and Vector are implementations of a List interface.
The Collection API also supports maps, but within a hierarchy distinct from the Collection interface. A Map is an
object that maps keys to values, where the list of keys is itself a collection object. A map can contain duplicate
values, but the keys in a map must be distinct. HashMap, TreeMap and Hashtable are implementations of a Map
How to implement collection ordering? SortedSet and SortedMap interfaces maintain sorted order. The
classes, which implement the Comparable interface, impose natural order. For classes that don’t implement
comparable interface, or when one needs even more control over ordering based on multiple attributes, a
Comparator interface should be used.
Design pattern: What is an Iterator? An Iterator is a use once object to access the objects stored in a collection.
Iterator design pattern (aka Cursor) is used, which is a behavioural design pattern that provides a way to access
elements of a collection sequentially without exposing its internal representation.
JAVA collection framework






(Diagram sourced from:
What are the benefits of the Java collection framework? Collection framework provides flexibility,
performance, and robustness.
􀂃 Polymorphic algorithms – sorting, shuffling, reversing, binary search etc.
􀂃 Set algebra - such as finding subsets, intersections, and unions between objects.
􀂃 Performance - collections have much better performance compared to the older Vector and Hashtable
classes with the elimination of synchronization overheads.
􀂃 Thread-safety - when synchronization is required, wrapper implementations are provided for temporarily
synchronizing existing collection objects.
􀂃 Immutability - when immutability is required wrapper implementations are provided for making a collection
􀂃 Extensibility - interfaces and abstract classes provide an excellent starting point for adding functionality and
features to create specialized object collections.
Q 15: What are some of the best practices relating to Java collection? BP PI CI
A 15:
􀂃 Use ArrayLists, HashMap etc as opposed to Vector, Hashtable etc, where possible to avoid any
synchronization overhead. Even better is to use just arrays where possible. If multiple threads concurrently
access a collection and at least one of the threads either adds or deletes an entry into the collection,
then the collection must be externally synchronized. This is achieved by:
Map myMap = Collections.synchronizedMap (myMap);
List myList = Collections.synchronizedList (myList);
􀂃 Set the initial capacity of a collection appropriately (e.g. ArrayList, HashMap etc). This is because collection
classes like ArrayList, HashMap etc must grow periodically to accommodate new elements. But if you have a
very large array, and you know the size in advance then you can speed things up by setting the initial size
For example: HashMaps/Hashtables need to be created with sufficiently large capacity to minimise
rehashing (which happens every time the table grows). HashMap has two parameters initial capacity and
load factor that affect its performance and space requirements. Higher load factor values (default load factor
of 0.75 provides a good trade off between performance and space) will reduce the space cost but will
increase the lookup cost of myMap.get(…) and myMap.put(…) methods. When the number of entries in the
HashMap exceeds the current capacity * loadfactor then the capacity of the HasMap is roughly doubled by
calling the rehash function. It is also very important not to set the initial capacity too high or load factor too
low if iteration performance or reduction in space is important.
􀂃 Program in terms of interface not implementation: For example you might decide a LinkedList is the best
choice for some application, but then later decide ArrayList might be a better choice for performance reason.
List list = new ArrayList(100); //program in terms of interface & set the initial capacity.
Instead of:
ArrayList list = new ArrayList();
􀂃 Avoid storing unrelated or different types of objects into same collection: This is analogous to storing
items in pigeonholes without any labelling. To store items use value objects or data objects (as oppose to
storing every attribute in an ArrayList or HashMap). Provide wrapper classes around your collection API
classes like ArrayList, Hashmap etc as shown in better approach column. Also where applicable consider
using composite design pattern, where an object may represent a single object or a collection of objects.
Refer Q52 in Java section for UML diagram of a composite design pattern. CO
Avoid where possible Better approach
The code below is hard to maintain and understand by
others. Also gets more complicated as the requirements
grow in the future because we are throwing different
types of objects like Integer, String etc into a list just
based on the indices and it is easy to make mistakes
while casting the objects back during retrieval.
List myOrder = new ArrayList()
ResultSet rs = …
While (rs.hasNext()) {
List lineItem = new ArrayList();
lineItem.add (new Integer(rs.getInt(“itemId”)));
lineItem.add (rs.getString(“description”));
myOrder.add( lineItem);
return myOrder;
Example 2:
List myOrder = new ArrayList(10);
//create an order
OrderVO header = new OrderVO();

//add all the line items
LineItemVO line1 = new LineItemVO();
LineItemVO line2 = new LineItemVO();
When storing items into a collection define value objects as shown
below: (VO is an acronym for Value Object).
public class LineItemVO {
private int itemId;
private String productName;
public int getLineItemId(){return accountId ;}
public int getAccountName(){return accountName;}
public void setLineItemId(int accountId ){
this.accountId = accountId
//implement other getter & setter methods
Now let’s define our base wrapper class, which represents an order:
public abstract class Order {
int orderId;
List lineItems = null;
public abstract int countLineItems();
public abstract boolean add(LineItemVO itemToAdd);
public abstract boolean remove(LineItemVO itemToAdd);
public abstract Iterator getIterator();
public int getOrderId(){return this.orderId; }
Now a specific implementation of our wrapper class:
public class OverseasOrder extends Order {
public OverseasOrder(int inOrderId) {
this.lineItems = new ArrayList(10);
this.orderId = inOrderId;
List lineItems = new ArrayList();
//to store objects
myOrder.add(order);// index 0 is an OrderVO object
myOrder.add(lineItems);//index 1 is a List of line items
//to retrieve objects
Above approaches are bad because disparate objects
are stored in the lineItem collection in example-1 and
example-2 relies on indices to store disparate objects.
The indices based approach and storing disparate
objects are hard to maintain and understand because
indices are hard coded and get scattered across the
code. If an index position changes for some reason, then
you will have to change every occurrence, otherwise it
breaks your application.
The above coding approaches are analogous to storing
disparate items in a storage system without proper
labelling and just relying on its grid position.
public int countLineItems() { //logic to count }
public boolean add(LineItemVO itemToAdd){
…//additional logic or checks
return lineItems.add(itemToAdd);
public boolean remove(LineItemVO itemToAdd){
return lineItems.remove(itemToAdd);
public ListIterator getIterator(){ return lineItems.Iterator();}
Now to use:
Order myOrder = new OverseasOrder(1234) ;
LineItemVO item1 = new LineItemVO();
LineItemVO item2 = new LineItemVO();
Item1.setProductName(“Outdoor chair”);
//to add line items to order

Q 16: When providing a user defined key class for storing objects in the Hashmaps or Hashtables, what methods do you
have to provide or override (i.e. method overriding)? LF PI CO
A 16: You should override the equals() and hashCode() methods from the Object class. The default implementation of
the equals() and hashcode(), which are inherited from the java.lang.Object uses an object instance’s memory
location (e.g. MyObject@6c60f2ea). This can cause problems when two instances of the car objects have the
same colour but the inherited equals() will return false because it uses the memory location, which is different for
the two instances. Also the toString() method can be overridden to provide a proper string representation of your
object. Points to consider:
• If a class overrides equals(), it must override hashCode().
• If 2 objects are equal, then their hashCode values must be equal as well.
• If a field is not used in equals(), then it must not be used in hashCode().
• If it is accessed often, hashCode() is a candidate for caching to enhance performance.
Note: Java 1.5 introduces enumerated constants, which improves readability and maintainability of your code.
Java programming language enums are more powerful than their counterparts in other languages. E.g. A class
like Weather can be built on top of simple enum type Season and the class Weather can be made immutable, and
only one instance of each Weather can be created, so that your Weather class does not have to override
equals() and hashCode() methods.
public class Weather {
public enum Season {WINTER, SPRING, SUMMER, FALL}
private final Season season;
private static final List listWeather = new ArrayList ();
private Weather (Season season) { this.season = season;}
public Season getSeason () { return season;}
static {
for (Season season : Season.values()) {
listWeather.add(new Weather(season));
public static ArrayList getWeatherList () { return listWeather; }
public String toString(){ return season;} // takes advantage of toString() method of Season.
Q 17: What is the main difference between a String and a StringBuffer class? LF PI CI CO
A 17:
String StringBuffer / StringBuilder
String is immutable: you can’t modify a string
object but can replace it by creating a new
instance. Creating a new instance is rather
//Inefficient version using immutable String
String output = “Some text”
Int count = 100;
for(int I =0; ioutput += i;
return output;
The above code would build 99 new String
objects, of which 98 would be thrown away
immediately. Creating new objects is not
StringBuffer is mutable: use StringBuffer or StringBuilder when you want to
modify the contents. StringBuilder was added in Java 5 and it is identical in
all respects to StringBuffer except that it is not synchronised, which makes
it slightly faster at the cost of not being thread-safe.
//More efficient version using mutable StringBuffer
StringBuffer output = new StringBuffer(110);
Output.append(“Some text”);
for(int I =0; ioutput.append(i);
return output.toString();
The above code creates only two new objects, the StringBuffer and the final
String that is returned. StringBuffer expands as needed, which is costly
however, so it would be better to initilise the StringBuffer with the correct size
from the start as shown.
Another important point is that creation of extra strings is not limited to ‘overloaded mathematical operators’ (“+”) but
there are several methods like concat(), trim(), substring(), and replace() in String classes that generate new
string instances. So use StringBuffer or StringBuilder for computation intensive operations, which offer better
Q 18: What is the main difference between pass-by-reference and pass-by-value? LF PI
A 18: Other languages use pass-by-reference or pass-by-pointer. But in Java no matter what type of argument you
pass the corresponding parameter (primitive variable or object reference) will get a copy of that data, which is
exactly how pass-by-value (i.e. copy-by-value) works.
In Java, if a calling method passes a reference of an object as an argument to the called method then the passedin
reference gets copied first and then passed to the called method. Both the original reference that was
passed-in and the copied reference will be pointing to the same object. So no matter which reference you use, you
will be always modifying the same original object, which is how the pass-by-reference works as well.
ref d
Pass-by-value for primitive variables vs O bject references
public void first(){
int i= 10;
int x = second(i);
//At this point
//value of i is still 10
//value of x is 11
public int second(int k) {
return k ;
i = 10
k = 10
k = 11
Copy of i
s t o res i
co p ies i
acts on k
public void first(){
Car c = new Car("red")
//At this point
//color is Red
//At this point
//color is B lue
public void second(Car d)
//color is blue
Car object
String color = red
ref c
copy of c
Prim itive variables Object references
modifies the original
object through copied
modifies the copy k
but not the original.
color = b lue
If your method call involves inter-process (e.g. between two JVMs) communication, then the reference of the
calling method has a different address space to the called method sitting in a separate process (i.e. separate
JVM). Hence inter-process communication involves calling method passing objects as arguments to called method
by-value in a serialized form, which can adversely affect performance due to marshalling and unmarshalling cost.
Note: As discussed in Q69 in Enterprise section, EJB 2.x introduced local interfaces, where enterprise beans that can be used
locally within the same JVM using Java’s form of pass-by-reference, hence improving performance.
Q 19: What is serialization? How would you exclude a field of a class from serialization or what is a transient variable?
What is the common use? LF SI PI
A 19: Serialization is a process of reading or writing an object. It is a process of saving an object’s state to a sequence of
bytes, as well as a process of rebuilding those bytes back into a live object at some future time. An object is
marked serializable by implementing the interface, which is only a marker interface -- it simply
allows the serialization mechanism to verify that the class can be persisted, typically to a file.
byte stream
write to
class Car implements Serializable {
String color = null;
transient File fh = null;
Car Object1
Class Car implements
Serializable {
String color = null;
Car Object 2
deserialize serialize
Transient variables cannot be serialized. The fields marked transient in a serializable object will not be
transmitted in the byte stream. An example would be a file handle or a database connection. Such objects are only
meaningful locally. So they should be marked as transient in a serializable class.
Serialization can adversely affect performance since it:
􀂃 Depends on reflection.
􀂃 Has an incredibly verbose data format.
􀂃 Is very easy to send surplus data.
When to use serialization? Do not use serialization if you do not have to. A common use of serialization is to use
it to send an object over the network or if the state of an object needs to be persisted to a flat file or a database.
(Refer Q57 on Enterprise section). Deep cloning or copy can be achieved through serialization. This may be fast
to code but will have performance implications (Refer Q22 in Java section).
The objects stored in an HTTP session should be serializable to support in-memory replication of sessions to
achieve scalability (Refer Q20 in Enterprise section). Objects are passed in RMI (Remote Method Invocation)
across network using serialization (Refer Q57 in Enterprise section).
Q 20: Explain the Java I/O streaming concept and the use of the decorator design pattern in Java I/O? LF DP PI SI
A 20: Java input and output is defined in terms of an abstract concept called a “stream”, which is a sequence of data.
There are 2 kinds of streams.
􀂃 Byte streams (8 bit bytes) 􀃆 Abstract classes are: InputStream and OutputStream
􀂃 Character streams (16 bit UNICODE) 􀃆 Abstract classes are: Reader and Writer
Design pattern:* classes use the decorator design pattern. The decorator design pattern attaches
responsibilities to objects at runtime. Decorators are more flexible than inheritance because the inheritance
attaches responsibility to classes at compile time. The* classes use the decorator pattern to construct
different combinations of behaviour at runtime based on some basic classes.
Attaching responsibilities to classes at
compile time using subclassing.
Attaching responsibilities to objects at runtime using a decorator
design pattern.
Inheritance (aka subclassing) attaches
responsibilities to classes at compile time.
When you extend a class, each individual
changes you make to child class will affect all
instances of the child classes. Defining many
classes using inheritance to have all possible
combinations is problematic and inflexible.
By attaching responsibilities to objects at runtime, you can apply changes
to each individual object you want to change.
File file = new File(“c:/temp”);
FileInputStream fis = new FileInputStream(file);
BufferedInputStream bis = new BufferedInputStream(fis);
Decorators decorate an object by enhancing or restricting functionality of an
object it decorates. The decorators add or restrict functionality to decorated
objects either before or after forwarding the request. At runtime the
BufferedInputStream (bis), which is a decorator (aka a wrapper around
decorated object), forwards the method call to its decorated object
FileInputStream (fis). The ‘bis’ will apply the additional functionality of
buffering around the lower level file (i.e. fis) I/O.
java.lang.System* class hierachy
Note: Only a few sub classes of abstract classes
like InputStream are shown for clarity.
The New I/O (NIO): more scalable and better performance
Java has long been not suited for developing programs that perform a lot of I/O operations. Furthermore,
commonly needed tasks such as file locking, non-blocking and asynchronous I/O operations and ability to map file
to memory were not available. Non-blocking I/O operations were achieved through work around such as
multithreading or using JNI. The New I/O API (aka NIO) in J2SE 1.4 has changed this situation.
A server’s ability to handle several client requests effectively depends on how it uses I/O streams. When a server
has to handle hundreds of clients simultaneously, it must be able to use I/O services concurrently. One way to
cater for this scenario in Java is to use threads but having almost one-to-one ratio of threads (100 clients will have
100 threads) is prone to enormous thread overhead and can result in performance and scalability problems
due to consumption of memory stacks and CPU context switching. To overcome this problem, a new set of
non-blocking I/O classes have been introduced to the Java platform in java.nio package. The non-blocking I/O
mechanism is built around Selectors and Channels. Channels, Buffers and Selectors are the core of the NIO.
A Channel class represents a bi-directional communication channel (similar to InputStrean and OutputStream)
between datasources such as a socket, a file, or an application component, which is capable of performing one or
more I/O operations such as reading or writing. Channels can be non-blocking, which means, no I/O operation will
wait for data to be read or written to the network. The good thing about NIO channels is that they can be
asynchronously interrupted and closed. So if a thread is blocked in an I/O operation on a channel, another thread
can interrupt that blocked thread.
Buffers hold data. Channels can fill and drain Buffers. Buffers replace the need for you to do your own buffer
management using byte arrays. There are different types of Buffers like ByteBuffer, CharBuffer, DoubleBuffer, etc.
A Selector class is responsible for multiplexing (combining multiple streams into a single stream) by allowing a
single thread to service multiple channels. Each Channel registers events with a Selector. When events arrive
from clients, the Selector demultiplexes (separating a single stream into multiple streams) them and dispatches
the events to corresponding Channels. To achieve non-blocking I/O a Channel class must work in conjunction with
a Selector class.
Design pattern: NIO uses a reactor design pattern, which demultiplexes events (separating single stream into
multiple streams) and dispatches them to registered object handlers. The reactor pattern is similar to an observer
pattern (aka publisher and subscriber design pattern), but an observer pattern handles only a single source of
events (i.e. a single publisher with multiple subscribers) where a reactor pattern handles multiple event sources
(i.e. multiple publishers with multiple subscribers). The intent of an observer pattern is to define a one-to-many
dependency so that when one object (i.e. the publisher) changes its state, all its dependents (i.e. all its
subscribers) are notified and updated correspondingly.
Another sought after functionality of NIO is its ability to map a file to memory. There is a specialized form of a
Buffer known as MappedByteBuffer, which represents a buffer of bytes mapped to a file. To map a file to
MappedByteBuffer, you must first get a channel for a file. Once you get a channel then you map it to a buffer and
subsequently you can access it like any other ByteBuffer. Once you map an input file to a CharBuffer, you can do
pattern matching on the file contents. This is similar to running “grep” on a UNIX file system.
Another feature of NIO is its ability to lock and unlock files. Locks can be exclusive or shared and can be held on a
contiguous portion of a file. But file locks are subject to the control of the underlying operating system.
Q 21: How can you improve Java I/O performance? PI BP
A 21: Java applications that utilise Input/Output are excellent candidates for performance tuning. Profiling of Java
applications that handle significant volumes of data will show significant time spent in I/O operations. This means
substantial gains can be had from I/O performance tuning. Therefore, I/O efficiency should be a high priority for
developers looking to optimally increase performance.
The basic rules for speeding up I/O performance are
􀂃 Minimise accessing the hard disk.
􀂃 Minimise accessing the underlying operating system.
􀂃 Minimise processing bytes and characters individually.
Let us look at some of the techniques to improve I/O performance. CO
􀂃 Use buffering to minimise disk access and underlying operating system. As shown below, with buffering
large chunks of a file are read from a disk and then accessed a byte or character at a time.
Without buffering : inefficient code
File f = new File("myFile.txt");
FileInputStream fis = new FileInputStream(f);
int count = 0;
int b = ;
while((b = != -1){
if(b== '\n') {
// fis should be closed in a finally block.
fis.close() ;
catch(IOException io){}
Note: is a native method call to the
underlying system.
With Buffering: yields better performance
File f = new File("myFile.txt");
FileInputStream fis = new FileInputStream(f);
BufferedInputStream bis = new BufferedInputStream(fis);
int count = 0;
int b = ;
while((b = != -1){
if(b== '\n') {
//bis should be closed in a finally block.
bis.close() ;
catch(IOException io){}
Note: takes the next byte from the input buffer and only
rarely access the underlying operating system.
Instead of reading a character or a byte at a time, the above code with buffering can be improved further by
reading one line at a time as shown below:
FileReader fr = new FileReader(f);
BufferedReader br = new BufferedReader(fr);
While (br.readLine() != null) count++;
By default the System.out is line buffered, which means that the output buffer is flushed when a new line
character is encountered. This is required for any interactivity between an input prompt and display of output.
The line buffering can be disabled for faster I/O operation as follows:
FileOutputStream fos = new FileOutputStream(file);
BufferedOutputStream bos = new BufferedOutputStream(fos, 1024);
PrintStream ps = new PrintStream(bos,false);
while (someConditionIsTrue)
It is recommended to use logging frameworks like Log4J or apache commons logging, which uses
buffering instead of using default behaviour of System.out.println(…..) for better performance. Frameworks
like Log4J are configurable, flexible, extensible and easy to use.
􀂃 Use the NIO package, if you are using JDK 1.4 or later, which uses performance-enhancing features like
buffers to hold data, memory mapping of files, non-blocking I/O operations etc.
􀂃 I/O performance can be improved by minimising the calls to the underlying operating systems. The Java
runtime itself cannot know the length of a file, querying the file system for isDirectory(), isFile(), exists() etc
must query the underlying operating system.
􀂃 Where applicable caching can be used to improve performance by reading in all the lines of a file into a Java
collection class like an ArrayList or a HashMap and subsequently access the data from an in-memory
collection instead of the disk.
Q 22: What is the main difference between shallow cloning and deep cloning of objects? DC LF MI PI
A 22: The default behaviour of an object’s clone() method automatically yields a shallow copy. So to achieve a deep
copy the classes must be edited or adjusted.
Shallow copy: If a shallow copy is performed on obj-1 as shown in fig-2 then it is copied but its contained objects
are not. The contained objects Obj-1 and Obj-2 are affected by changes to cloned Obj-2. Java supports shallow
cloning of objects by default when a class implements the java.lang.Cloneable interface.
Deep copy: If a deep copy is performed on obj-1 as shown in fig-3 then not only obj-1 has been copied but the
objects contained within it have been copied as well. Serialization can be used to achieve deep cloning. Deep
cloning through serialization is faster to develop and easier to maintain but carries a performance overhead.
Fig-2:Shallow cloning
Fig-3:Deep cloning
Shallow Vs Deep cloning
Fig-1:Original Object
contains contains
For example, invoking clone() method on a HashMap returns a shallow copy of HashMap instance, which means
the keys and values themselves are not cloned. If you want a deep copy then a simple method is to serialize
the HashMap to a ByteArrayOutputSream and then deserialize it. This creates a deep copy but does require that
all keys and values in the HashMap are Serializable. Its primary advantage is that it will deep copy any arbitrary
object graph.
List some of the methods supported by Java object class? clone(), toString(), equals(Object obj), hashCode()
􀃆 refer Q16 in Java section, wait(), notify() 􀃆 refer Q42 in Java section, finalize() etc.
Q 23: What is the difference between an instance variable and a static variable? Give an example where you might use
a static variable? LF
A 23:
Static variable Instance variable
Class variables are called static variables. There is only one
occurrence of a class variable per JVM per class loader.
When a class is loaded the class variables (aka static
variables) are initialised.
Instance variables are non-static and there is one
occurrence of an instance variable in each class instance
(i.e. each object).
A static variable is used in the singleton pattern. (Refer Q45 in Java section). A static variable is used with a final
modifier to define constants.
Q 24: Give an example where you might use a static method? LF
A 24: Static methods prove useful for creating utility classes, singleton classes and factory methods (Refer Q45,
Q46 in Java section). Utility classes are not meant to be instantiated. Improper coding of utility classes can lead to
procedural coding. java.lang.Math, java.util.Collections etc are examples of utility classes in Java.
Q 25: What are access modifiers? LF
A 25:
Modifier Used with Description
public Outer classes, interfaces,
constructors, Inner classes, methods
and field variables
A class or interface may be accessed from outside the
package. Constructors, inner classes, methods and field
variables may be accessed wherever their class is
protected Constructors, inner classes, methods,
and field variables.
Accessed by other classes in the same package or any
subclasses of the class in which they are referred (i.e. same
package or different package).
private Constructors, inner classes,
methods and field variables,
Accessed only within the class in which they are declared
No modifier:
(Package by
Outer classes, inner classes,
interfaces, constructors, methods, and
field variables
Accessed only from within the package in which they are
Q 26: Where and how can you use a private constructor? LF
A 26: Private constructor is used if you do not want other classes to instantiate the object. The instantiation is done by a
public static method within the same class.
􀂃 Used in the singleton pattern. (Refer Q45 in Java section).
􀂃 Used in the factory method pattern (Refer Q46 in Java section).
􀂃 Used in utility classes e.g. StringUtils etc.
Q 27: What is a final modifier? Explain other Java modifiers? LF
A 27: A final class can’t be extended i.e. A final class may not be subclassed. A final method can’t be overridden when
its class is inherited. You can’t change value of a final variable (i.e. it is a constant).
Modifier Class Method Property
static A static inner class is just an
inner class associated with
the class, rather than with an
cannot be instantiated, are called by
classname.method, can only access static
Only one instance
of the variable
abstract Cannot be instantiated, must
be a superclass, used
whenever one or more
methods are abstract.
Method is defined but contains no implementation
code (implementation code is included in the
subclass). If a method is abstract then the entire
class must be abstract.
synchronized N/A Acquires a lock on the class for static methods.
Acquires a lock on the instance for non-static
transient N/A N/A
Field should not
be serialized.
final Class cannot be inherited Method cannot be overridden Makes the variable
a constant.
native N/A Platform dependent. No body, only signature. N/A
Note: Be prepared for tricky questions on modifiers like, what is a “volatile”? Or what is a “const”? Etc. The reason it is tricky is
that Java does have these keywords “const” and “volatile” as reserved, which means you can’t name your variables with these
names but modifier “const” is not yet added in the language and the modifier “volatile” is very rarely used.
The “volatile” modifier is used on member variables that may be modified simultaneously by other threads. Since other threads
cannot see local variables, there is no need to mark local variables as volatile. E.g. volatile int number; volatile private List
listItems = null; etc. The modifier volatile only synchronizes the variable marked as volatile whereas “synchronized” modifier
synchronizes all variables.
Java uses the final modifier to declare constants. A final variable or constant declared as “final” has a value that is immutable
and cannot be modified to refer to any other objects other than one it was initialized to refer to. So the “final” modifier applies only
to the value of the variable itself, and not to the object referenced by the variable. This is where the “const” modifier can come in
very useful if added to the Java language. A reference variable or a constant marked as “const” refers to an immutable object
that cannot be modified. The reference variable itself can be modified, if it is not marked as “final”. The “const” modifier will be
applicable only to non-primitive types. The primitive types should continue to use the modifier “final”.
Q 28: What is the difference between final, finally and finalize() in Java? LF
A 28:
􀂃 final - constant declaration. Refer Q27 in Java section.
􀂃 finally - handles exception. The finally block is optional and provides a mechanism to clean up regardless of
what happens within the try block (except System.exit(0) call). Use the finally block to close files or to release
other system resources like database connections, statements etc. (Refer Q45 in Enterprise section)
􀂃 finalize() - method helps in garbage collection. A method that is invoked before an object is discarded by the
garbage collector, allowing it to clean up its state. Should not be used to release non-memory resources like
file handles, sockets, database connections etc because Java has only a finite number of these resources and
you do not know when the garbage collection is going to kick in to release these non-memory resources
through the finalize() method.
Q 29: How does Java allocate stack and heap memory? Explain re-entrant, recursive and idempotent
methods/functions? MI CI
A 29: Each time an object is created in Java it goes into the area of memory known as heap. The primitive variables like
int and double are allocated in the stack, if they are local method variables and in the heap if they are member
variables (i.e. fields of a class). In Java methods local variables are pushed into stack when a method is invoked
and stack pointer is decremented when a method call is completed. In a multi-threaded application each thread
will have its own stack but will share the same heap. This is why care should be taken in your code to avoid any
concurrent access issues in the heap space. The stack is threadsafe (each thread will have its own stack) but the
heap is not threadsafe unless guarded with synchronisation through your code.
A method in stack is re-entrant allowing multiple concurrent invocations that do not interfere with each other. A
function is recursive if it calls itself. Given enough stack space, recursive method calls are perfectly valid in Java
though it is tough to debug. Recursive functions are useful in removing iterations from many sorts of algorithms. All
recursive functions are re-entrant but not all re-entrant functions are recursive. Idempotent methods are methods,
which are written in such a way that repeated calls to the same method with the same arguments yield same
results. For example clustered EJBs, which are written with idempotent methods, can automatically recover from a
server failure as long as it can reach another server.
J a v a s t a c k & h e a p m e m o r y a l lo c a t io n
S ta c k
H e a p
p u b lic c la s s S ta c k R e f {
p u b l i c v o i d f i r s t ( ) {
s e c o n d ( ) ;
/ / a f te r
p u b l i c v o i d s e c o n d ( ) {
C a r c = n e w C a r ( ) ;
f i r s t ( )
s e c o n d ()
f i r s t ( )
s e c o n d () re f c
f i r s t ( )
C a r
R e f
p u b lic c la s s H e a p R e f {
C a r c = n e w C a r ( ) ;
p u b l i c v o i d f i r s t ( ) {
c = N e w C a r ( ) ;
c C a r
R e f
C a r
c C a r 2
Q 30: Explain Outer and Inner classes (or Nested classes) in Java? When will you use an Inner Class? LF
A 30: In Java not all classes have to be defined separate from each other. You can put the definition of one class inside
the definition of another class. The inside class is called an inner class and the enclosing class is called an outer
class. So when you define an inner class, it is a member of the outer class in much the same way as other
members like attributes, methods and constructors.
Where should you use inner classes? Code without inner classes is more maintainable and readable. When
you access private data members of the outer class, the JDK compiler creates package-access member functions
in the outer class for the inner class to access the private members. This leaves a security hole. In general we
should avoid using inner classes. Use inner class only when an inner class is only relevant in the context of the
outer class and/or inner class can be made private so that only outer class can access it. Inner classes are used
primarily to implement helper classes like Iterators, Comparators etc which are used in the context of an outer
class. CO
Member inner class Anonymous inner class
public class MyStack {
private Object[] items = null;

public Iterator iterator() {
return new StackIterator();
//inner class
class StackIterator implements Iterator{

public boolean hasNext(){…}
public class MyStack {
private Object[] items = null;

public Iterator iterator() {
return new Iterator {

public boolean hasNext() {…}
Explain outer and inner classes?
Class Type Description Example + Class name
member class
or interface
Top level class. Only type JVM
can recognize.
//package scope
class Outside{}
static nested
class or
Defined within the context of the
top-level class. Must be static &
can access static members of its
containing class. No relationship
between the instances of outside
and Inside classes.
//package scope
class Outside {
static class Inside{ }
Outside.class ,Outside$Inside.class
Member class Defined within the context of
outer class, but non-static. Until
an object of Outside class has
been created you can’t create
class Outside{
class Inside(){}
Outside.class , Outside$Inside.class
Local class Defined within a block of code.
Can use final local variables and
final method parameters. Only
visible within the block of code
that defines it.
class Outside {
void first() {
final int i = 5;
class Inside{}
Outside.class , Outside$1$Inside.class
Just like local class, but no
name is used. Useful when only
one instance is used in a
method. Most commonly used in
AWT event model.
class Outside{
void first() {
button.addActionListener ( new ActionListener()
public void actionPerformed(ActionEvent e) {
System.out.println(“The button was pressed!”);
Outside.class , Outside$1.class
Q 31: What is type casting? Explain up casting vs. down casting? When do you get ClassCastException? LF DP
A 31: Type casting means treating a variable of one type as though it is another type.
When up casting primitives as shown below from left to right, automatic conversion occurs. But if you go from
right to left, down casting or explicit casting is required. Casting in Java is safer than in C or other languages that
allow arbitrary casting. Java only lets casts occur when they make sense, such as a cast between a float and an
int. However you can't cast between an int and a String (is an object in Java).
byte 􀃆 short 􀃆 int 􀃆 long 􀃆 float 􀃆 double
int i = 5;
long j = i; //Right. Up casting or implicit casting
byte b1 = i; //Wrong. Compile time error “Type Mismatch”.
byte b2 = (byte) i ; //Right. Down casting or explicit casting is required.
When it comes to object references you can always cast from a subclass to a superclass because a subclass
object is also a superclass object. You can cast an object implicitly to a super class type (i.e. upcasting). If this
were not the case polymorphism wouldn’t be possible.
O bjec t
V e h ic le
B us C a r
V e h ic le v 1 = n ew C a r ( ); / / R igh t .u p c a s t in g o r im p lic it ca s t in g
V e h ic le v 2 = n ew V e h ic le ( );
C a r c 0 = v 1 ; / /W ro n g . c om p ile t im e e r ro r "T ype M ism atch " .
/ / E x p lic it o r d ow n ca s t in g is re q u ir ed
C a r c 1 = (C a r)v 1 ; / / R igh t . d ow n c as tin g o r e xp lic it c a stin g .
/ / v 1 h as kn ow le d ge o f C a r d u e to lin e 1
C a r c 2 = (C a r)v 2 ; / /W ro n g. R un t im e e x c e p tio n C la s sC a s tE x c e p t ion
/ / v 2 h a s n o k n o w le d ge o f C ar .
B u s b 1 = n ew BM W ( ); / /W ro n g . com p ile t im e e r ro r "T yp e M ism atc h"
C a r c 3 = n ew BM W () ; / /R ig h t.u p c ast in g o r im p lic it c as tin g
C a r c 4 = (BM W )v 1 ; / /W ro n g . R u n tim e e x c ep tio n C la s sC a s tE x c e p t io n
O b je c t o = v 1 ; / / v 1 c a n o n ly b e u p c a st to its p a re n t o r
C a r c 5 = (C a r) v 1 ; / / v 1 c an b e d o w n c a s t to C a r d u e to lin e 1 .
U p c a s tin g v s D ow n c a s tin g O b je c ts
You can cast down the hierarchy as well but you must explicitly write the cast and the object must be a
legitimate instance of the class you are casting to. The ClassCastException is thrown to indicate that code
has attempted to cast an object to a subclass of which it is not an instance. We can deal with the problem of
incorrect casting in two ways:
􀂃 Use the exception handling mechanism to catch ClassCastException.
Object o = new Integer(1);
System.out.println((String) o);
catch(ClassCastException cce) {
logger.log(“Invalid casting, String is expected…Not an Integer”);
System.out.println(((Integer) o).toString());
􀂃 Use the instanceof statement to guard against incorrect casting.
If(v2 instanceof Car) {
Car c2 = (Car) v2;
Design pattern: The “instanceof” and “typecast” constructs are shown for the illustration purpose only.
Using these constructs can be unmaintainable due to large if and elseif statements and can affect
performance if used in frequently accessed methods or loops. Look at using visitor design pattern to avoid
these constructs. (Refer Q11 in How would you go about section…).
Points-to-ponder: We can also get a ClassCastException when two different class loaders load the same class because they
are treated as two different classes.
Q 32: What do you know about the Java garbage collector? When does the garbage collection occur? Explain different
types of references in Java? LF MI
A 32: Each time an object is created in Java, it goes into the area of memory known as heap. The Java heap is called
the garbage collectable heap. The garbage collection cannot be forced. The garbage collector runs in low
memory situations. When it runs, it releases the memory allocated by an unreachable object. The garbage
collector runs on a low priority daemon (background) thread. You can nicely ask the garbage collector to collect
garbage by calling System.gc() but you can’t force it.
What is an unreachable object? An object’s life has no meaning unless something has reference to it. If you
can’t reach it then you can’t ask it to do anything. Then the object becomes unreachable and the garbage collector
will figure it out. Java automatically collects all the unreachable objects periodically and releases the memory
consumed by those unreachable objects to be used by the future reachable objects.
G a r b a g e C o l l e c t io n & U n r e a c h a b l e O b je c t s
r e a c h a b l e
r e a c h a b l e
C a s e 1
C a r a = n e w C a r ( ) ;
C a r b = n e w C a r( )
1 r e f a
C a r o b je c t
2 r e f b
C a r o b je c t
C a s e 2
a = n e w C a r( ) r e a c h a b l e
r e a c h a b l e
u n r e a c h a b le
2 r e f b
C a r o b je c t
3 r e f a
C a r o b je c t
C a r o b je c t
C a s e 3
a = b
u n r e a c h a b le
r e a c h a b l e
u n r e a c h a b le
r e f a
C a r o b je c t
r e f b
C a r o b je c t
C a r o b je c t
C a s e 4
a = n u ll ;
b = n u ll ;
u n r e a c h a b le
u n r e a c h a b le
u n r e a c h a b le
C a r o b je c t
C a r o b je c t
C a r o b je c t
H e a p
We can use the following options with the Java command to enable tracing for garbage collection events.
-verbose:gc reports on each garbage collection event.
Explain types of references in Java? java.lang.ref package can be used to declare soft, weak and phantom
􀂃 Garbage Collector won’t remove a strong reference.
􀂃 A soft reference will only get removed if memory is low. So it is useful for implementing caches while
avoiding memory leaks.
􀂃 A weak reference will get removed on the next garbage collection cycle. Can be used for implementing
canonical maps. The java.util.WeakHashMap implements a HashMap with keys held by weak references.
􀂃 A phantom reference will be finalized but the memory will not be reclaimed. Can be useful when you want to
be notified that an object is about to be collected.
Q 33: If you have a circular reference of objects, but you no longer reference it from an execution thread, will this object
be a potential candidate for garbage collection? LF MI
A 33: Yes. Refer diagram below.
sample code
public void buildCar() {
Car c = new Car();
Engine e = new Engine();
//lets create a circular reference
c.engine = e; = c;
Stack Heap
Before buildCar() returns
Stack Heap
After buildCar() returns
Both the Car & Engine are not reachable
and potential candidate for Garbage
Garbage Collecting Circular References
Q 34: Discuss the Java error handling mechanism? What is the difference between Runtime (unchecked) exceptions
and checked exceptions? What is the implication of catching all the exceptions with the type “Exception”? EH BP
A 34:
Errors: When a dynamic linking failure or some other “hard” failure in the virtual machine occurs, the virtual
machine throws an Error. Typical Java programs should not catch Errors. In addition, it’s unlikely that typical Java
programs will ever throw Errors either.
Exceptions: Most programs throw and catch objects that derive from the Exception class. Exceptions indicate
that a problem occurred but that the problem is not a serious JVM problem. An Exception class has many
subclasses. These descendants indicate various types of exceptions that can occur. For example,
NegativeArraySizeException indicates that a program attempted to create an array with a negative size. One
exception subclass has special meaning in the Java language: RuntimeException. All the exceptions except
RuntimeException are compiler checked exceptions. If a method is capable of throwing a checked exception it
must declare it in its method header or handle it in a try/catch block. Failure to do so raises a compiler error. So
checked exceptions can, at compile time, greatly reduce the occurrence of unhandled exceptions surfacing at
runtime in a given application at the expense of requiring large throws declarations and encouraging use of poorlyconstructed
try/catch blocks. Checked exceptions are present in other languages like C++, C#, and Python.
Throw able a n d its subclasses
IOException RuntimeException
NullPoin terException
Runtime Exceptions (unchecked exception)
A RuntimeException class represents exceptions that occur within the Java virtual machine (during runtime). An
example of a runtime exception is NullPointerException. The cost of checking for the runtime exception often
outweighs the benefit of catching it. Attempting to catch or specify all of them all the time would make your code
unreadable and unmaintainable. The compiler allows runtime exceptions to go uncaught and unspecified. If you
like, you can catch these exceptions just like other exceptions. However, you do not have to declare it in your
“throws" clause or catch it in your catch clause. In addition, you can create your own RuntimeException
subclasses and this approach is probably preferred at times because checked exceptions can complicate method
signatures and can be difficult to follow.
Exception handling best practices: BP
Why is it not advisable to catch type “Exception”? CO
Exception handling in Java is polymorphic in nature. For example if you catch type Exception in your code then it
can catch or throw its descendent types like IOException as well. So if you catch the type Exception before the
type IOException then the type Exception block will catch the entire exceptions and type IOException block is
never reached. In order to catch the type IOException and handle it differently to type Exception, IOException
should be caught first (remember that you can’t have a bigger basket above a smaller basket).
The diagram below is an example for illustration only. In practice it is not recommended to catch type
“Exception”. We should only catch specific subtypes of the Exception class. Having a bigger basket (i.e.
Exception) will hide or cause problems. Since the RunTimeException is a subtype of Exception, catching the type
Exception will catch all the run time exceptions (like NullpointerException, ArrayIndexOut-OfBounds-Exception) as
Catching Exceptions
catch(Exception ex){
//this block is reached
catch(IOException ioe) {
//this block is never reached
//There is a bigger basket
//above me who will catch it
//before I can.
catch(IOException ioe){
catch(Exception ex) {
Wrong approach
Right approach
Hint: as shown in the
figure, think of catching an
exception in a basket. You
should always have the
smaller basket above the
bigger one. Otherwise the
bigger basket will catch all
the exceptions and smaller
baskets will not catch any.
Why should you throw an exception early? CO
The exception stack trace helps you pinpoint where an exception occurred by showing us the exact sequence of
method calls that lead to the exception. By throwing your exception early, the exception becomes more accurate
and more specific. Avoid suppressing or ignoring exceptions. Also avoid using exceptions just to get a flow control.
Instead of:

InputStream in = new FileInputStream(fileName); // assume this line throws an exception because filename == null.

Use the following code because you get a more accurate stack trace:

if(filename == null) {
throw new IllegalArgumentException(“file name is null”);
InputStream in = new FileInputStream(fileName);

Why should you catch a checked exception late in a catch {} block?
You should not try to catch the exception before your program can handle it in an appropriate manner. The natural
tendency when a compiler complains about a checked exception is to catch it so that the compiler stops reporting
errors. The best practice is to catch the exception at the appropriate layer (e.g. an exception thrown at an
integration layer can be caught at a presentation layer in a catch {} block), where your program can either
meaningfully recover from the exception and continue to execute or log the exception only once in detail, so that
user can identify the cause of the exception.
Note: Due to heavy use of checked exceptions and minimal use of unchecked exceptions, there has been a hot debate in the
Java community regarding true value of checked exceptions. Use checked exceptions when the client code can take some
useful recovery action based on information in exception. Use unchecked exception when client code cannot do anything. For
example, convert your SQLException into another checked exception if the client code can recover from it and convert your
SQLException into an unchecked (i.e. RuntimeException) exception, if the client code cannot do anything about it.
A note on key words for error handling:
throw / throws – used to pass an exception to the method that called it.
try – block of code will be tried but may cause an exception.
catch – declares the block of code, which handles the exception.
finally – block of code, which is always executed (except System.exit(0) call) no matter what program flow, occurs when dealing
with an exception.
assert – Evaluates a conditional expression to verify the programmer’s assumption.
Q 35: What is a user defined exception? EH
A 35: User defined exceptions may be implemented by defining a new exception class by extending the Exception class.
public class MyException extends Exception {
/* class definition of constructors goes here */
public MyException() {
public MyException (String errorMessage) {
super (errorMessage);
Throw and/or throws statement is used to signal the occurrence of an exception. Throw an exception:
throw new MyException(“I threw my own exception.”)
To declare an exception: public myMethod() throws MyException {…}
Q 36: What is the difference between processes and threads? LF MI CI
A 36: A process is an execution of a program but a thread is a single execution sequence within the process. A process
can contain multiple threads. A thread is sometimes called a lightweight process.
Process (JVM)
Stack Stack Stack
Each thread has its
own stack memory
Thread 1 Thread 2 Thread 3
method1() method1() method1()
Process vs Threads
Object1 Object
Single heap per process
shared by all the threads
A JVM runs in a single process and threads in a JVM share the heap belonging to that process. That is why
several threads may access the same object. Threads share the heap and have their own stack space. This is
how one thread’s invocation of a method and its local variables are kept thread safe from other threads. But the
heap is not thread-safe and must be synchronized for thread safety.
Q 37: Explain different ways of creating a thread? LF
A 37: Threads can be used by either :
􀂃 Extending the Thread class
􀂃 Implementing the Runnable interface.
class Counter extends Thread {
//method where the thread execution will start
public void run(){
//logic to execute in a thread
//let’s see how to start the threads
public static void main(String[] args){
Thread t1 = new Counter();
Thread t2 = new Counter();
t1.start(); //start the first thread. This calls the run() method
t2.start(); //this starts the 2nd thread. This calls the run() method
class Counter extends Base implements Runnable {
//method where the thread execution will start
public void run(){
//logic to execute in a thread
//let us see how to start the threads
public static void main(String[] args){
Thread t1 = new Thread(new Counter());
Thread t2 = new Thread(new Counter());
t1.start(); //start the first thread. This calls the run() method
t2.start(); //this starts the 2nd thread. This calls the run() method
The runnable interface is preferred, as it does not require your object to inherit a thread because when you need
multiple inheritance, only interfaces can help you. In the above example we had to extend the Base class so
implementing runnable interface is an obvious choice. Also note how the threads are started in each of the
different cases as shown in the code sample.
Q 38: Briefly explain high-level thread states? LF
A 38: The state chart diagram below describes the thread states. (Refer Q107 in Enterprise section for state chart
Thread states(StateMachine diagram)
done (executing)
Object.notifyAll(); Sleeping
Scheduler swap
or Thread.yield();
chosen by
Blocked on I/O
another thread closes socket
(Diagram sourced from:
􀂃 Runnable — waiting for its turn to be picked for execution by the thread schedular based on thread priorities.
􀂃 Running: The processor is actively executing the thread code. It runs until it becomes blocked, or voluntarily
gives up its turn with this static method Thread.yield(). Because of context switching overhead, yield() should
not be used very frequently.
􀂃 Waiting: A thread is in a blocked state while it waits for some external processing such as file I/O to finish.
􀂃 Sleeping: Java threads are forcibly put to sleep (suspended) with this overloaded method:
Thread.sleep(milliseconds), Thread.sleep(milliseconds, nanoseconds);
􀂃 Blocked on I/O: Will move to runnable after I/O condition like reading bytes of data etc changes.
􀂃 Blocked on synchronization: Will move to Runnable when a lock is acquired.
􀂃 Dead: The thread is finished working.
Q 39: What is the difference between yield and sleeping? LF
A 39: When a task invokes yield(), it changes from running state to runnable state. When a task invokes sleep(), it
changes from running state to waiting/sleeping state.
Q 40: How does thread synchronization occurs inside a monitor? What levels of synchronization can you apply? What is
the difference between synchronized method and synchronized block? LF CI PI
A 40: In Java programming, each object has a lock. A thread can acquire the lock for an object by using the
synchronized keyword. The synchronized keyword can be applied in method level (coarse grained lock – can
affect performance adversely) or block level of code (fine grained lock). Often using a lock on a method level is
too coarse. Why lock up a piece of code that does not access any shared resources by locking up an entire
method. Since each object has a lock, dummy objects can be created to implement block level synchronization.
The block level is more efficient because it does not lock the whole method.
class MethodLevel {
//shared among threads
SharedResource x, y ;
pubic void synchronized
method1() {
//multiple threads can't access
pubic void synchronized
method2() {
//multiple threads can't access
public void method3() {
//not synchronized
//multiple threads can access
class BlockLevel {
//shared among threads
SharedResource x, y ;
//dummy objects for locking
Object xLock = new Object(), yLock = new Object();
pubic void method1() {
//access x here. thread safe
//do something here but don't use
SharedResource x, y ;
synchronized(xLock) {
synchronized(yLock) {
//access x,y here. thread safe
//do something here but don't use
SharedResource x, y ;
The JVM uses locks in conjunction with monitors. A monitor is basically a guardian who watches over a sequence
of synchronized code and making sure only one thread at a time executes a synchronized piece of code. Each
monitor is associated with an object reference. When a thread arrives at the first instruction in a block of code it
must obtain a lock on the referenced object. The thread is not allowed to execute the code until it obtains the lock.
Once it has obtained the lock, the thread enters the block of protected code. When the thread leaves the block, no
matter how it leaves the block, it releases the lock on the associated object.
Why synchronization is important? Without synchronization, it is possible for one thread to modify a shared
object while another thread is in the process of using or updating that object’s value. This often causes dirty data
and leads to significant errors. The disadvantage of synchronization is that it can cause deadlocks when two
threads are waiting on each other to do something. Also synchronized code has the overhead of acquiring lock,
which can adversely the performance.
Q 41: What is a daemon thread? LF
A 41: Daemon threads are sometimes called "service" threads. These are threads that normally run at a low priority and
provide a basic service to a program or programs when activity on a machine is reduced. An example of a daemon
thread that is continuously running is the garbage collector thread. This thread is provided by the JVM.
Q 42: How can threads communicate with each other? How would you implement a producer (one thread) and a
consumer (another thread) passing data (via stack)? LF
A 42: The wait(), notify(), and notifyAll() methods are used to provide an efficient way for threads to communicate with
each other. This communication solves the ‘consumer-producer problem’. This problem occurs when the
producer thread is completing work that the other thread (consumer thread) will use.
Example: If you imagine an application in which one thread (the producer) writes data to a file while a second
thread (the consumer) reads data from the same file. In this example the concurrent threads share the same
resource file. Because these threads share the common resource file they should be synchronized. Also these
two threads should communicate with each other because the consumer thread, which reads the file, should wait
until the producer thread, which writes data to the file and notifies the consumer thread that it has completed its
writing operation.
Let’s look at a sample code where count is a shared resource. The consumer thread will wait inside the
consume() method on the producer thread, until the producer thread increments the count inside the produce()
method and subsequently notifies the consumer thread. Once it has been notified, the consumer thread waiting
inside the consume() method will give up its waiting state and completes its method by consuming the count (i.e.
decrementing the count).
T h read com m u n icatio n (C o n sum er v s P ro d u cer th read s)
C lass C onsum erProd ucer {
p r iv a te in t co u n t;
p u b lic syn ch ro nized vo id co nsum e(){
w h ile (co u n t = = 0 ) {
w ait()
c a tch (In te r ru p tedE x c ep tio n ie ) {
/ / k e ep try in g
co u n t --; / / co n sum ed
p rivate syn ch ro nized void p ro d u ce(){
co u n t+ + ;
n o tify ( ) ; / / n o tify th e c o n sum e r th at co u n t h as b een in crem en ted .
Q 43: If 2 different threads hit 2 different synchronized methods in an object at the same time will they both continue?
A 43: No. Only one method can acquire the lock.
synchronized method1() {}
synchronized method2() {}
method3() {}
Car1 object
synchronized method1() {}
synchronized method2() {}
method3() {}
Car2 object
Thread synchronization
4. Always ok. method3() is not synchronized
6.Always ok. method3() is not synchronized
1. ok. method1() is not busy.
3. ok. Method2() is not busy
2. No. method2() is busy
5. No. method1() is busy.
Q 44: Explain threads blocking on I/O? LF
A 44: Occasionally threads have to block on conditions other than object locks. I/O is the best example of this. Threads
block on I/O (i.e. enters the waiting state) so that other threads may execute while the I/O operation is performed.
When threads are blocked (say due to time consuming reads or writes) on an I/O call inside an object’s
synchronized method and also if the other methods of the object are also synchronized then the object is
essentially frozen while the thread is blocked.
Be sure to not synchronize code that makes blocking calls, or make sure that a non-synchronized method
exists on an object with synchronized blocking code. Although this technique requires some care to ensure that
the resulting code is still thread safe, it allows objects to be responsive to other threads when a thread holding its
locks is blocked.
Note: The java.nio.* package was introduced in JDK1.4. The coolest addition is nonblocking I/O (aka NIO that
stands for New I/O). Refer Q20 in Java section for NIO.
Note: Q45 & Q46 are very popular questions on design patterns.
Q 45: What is a singleton pattern? How do you code it in Java? DP MI CO
A 45: A singleton is a class that can be instantiated only one time in a JVM per class loader. Repeated calls always
return the same instance. Ensures that a class has only one instance, and provide a global point of access. It
can be an issue if singleton class gets loaded by multiple class loaders.
public class OnlyOne {
private static OnlyOne one = new OnlyOne();
private OnlyOne(){… } //private constructor. This class cannot be instantiated from outside.
public static OnlyOne getInstance() {
return one;
To use it:
//No matter how many times you call, you get the same instance of the object.
OnlyOne myOne = OnlyOne.getInstance();
Note: The constructor must be explicitly declared and should have the private access modifier, so that it cannot be
instantiated from out side the class. The only way to instantiate an instance of class OnlyOne is through the
getInstance() method with a public access modifier.
When to use: Use it when only a single instance of an object is required in memory for a single point of access.
For example the following situations require a single point of access, which gets invoked from various parts of
the code.
􀂃 Accessing application specific properties through a singleton object, which reads them for the first time from
a properties file and subsequent accesses are returned from in-memory objects. Also there could be
another piece of code, which periodically synchronizes the in-memory properties when the values get
modified in the underlying properties file. This piece of code accesses the in-memory objects through the
singleton object (i.e. global point of access).
􀂃 Accessing in-memory object cache or object pool, or non-memory based resource pools like sockets,
connections etc through a singleton object (i.e. global point of access).
What is the difference between a singleton class and a static class? Static class is one approach to make a class singleton
by declaring the class as “final” so that it cannot be extended and declaring all the methods as static so that you can’t create any
instance of the class and can call the static methods directly.
Q 46: What is a factory pattern? DP CO
A 46: A Factory method pattern (aka Factory pattern) is a creational pattern. The creational patterns abstract the
object instantiation process by hiding how the objects are created and make the system independent of the object
creation process. An Abstract factory pattern is one level of abstraction higher than a factory method pattern,
which means it returns the factory classes.
Factory method pattern (aka Factory pattern) Abstract factory pattern
Factory for what? Factory pattern returns one of the
several product subclasses. You should use a factory
pattern If you have a super class and a number of subclasses,
and based on some data provided, you have to
return the object of one of the subclasses. Let’s look at
a sample code:
Factory pattern
Factory Product hierachy
+getShape(int shapeId)()
+getShape (int shapeId)()
public interface Const {
public static final int SHAPE_CIRCLE =1;
public static final int SHAPE_SQUARE =2;
public static final int SHAPE_HEXAGON =3;
public class ShapeFactory {
public abstract Shape getShape(int shapeId);
public class SimpleShapeFactory extends
ShapeFactory throws BadShapeException {
An Abstract factory pattern is one level of abstraction higher than
a factory method pattern, which means the abstract factory
returns the appropriate factory classes, which will later on
return one of the product subclasses. Let’s look at a sample code:
public class ComplexShapeFactory extends ShapeFactory {
throws BadShapeException {
public Shape getShape(int shapeTypeId){
Shape shape = null;
if(shapeTypeId == Const.SHAPE_HEXAGON) {
shape = new Hexagon();//complex shape
else throw new BadShapeException
(“shapeTypeId=” + shapeTypeId);
return shape;
Now let’s look at the abstract factory, which returns one of the
types of ShapeFactory:
public class ShapeFactoryType
throws BadShapeFactoryException {
public static final int TYPE_SIMPLE = 1;
public static final int TYPE_COMPLEX = 2;
public ShapeFactory getShapeFactory(int type) {
ShapeFactory sf = null;
if(type == TYPE_SIMPLE) {
sf = new SimpleShapeFactory();
else if (type == TYPE_COMPLEX) {
sf = new ComplexShapeFactory();
else throw new BadShapeFactoryException(“No factory!!”);
public Shape getShape(int shapeTypeId){
Shape shape = null;
if(shapeTypeId == Const.SHAPE_CIRCLE) {
//in future can reuse or cache objects.
shape = new Circle();
else if(shapeTypeId == Const.SHAPE_SQUARE) {
//in future can reuse or cache objects
shape = new Square();
else throw new BadShapeException
(“ShapeTypeId=”+ shapeTypeId);
return shape;
Now let’s look at the calling code, which uses the
ShapeFactory factory = new SimpleShapeFactory();
//returns a Shape but whether it is a Circle or a
//Square is not known to the caller.
Shape s = factory.getShape(1);
s.draw(); // circle is drawn
//returns a Shape but whether it is a Circle or a
//Square is not known to the caller.
s = factory.getShape(2);
s.draw(); //Square is drawn
return sf;
Now let’s look at the calling code, which uses the factory:
ShapeFactoryType abFac = new ShapeFactoryType();
ShapeFactory factory = null;
Shape s = null;
//returns a ShapeFactory but whether it is a
//SimpleShapeFactory or a ComplexShapeFactory is not
//known to the caller.
factory = abFac.getShapeFactory(1);//returns SimpleShapeFactory
//returns a Shape but whether it is a Circle or a Pentagon is
//not known to the caller.
s = factory.getShape(2); //returns square.
s.draw(); //draws a square
//returns a ShapeFactory but whether it is a
//SimpleShapeFactory or a ComplexShapeFactory is not
//known to the caller.
factory = abFac.getShapeFactory(2);
//returns a Shape but whether it is a Circle or a Pentagon is
//not known to the caller.
s = factory.getShape(3); //returns a pentagon.
s.draw(); //draws a pentagon
Why use factory pattern or abstract factory pattern? Factory pattern returns an instance of several (product
hierarchy) subclasses (like Circle, Square etc), but the calling code is unaware of the actual implementation class.
The calling code invokes the method on the interface (for example Shape) and using polymorphism the correct
draw() method gets invoked [Refer Q8 in Java section for polymorphism]. So, as you can see, the factory pattern
reduces the coupling or the dependencies between the calling code and called objects like Circle, Square etc. This
is a very powerful and common feature in many frameworks. You do not have to create a new Circle or a new
Square on each invocation as shown in the sample code, which is for the purpose of illustration and simplicity. In
future, to conserve memory you can decide to cache objects or reuse objects in your factory with no changes
required to your calling code. You can also load objects in your factory based on attribute(s) read from an external
properties file or some other condition. Another benefit going for the factory is that unlike calling constructors
directly, factory patterns have more meaningful names like getShape(…), getInstance(…) etc, which may make
calling code more clear.
Can we use the singleton pattern within our factory pattern code? Yes. Another important aspect to consider
when writing your factory class is that, it does not make sense to create a new factory object for each invocation
as it is shown in the sample code, which is just fine for the illustration purpose.
ShapeFactory factory = new SimpleShapeFactory();
To overcome this, you can incorporate the singleton design pattern into your factory pattern code. The singleton
design pattern will create only a single instance of your SimpleShapeFactory class. Since an abstract factory
pattern is unlike factory pattern, where you need to have an instance for each of the two factories (i.e.
SimpleShapeFactory and ComplexShapeFactory) returned, you can still incorporate the singleton pattern as an
access point and have an instance of a HashMap, store your instances of both factories. Now your calling method
uses a static method to get the same instance of your factory, hence conserving memory and promoting object
ShapeFactory factory = ShapeFactory. Ge/tFactoryInstance();
Note: Since questions on singleton pattern and factory pattern are commonly asked in the interviews, they are included as part
of this section. To learn more about design patterns refer Q11 in How would you go about section…?
Q 47: What is a socket? How do you facilitate inter process communication in Java? LF
A 47: A socket is a communication channel, which facilitates inter-process communication (For example
communicating between two JVMs, which may or may not be running on two different physical machines). A
socket is an endpoint for communication. There are two kinds of sockets, depending on whether one wishes to
use a connectionless or a connection-oriented protocol. The connectionless communication protocol of the
Internet is called UDP. The connection-oriented communication protocol of the Internet is called TCP. UDP
sockets are also called datagram sockets. Each socket is uniquely identified on the entire Internet with two
numbers. The first number is a 128-bit integer called the Internet Address (or IP address). The second number is
a 16-bit integer called the port of the socket. The IP address is the location of the machine, which you are trying to
connect to and the port number is the port on which the server you are trying to connect is running. The port
numbers 0 to 1023 are reserved for standard services such as e-mail, FTP, HTTP etc.
The lifetime of the socket is made of 3 phases: Open Socket 􀃆 Read and Write to Socket 􀃆 Close Socket
To make a socket connection you need to know two things: An IP address and port on which to listen/connect. In
Java you can use the Socket (client side) and ServerSocket (Server side) classes.
Sending Process(JVM) Receiving Process(JVM)
Operating System Operating System
IP address:
port: 6678
port: 6678
Network communication
Q 48: How will you call a Web server from a stand alone Java application? LF
A 48: Using the and its subclasses like HttpURLConnection and JarURLConnection.
URLConnection HttpClient (browser)
Does not support cookies. Does support cookies.
Can handle protocols other than http like ftp, gopher, mailto
and file.
Handles only http.
Java – Swing
Q 49: What is the difference between AWT and Swing? LF DC
A 49: Swing provides a richer set of components than AWT. They are 100% Java-based. There are a few other
advantages to Swing over AWT:
• Swing provides both additional components like JTable, JTree etc and added functionality to AWT-replacement
• Swing components can change their appearance based on the current “look and feel” library that’s being used.
• Swing components follow the Model-View-Controller (MVC) paradigm, and thus can provide a much more
flexible UI.
• Swing provides “extras” for components, such as: icons on many components, decorative borders for
components, tool tips for components etc.
• Swing components are lightweight (less resource intensive than AWT).
• Swing provides built-in double buffering (which means an off-screen buffer [image] is used during drawing
and then the resulting bits are copied onto the screen. The resulting image is smoother, less flicker and quicker
than drawing directly on the screen).
• Swing provides paint debugging support for when you build your own component i.e.-slow motion rendering.
Swing also has a few disadvantages:
• If you’re not very careful when programming, it can be slower than AWT (all components are drawn).
• Swing components that look like native components might not behave exactly like native components.
Q 50: Explain the Swing Action architecture? LF DP
A 50: The Swing Action architecture is used to implement shared behaviour between two or more user interface
components. For example, the menu items and the tool bar buttons will be performing the same action no matter
which one is clicked. Another distinct advantage of using actions is that when an action is disabled then all the
components, which use the Action, become disabled.
Design pattern: The javax.swing.Action interface extends the ActionListener interface and is an abstraction of a
command that does not have an explicit UI component bound to it. The Action architecture is an implementation of
a command design pattern. This is a powerful design pattern because it allows the separation of controller logic
of an application from its visual representation. This allows the application to be easily configured to use different
UI elements without having to re-write the control or call-back logic.
Defining action classes:
class FileAction extends AbstractAction {
FileAction(String name) {
public void actionPerformed(ActionEvent ae){
//add action logic here
To add an action to a menu bar:
JMenu fileMenu = new JMenu(“File”);
FileAction newAction = new FileAction(“New”);
JMenuItem item = fileMenu.add(newAction);
item.setAccelaror(KeyStroke.getKeyStroke(‘N’, Event.CTRL_MASK));
To add action to a toolbar
private JToolBar toolbar = new JToolBar();
So, an action object is a listener as well as an action.
Q 51: If you add a component to the CENTER of a border layout, which directions will the component stretch? LF
A 51: The component will stretch both horizontally and vertically. It will occupy the whole space in the middle.
Q 52: What is the base class for all Swing components? LF
A 52: All the Swing components start with ‘J’. The hierarchy diagram is shown below. JComponent is the base class.
Swing Hierarchy
Frame Dialog
JFrame JDialog
JToggleButton JButton JmenuItem
(Diagram source:
Design pattern: As you can see from the above diagram, containers collect components. Sometimes you want
to add a container to another container. So, a container should be a component. For example
container.getPreferredSize() invokes getPreferredSize() of all contained components. Composite design
pattern is used in GUI components to achieve this. A composite object is an object, which contains other
objects. Composite design pattern manipulates composite objects just like you manipulate individual
components. Refer Q11 in How would you go about…? section.
Composite Design Pattern
Q 53: Explain the Swing event dispatcher mechanism? LF CI PI
A 53: Swing components can be accessed by the Swing event dispatching thread. A few operations are guaranteed to
be thread-safe but most are not. Generally the Swing components should be accessed through this eventJava
dispatching thread. The event-dispatching thread is a thread that executes drawing of components and eventhandling
code. For example the paint() and actionPerformed() methods are automatically executed in the eventdispatching
thread. Another way to execute code in the event-dispatching thread from outside event-handling or
drawing code, is using SwingUtilities invokeLater() or invokeAndWait() method. Swing lengthy initialization
tasks (e.g. I/O bound and computationally expensive tasks), should not occur in the event-dispatching
thread because this will hold up the dispatcher thread. If you need to create a new thread for example, to
handle a job that’s computationally expensive or I/O bound then you can use the thread utility classes such as
SwingWorker or Timer without locking up the event-dispatching thread.
• SwingWorker – creates a background thread to execute time consuming operations.
• Timer – creates a thread that executes at certain intervals.
However after the lengthy initialization the GUI update should occur in the event dispatching thread, for thread
safety reasons. We can use invokeLater() to execute the GUI update in the event-dispatching thread. The other
scenario where invokeLater() will be useful is that the GUI must be updated as a result of non-AWT event.
Q 54: What do you understand by MVC as used in a JTable? LF DP
A 54: MVC stands for Model View Controller architecture. Swing “J” components (e.g. JTable, JList, JTree etc) use a
modified version of MVC. MVC separates a model (or data source) from a presentation and the logic that
manages it.
(Eg: JTable):View & controller
Swing MVC architecture (e.g. JTable)
Eg: TableModel
for JTable
• Component (e.g. JTable, JTree, and JList): coordinates actions of model and the UI delegate. Each generic
component class handles its own individual view-and-controller responsibilities.
• Model (e.g. TableModel): charged with storing the data.
• UIDelegate: responsible for getting the data from model and rendering it to screen. It delegates any look-andfeel
aspect of the component to the UI Manager.
Q 55: Explain layout managers? LF
A 55: Layout managers are used for arranging GUI components in windows. The standard layout managers are:
• FlowLayout: Default layout for Applet and Panel. Lays out components from left to right, starting new rows if
• BorderLayout: Default layout for Frame and Dialog. Lays out components in north, south, east, west and
center. All extra space is placed on the center.
• CardLayout: stack of same size components arranged inside each other. Only one is visible at any time. Used
in TABs.
• GridLayout: Makes a bunch of components equal in size and displays them in the requested number of rows
and columns.
• GridBagLayout: Most complicated but the most flexible. It aligns components by placing them within a grid of
cells, allowing some components to span more than one cell. The rows in the grid aren’t necessarily all the
same height, similarly, grid columns can have different widths as well.
• BoxLayout: is a full-featured version of FlowLayout. It stacks the components on top of each other or places
them in a row.
Complex layouts can be simplified by using nested containers for example having panels within panels and each
panel can use its own LayoutManager. It is also possible to write your own layout manager or use manual
positioning of the GUI components. Note: Further reading on each LayoutManagers is recommended for Swing
Design pattern: The AWT containers like panels, dialog boxes, windows etc do not perform the actual laying out
of the components. They delegate the layout functionality to layout managers. The layout managers make use of
the strategy design pattern, which encapsulates family of algorithms for laying out components in the containers.
If a particular layout algorithm is required other than the default algorithm, an appropriate layout manager can be
instantiated and plugged into the container (e.g. panels by default uses the FlowLayout but it can be changed by
executing 􀃆 panel.setLayout(new GridLayout(4,5))). This enables the layout algorithms to vary independently
from the containers that uses them, this is one of the key benefits of the strategy pattern.
Q 56: Explain the Swing delegation event model? LF
A 56: In this model, the objects that receive user events notify the registered listeners of the user activity. In most cases
the event receiver is a component.
• Event Types: ActionEvent, KeyEvent, MouseEvent, WindowEvent etc.
• Event Processors: JButton, JList etc.
• EventListeners: ActionListener, ComponentListener, KeyListener etc.
Swing Event Delegation Model
(eg JButton, JList etc)
(eg ActionListener etc)

Java – Applet
Q 57: How will you initialize an applet? LF
A 57: By writing your initialization code in the applet’s init() method or applet’s constructor.
Q 58: What is the order of method invocation in an applet? LF
A 58: The Applet’s life cycle methods are as follows:
• public void init() : Initialization method called only once by the browser.
• public void start() : Method called after init() and contains code to start processing. If the user leaves the
page and returns without killing the current browser session, the start () method is called without being
preceded by init ().
• public void stop() : Stops all processing started by start (). Done if user moves off page.
• public void destroy() : Called if current browser session is being terminated. Frees all resources used by the
Q 59: How would you communicate between applets and servlets? LF
A 59: We can use the and classes to open a standard HTTP connection and
“tunnel” to a Web server. The server then passes this information to the servlet. Basically, the applet pretends to
be a Web browser, and the servlet doesn’t know the difference. As far as the servlet is concerned, the applet is
just another HTTP client. Applets can communicate with servlets using GET or POST methods.
The parameters can be passed between the applet and the servlet as name value pairs.
Objects can also be passed between applet and servlet using object serialization. Objects are serialized to and
from the inputstream and outputstream of the connection respectively.
Q 60: How will you communicate between two Applets? LF
A 60: All the applets on a given page share the same AppletContext. We obtain this applet context as follows:
AppletContext ac = getAppletContext();
AppletContext provides applets with methods such as getApplet(name), getApplets(),getAudioClip, getImage,
showDocument and showStatus().
Q 61: What is a signed Applet? LF SE
A 61: A signed Applet is a trusted Applet. By default, and for security reasons, Java applets are contained within a
“sandbox”. Refer to the diagram below:
This means that the applets can’t do anything, which might be construed as threatening to the user’s machine
(e.g. reading, writing or deleting local files, putting up message windows, or querying various system parameters).
Early browsers had no provisions for Java applets to reach outside of the sandbox. Recent browsers, however
(Internet Explorer 4 on Windows etc), have provisions to give “trusted” applets the ability to work outside the
sandbox. For this power to be granted to one of your applets, the applet’s code must be digitally signed with your
unforgeable digital ID, and then the user must state that he trusts applets signed with your ID. The untrusted
applet can request to have privileges outside the sand box but will have to request the user for privileges every
time it executes. But with the trusted applet the user can choose to remember their answer to the request, which
means they won’t be asked again.
Signed Applet
Valuable resources like files
localcode RemoteCode
Signed unsigned
JVM Sandbox
Q 62: What is the difference between an applet and an application? LF
A 62:
Applet Application
Applets don’t have a main method. They operate on life
cycle methods init(), start(), stop(), destroy() etc.
Has a static main() method.
Applets can be embedded in HTML pages and Has no support for embedding or downloading. Has
downloaded over the Internet. Has a sand box security
no inherent security restriction.
Can only be executed within a Java compatible
container like browser, appletviewer etc.
Applications are executed at command line by
Java – Performance and Memory leaks
Q 63: How would you improve performance of a Java application? PI BP
A 63:
􀂃 Pool valuable system resources like threads, database connections, socket connections etc. Emphasise on
reuse of threads from a pool of threads. Creating new threads and discarding them after use can adversely
affect performance. Also consider using multi-threading in your single-threaded applications where possible to
enhance performance. Optimze the pool sizes based on system and application specifications and
􀂃 Optimize your I/O operations: use buffering (Refer Q21 in Java section) when writing to and reading from
files and/or streams. Avoid writers/readers if you are dealing with only ASCII characters. You can use streams
instead, which are faster. Avoid premature flushing of buffers. Also make use of the performance and
scalability enhancing features such as non-blocking and asynchronous I/O, mapping of file to memory etc
offered by the NIO (New I/O).
􀂃 Minimize network overheads by retrieving several related items simultaneously in one remote invocation if
possible. Remote method invocations involve a network round-trip, marshalling and unmarshalling of
parameters, which can cause huge performance problems if the remote interface is poorly designed. (Refer
Q125 in Enterprise section).
􀂃 Establish whether you have a potential memory problem and manage your objects efficiently: remove
references to the short-lived objects from long-lived objects like Java collections etc (Refer Q64 in Java
section) to minimise any potential memory leaks. Also reuse objects where possible. It is cheaper to recycle
objects than creating new objects each time. Avoid creating extra objects unnecessarily. For example use
mutable StringBuffer/StringBuilder classes instead of immutable String objects in computation expensive
loops as discussed in Q17 in Java section. Automatic garbage collection is one of the most highly touted
conveniences of Java. However, it comes at a price. Creating and destroying objects occupies a significant
chunk of the JVM's time. Wherever possible, you should look for ways to minimise the number of objects
created in your code:
􀂃 If repeating code within a loop, avoid creating new objects for each iteration. Create objects before
entering the loop (i.e. outside the loop) and reuse them if possible.
􀂃 For complex objects that are used frequently, consider creating a pool of recyclable objects rather than
always instantiating new objects. This adds additional burden on the programmer to manage the pool,
but in select cases can represent an order of magnitude performance gain.
􀂃 Use lazy initialization when you want to distribute the load of creating large amounts of objects. Use lazy
initialization only when there is merit in the design.
􀂃 Where applicable apply the following performance tips in your code:
􀂃 Use ArrayLists, HashMap etc as opposed to Vector, Hashtable etc where possible. This is because the
methods in ArrayList, HashMap etc are not synchronized (Refer Q13 in Java Section). Even better is to
use just arrays where possible.
􀂃 Set the initial capacity of a collection (e.g. ArrayList, HashMap etc) and StringBuffer/StringBuilder
appropriately. This is because these classes must grow periodically to accommodate new elements.
So, if you have a very large ArrayList or a StringBuffer, and you know the size in advance then you can
speed things up by setting the initial size appropriately. (Refer Q15, Q17 in Java Section).
􀂃 Minimise the use of casting or runtime type checking like instanceof in frequently executed methods
or in loops. The “casting” and “instanceof” checks for a class marked as final will be faster. Using
“instanceof” construct is not only ugly but also unmaintainable. Look at using visitor pattern (Refer
Q11 in How would you go about…? section) to avoid “instanceof” construct.
􀂃 Do not compute constants inside a large loop. Compute them outside the loop. For applets compute it
in the init() method.
􀂃 Exception creation can be expensive because it has to create the full stack trace. The stack trace is
obviously useful if you are planning to log or display the exception to the user. But if you are using your
exception to just control the flow, which is not recommended, then throw an exception, which is precreated.
An efficient way to do this is to declare a public static final Exception in your exception class
􀂃 Avoid using System.out.println and use logging frameworks like Log4J etc, which uses I/O buffers
(Refer Q21 in Java section).
􀂃 Minimise calls to Date, Calendar, etc related classes.
􀂃 Minimise JNI calls in your code.
Note: Set performance requirements in the specifications, include a performance focus in the analysis and design
and also create a performance test environment.
Q 64: How would you detect and minimise memory leaks in Java? MI BP
A 64: In Java memory leaks are caused by poor program design where object references are long lived and the garbage
collector is unable to reclaim those objects.
Detecting memory leaks:
􀂃 Use tools like JProbe, OptimizeIt etc to detect memory leaks.
􀂃 Use operating system process monitors like task manager on NT systems, ps, vmstat, iostat, netstat etc on
UNIX systems.
􀂃 Write your own utility class with the help of totalMemory() and freeMemory() methods in the Java Runtime
class. Place these calls in your code strategically for pre and post memory recording where you suspect to be
causing memory leaks. An even better approach than a utility class is using dynamic proxies (Refer Q11 in
How would you go about section…) or Aspect Oriented Programming (AOP) for pre and post memory
recording where you have the control of activating memory measurement only when needed. (Refer Q3 – Q5
in Emerging Technologies/Frameworks section).
Minimising memory leaks:
In Java, typically memory leak occurs when an object of a longer lifecycle has a reference to objects of a short life cycle.
This prevents the objects with short life cycle being garbage collected. The developer must remember to remove the references
to the short-lived objects from the long-lived objects. Objects with the same life cycle do not cause any issues because the
garbage collector is smart enough to deal with the circular references (Refer Q33 in Java section).
􀂃 Design applications with an object’s life cycle in mind, instead of relying on the clever features of the JVM.
Letting go of the object’s reference in one’s own class as soon as possible can mitigate memory problems.
Example: myRef = null;
􀂃 Unreachable collection objects can magnify a memory leak problem. In Java it is easy to let go of an entire
collection by setting the root of the collection to null. The garbage collector will reclaim all the objects (unless
some objects are needed elsewhere).
􀂃 Use weak references (Refer Q32 in Java section) if you are the only one using it. The WeakHashMap is a
combination of HashMap and WeakReference. This class can be used for programming problems where you
need to have a HashMap of information, but you would like that information to be garbage collected if you are
the only one referencing it.
􀂃 Free native system resources like AWT frame, files, JNI etc when finished with them. Example: Frame,
Dialog, and Graphics classes require that the method dispose() be called on them when they are no longer
used, to free up the system resources they reserve.
Q 65: Why does the JVM crash with a core dump or a Dr.Watson error? MI
A 65: Any problem in pure Java code throws a Java exception or error. Java exceptions or errors will not cause a core
dump (on UNIX systems) or a Dr.Watson error (on WIN32systems). Any serious Java problem will result in an
OutOfMemoryError thrown by the JVM with the stack trace and consequently JVM will exit. These Java stack
traces are very useful for identifying the cause for an abnormal exit of the JVM. So is there a way to know that
OutOfMemoryError is about to occur? The Java JDK 1.5 has a package called which has
useful JMX beans that we can use to manage the JVM. One of these beans is the MemoryMXBean.
An OutOfMemoryError can be thrown due to one of the following 4 reasons:
􀂃 JVM may have a memory leak due to a bug in its internal heap management implementation. But this is highly
unlikely because JVMs are well tested for this.
􀂃 The application may not have enough heap memory allocated for its running. You can allocate more JVM
heap size (with –Xmx parameter to the JVM) or decrease the amount of memory your application takes to
overcome this. To increase the heap space:
Java -Xms1024M -Xmx1024M
Care should be taken not to make the –Xmx value too large because it can slow down your application. The
secret is to make the maximum heap size value the right size.
􀂃 Another not so prevalent cause is the running out of a memory area called the “perm” which sits next to the
heap. All the binary code of currently running classes is archived in the “perm” area. The ‘perm’ area is
important if your application or any of the third party jar files you use dynamically generate classes. For
example: “perm” space is consumed when XSLT templates are dynamically compiled into classes, J2EE
application servers, JasperReports, JAXB etc use Java reflection to dynamically generate classes and/or
large amount of classes in your application. To increase perm space:
Java -XX:PermSize=256M -XX:MaxPermSize=256M
􀂃 The fourth and the most common reason is that you may have a memory leak in your application as
discussed in Q64 in Java section.
[Good read/reference: “Know Your Worst Friend, the Garbage Collector” http://java.syscon.
com/read/84695.htm by Romain Guy]
So why does the JVM crash with a core dump or Dr.Watson error?
Both the core dump on UNIX operating system and Dr.Watson error on WIN32 systems mean the same thing. The
JVM is a process like any other and when a process crashes a core dump is created. A core dump is a memory
map of a running process. This can happen due to one of the following reasons:
􀂃 Using JNI (Java Native Interface) code, which has a fatal bug in its native code. Example: using Oracle OCI
drivers, which are written partially in native code or jdbc-odbc bridge drivers, which are written in non Java
code. Using 100% pure Java drivers (communicates directly with the database instead of through client
software utilizing the JNI) instead of native drivers can solve this problem. We can use Oracle thin driver,
which is a 100% pure Java driver.
􀂃 The operating system on which your JVM is running might require a patch or a service pack.
􀂃 The JVM implementation you are using may have a bug in translating system resources like threads, file
handles, sockets etc from the platform neutral Java byte code into platform specific operations. If this JVM’s
translated native code performs an illegal operation then the operating system will instantly kill the
process and mostly will generate a core dump file, which is a hexadecimal file indicating program’s state
in memory at the time of error. The core dump files are generated by the operating system in response to
certain signals. Operating system signals are responsible for notifying certain events to its threads and
processes. The JVM can also intercept certain signals like SIGQUIT which is kill -3 <> from the
operating system and it responds to this signal by printing out a Java stack trace and then continue to run.
The JVM continues to run because the JVM has a special built-in debug routine, which will trap the signal -3.
On the other hand signals like SIGSTOP (kill -23 ) and SIGKILL (kill -9 ) will cause
the JVM process to stop or die. The following JVM argument will indicate JVM not to pause on SIGQUIT
signal from the operating system.
Java –Xsqnopause
Java – Personal
Q 66: Did you have to use any design patterns in your Java project? DP
A 66: Yes. Refer Q10 [Strategy], Q14 [Iterator], Q20 [Decorator], Q31 [Visitor], Q45 [Singleton], Q46 [Factory],
Q50 [Command], and Q54 [MVC] in Java section and Q11 in How would you go about… section. Note: Learning
of other patterns recommended (Gang of Four Design Patterns).
Why use design patterns, you may ask (Refer Q5 in Enterprise section). Design patterns are worthy of mention in
your CV and interview. Design patterns have a number of advantages:
􀂃 Capture design experience from the past.
􀂃 Promote reuse without having to reinvent the wheel.
􀂃 Define the system structure better.
􀂃 Provide a common design vocabulary.
Some advice if you are just starting on your design pattern journey:
􀂃 If you are not familiar with UML, now is the time. UML is commonly used to describe patterns in pattern
catalogues, including class diagrams, sequence diagrams etc. (Refer Q106 - Q109 in Enterprise section).
􀂃 When using patterns, it is important to define a naming convention. It will be much easier to manage a project
as it grows to identify exactly what role an object plays with the help of a naming convention e.g.
AccountFacilityBusinessDelegate, AccountFacilityFactory, AccountFacilityValueObject, AccountDecorator,
AccountVisitor, AccountTransferObject (or AccountFacilityVO or AccountTO).
􀂃 Make a list of requirements that you will be addressing and then try to identify relevant patterns that are
Q 67: Tell me about yourself or about some of the recent projects you have worked with? What do you consider your
most significant achievement? Why do you think you are qualified for this position? Why should we hire you and
what kind of contributions will you make?
A 67: [Hint:] Pick your recent projects and brief on it. Also is imperative that during your briefing, you demonstrate how
you applied your skills and knowledge in some of the following areas:
􀂃 Design concepts and design patterns: How you understand and applied them.
􀂃 Performance and memory issues: How you identified and fixed them.
􀂃 Exception handling and best practices: How you understand and applied them.
􀂃 Multi-threading and concurrent access: How you identified and fixed them.
Some of the questions in this section can help you prepare your answers by relating them to your current or past
work experience. For example:
􀂃 Design Concepts: Refer Q5, Q6, Q7, Q8, Q9 etc
􀂃 Design Patterns: Refer Q10, Q14, Q20, Q31, Q45, Q46, Q50 etc [Refer Q11 in How would you go about…?
􀂃 Performance issues: Refer Q21, Q63 etc
􀂃 Memory issues: Refer Q32, Q64, Q65 etc
􀂃 Exception Handling: Refer Q34, Q35 etc
􀂃 Multi-threading (Concurrency issues): Refer Q29, Q40 etc
Demonstrating your knowledge in the above mentioned areas will improve your chances of being successful in
your Java/J2EE interviews. 90% of the interview questions are asked based on your own resume. So in my view it
is also very beneficial to mention how you demonstrated your knowledge/skills by stepping through a recent
project on your resume.
The two other areas, which I have not mentioned in this section, which are also very vital, are transactions and
security. These two areas will be covered in the next section, which is the Enterprise section (J2EE, JDBC, EJB,
JMS, SQL, XML etc).
Even if you have not applied these skills knowingly or you have not applied them at all, just demonstrating that you
have the knowledge and an appreciation will help you improve your chances in the interviews. Also mention any
long hours worked to meet the deadline, working under pressure, fixing important issues like performance issues,
running out of memory issues etc.
Q 68: Why are you leaving your current position?
A 68: [Hint]
􀂃 Do not criticize your previous employer or coworkers or sound too opportunistic.
􀂃 It is fine to mention a major problem like a buy out, budget constraints, merger or liquidation.
􀂃 You may also say that your chance to make a contribution is very low due to company wide changes or
looking for a more challenging senior or designer role.
Q 69: What do you like and/or dislike most about your current and/or last position?
A 69: [Hint]
The interviewer is trying to find the compatibility with the open position. So
Do not say anything like:
􀂃 You dislike overtime.
􀂃 You dislike management or coworkers etc.
It is safe to say:
􀂃 You like challenges.
􀂃 Opportunity to grow into design, architecture, performance tuning etc.
􀂃 You dislike frustrating situations like identifying a memory leak problem or a complex transactional or a
concurrency issue. You want to get on top of it as soon as possible.
Q 70: How do you handle pressure? Do you like or dislike these situations?
A 70: [Hint]
These questions could mean that the open position is pressure-packed and may be out of control. Know what you
are getting into. If you do perform well under stress then give a descriptive example. High achievers tend to
perform well in pressure situations.
Q 71: What are your strengths and weaknesses? Can you describe a situation where you took initiative? Can you
describe a situation where you applied your problem solving skills?
A 71: [Hint]
􀂃 Taking initiatives and being pro-active: You can illustrate how you took initiative to fix a transactional issue,
a performance problem or a memory leak problem.
􀂃 Design skills: You can illustrate how you designed a particular application using OO concepts.
􀂃 Problem solving skills: Explain how you will break a complex problem into more manageable sub-sections
and then apply brain storming and analytical skills to solve the complex problem. Illustrate how you went
about identifying a scalability issue or a memory leak problem.
􀂃 Communication skills: Illustrate that you can communicate effectively with all the team members, business
analysts, users, testers, stake holders etc.
􀂃 Ability to work in a team environment as well as independently: Illustrate that you are technically sound
to work independently as well as have the interpersonal skills to fit into any team environment.
􀂃 Hard working, honest, and conscientious etc are the adjectives to describe you.
Select a trait and come up with a solution to overcome your weakness. Stay away from personal qualities and
concentrate more on professional traits for example:
􀂃 I pride myself on being an attention to detail guy but sometimes miss small details. So I am working on
applying the 80/20 principle to manage time and details. Spend 80% of my effort and time on 20% of the
tasks, which are critical and important to the task at hand.
􀂃 Some times when there is a technical issue or a problem I tend to work continuously until I fix it without having
a break. But what I have noticed and am trying to practise is that taking a break away from the problem and
thinking outside the square will assist you in identifying the root cause of the problem sooner.
Q 72: What are your career goals? Where do you see yourself in 5-10 years?
A 72: [Hint] Be realistic. For example
􀂃 Next 2-3 years to become a senior developer or a team lead.
􀂃 Next 3-5 years to become a solution designer or an architect.
Note: For Q66 – Q72 tailor your answers to the job. Also be prepared for questions like:
􀂃 What was the last Java related book or article you read? [Hint]
􀂃 Mastering EJB by Ed Roman.
􀂃 EJB design patterns by Floyd Marinescu.
􀂃 Bitter Java by Bruce Tate.
􀂃 Thinking in Java by Bruce Eckel.
􀂃 Which Java related website(s) do you use to keep your knowledge up to date? [Hint]
􀂃 What past accomplishments gave you satisfaction? What makes you want to work hard? [Hint]
􀂃 Material rewards such as salary, perks, benefits etc naturally come into play but focus on your achievements or
accomplishments than on rewards.
􀂃 Do you have any role models in software development? [Hint]
􀂃 Scott W. Ambler, Martin Fowler, Ed Roman, Floyd Marinescu, Grady Booch etc.
􀂃 Why do you want to work for us? (Research the company prior to the interview).
Java – Key Points
􀂃 Java is an object oriented (OO) language, which has built in support for multi-threading, socket communication,
automatic memory management (i.e. garbage collection) and also has better portability than other languages across
operating systems.
􀂃 Java class loaders are hierarchical and use a delegation model. The classes loaded by a child class loader have
visibility into classes loaded by its parents up the hierarchy but the reverse is not true.
􀂃 Java does not support multiple implementation inheritance but supports multiple interface inheritance.
􀂃 Polymorphism, inheritance and encapsulation are the 3 pillar of an object-oriented language.
􀂃 Code reuse can be achieved through either inheritance (“is a” relationship) or object composition (“has a”
relationship). Favour object composition over inheritance.
􀂃 When using implementation inheritance, make sure that the subclasses depend only on the behaviour of the
superclass, not the actual implementation. An abstract base class usually provides an implementation inheritance.
􀂃 Favour interface inheritance to implementation inheritance because it promotes the deign concept of coding to
interface and reduces coupling. The interface inheritance can achieve code reuse through object composition.
􀂃 Design by contract specifies the obligations of a calling-method and called-method to each other using preconditions,
post-conditions and class invariants.
􀂃 When using Java collection API, prefer using ArrayList or HashMap as opposed to Vector or Hashtable to avoid any
synchronization overhead. The ArrayList or HashMap can be externally synchronized for concurrent access by
multiple threads.
􀂃 Set the initial capacity of a collection appropriately and program in terms of interfaces as opposed to
􀂃 When providing a user defined key class for storing objects in HashMap, you should override equals(), and
hashCode() methods from the Object class.
􀂃 String class is immutable and StringBuffer and StringBuilder classes are mutable. So it is more efficient to use a
StringBuffer or a StringBuilder as opposed to a String in a computation intensive situations (ie. in for, while loops).
􀂃 Serialization is a process of writing an object to a file or a stream. Transient variables cannot be serialized.
􀂃 Java I/O performance can be improved by using buffering, minimising access to the underlying hard disk and
operating systems. Use the NIO package for performance enhancing features like non-blocking I/O operation, buffers
to hold data, and memory mapping of files.
􀂃 Each time an object is created in Java it goes into the area of memory known as heap. The primitive variables are
allocated in the stack if they are local method variables and in the heap if they are class member variables.
􀂃 Threads share the heap spaces so it is not thread-safe and the threads have their own stack space, which is
􀂃 The garbage collection cannot be forced, but you can nicely ask the garbage collector to collect garbage.
􀂃 There two types of exceptions checked (ie compiler checked) and unchecked (Runtime Exceptions). It is not
advisable to catch type Exception.
􀂃 A process is an execution of a program (e.g. JVM process) but a thread is a single execution sequence within the
􀂃 Threads can be created in Java by either extending the Thread class or implementing the Runnable interface.
􀂃 In Java each object has a lock and a thread can acquire a lock by using the synchronized key word. The
synchronization key word can be applied in method level (coarse-grained lock) or block level (fine-grained lock
which offers better performance) of code.
􀂃 Threads can communicate with each other using wait(), notify(), and notifyAll() methods. This communication
solves the consumer-producer problem.
􀂃 Sockets are communication channels, which facilitate inter-process communication.
􀂃 Swing uses the MVC paradigm to provide loose coupling and action architecture to implement a shared behaviour
between two or more user interface components.
􀂃 Swing components should be accessed through an event-dispatching thread. There is a way to access the Swing
event-dispatching thread from outside event-handling or drawing code, is using SwingUtilities’ invokeLater() and
invokeAndWait() methods.
􀂃 A signed applet can become a trusted applet, which can work outside the sandbox.
􀂃 In Java typically memory leak occurs when an object of longer life cycle has a reference to objects of a short life
􀂃 You can improve performance in Java by :
1. Pooling your valuable resources like threads, database and socket connections.
2. Optimizing your I/O operations.
3. Minimising network overheads, calls to Date, Calendar related classes, use of “casting” or runtime type
checking like “instanceof” in frequently executed methods/loops, JNI calls, etc
4. Managing your objects efficiently by caching or recycling them without having to rely on garbage collection.
5. Using a StringBuffer as opposed to String and ArrayList or HashMap as oppose to Vector or Hashtable
6. Applying multi-threading where applicable.
7. Minimise any potential memory leaks.
􀂃 Finally, very briefly familiarise yourself with some of the key design patterns like:
1. Decorator design pattern: used by Java I/O API. A popular design pattern.
2. Reactor design pattern/Observer design pattern: used by Java NIO API.
3. Visitor design pattern: to avoid instanceof and typecast constructs.
4. Factory method/abstract factory design pattern: popular pattern, which gets frequently asked in interviews.
5. Singleton pattern: popular pattern, which gets frequently asked in interviews.
6. Composite design pattern: used by GUI components and also a popular design pattern
7. MVC design pattern/architecture: used by Swing components and also a popular pattern.
8. Command pattern: used by Swing action architecture and also a popular design pattern.
9. Strategy design pattern: A popular design pattern used by AWT layout managers.
Refer Q11 in “How would you go about…” section for a detailed discussion and code samples on GOF (Gang of Four)
design patterns.
Recommended reading on design patterns:
􀂃 The famous Gang of Four book: Design Patterns, Eric Gamma, Richard Helm, Ralph Johnson, and John Vlissides
(Addiso-Wesley Publishing Co., 1995; ISBN: 0201633612).
􀂃 Try to find out the needs of the project in which you will be working and the needs of the people within the
􀂃 80% of the interview questions are based on your own resume.
􀂃 Where possible briefly demonstrate how you applied your skills/knowledge in the key areas [design
concepts, transactional issues, performance issues, memory leaks etc] as described in this book. Find the
right time to raise questions and answer those questions to show your strength.
􀂃 Be honest to answer technical questions, you are not expected to remember everything (for example you
might know a few design patterns but not all of them etc). If you have not used a design pattern in question,
request the interviewer, if you could describe a different design pattern.
􀂃 Do not be critical, focus on what you can do. Also try to be humorous to show your smartness.
􀂃 Do not act superior.
Enterprise Java
Enterprise Java – Interview questions & answers
􀂃 Specification Fundamentals SF
􀂃 Design Concepts DC
􀂃 Design Patterns DP
􀂃 Concurrency Issues CI
􀂃 Performance Issues PI
􀂃 Memory Issues MI
􀂃 Exception Handling EH
􀂃 Transactional Issues TI
􀂃 Security SE
􀂃 Scalability Issues SI
􀂃 Best Practices BP
􀂃 Coding1 CO
Popular Questions: Q02, Q03, Q10, Q16, Q19, Q20, Q24, Q25, Q30, Q31, Q36, Q39, Q40, Q45, Q46, Q48, Q49, Q53, Q58, Q63, Q64,
Q65, Q66, Q71, Q72, Q73, Q76, Q77, Q78, Q79, Q83, Q84, Q85, Q86, Q87, Q89, Q90, Q91, Q93, Q96, Q97, Q98, Q100, Q102, Q106,
Q107, Q110, Q123, Q124, Q125, Q129, Q131, Q136.
1 Unlike other key areas, the CO is not always shown against the question but shown above the actual subsection of relevance within a
Enterprise Java
Enterprise - J2EE
Q 01: What is J2EE? What are J2EE components and services? SF
A 01: J2EE (Java 2 Enterprise Edition) is an environment for developing and deploying enterprise applications. The
J2EE platform consists of J2EE components, services, Application Programming Interfaces (APIs) and protocols
that provide the functionality for developing multi-tiered and distributed Web based applications.
W eb
S e rv e r
J 2 E E P h y s ic a l T ie rs , C o n ta in e rs , C om p o n e n ts , S e rv ic e s & A P Is
C lie n t T ie r A p p lic a tio n T ie r (M id d le T ie r) D a ta (E IS ) T ie r
J 2 E E A p p lic a tio n S e rv e r
(X )H T M L ,
(B row s e r)
A p p le t
C lie n t A p p lic a tio n
(s ta n d a lo n e Ja va
p ro g ram )
H T T P ( S )
H T T P ( S )
J a v a
A p p lic a tio n
C o rb a S e rv e r
M e s s a g in g
D ire ctory
S erv ice
J a v a M a il
C lien t
A p p lic a tio n S e rv e r D a ta b a s e S e rv e r
F irew a ll F irew a ll
W eb S erver
in te rn e t
O th e r S e rv ic e s + A P I s p ro v id e d b y se rv e r/ c o n ta in e r :
S e cu r ity (S S L , A C L , JA A S ,X .5 0 9 )
tran sa c tio n s, th re ad in g , R e so u rc e p o o lin g (E g : C o n n e c tio n p o o lin g ) e tc
,F a u lt T o le r a n c e , L o a d B a la n c in g , c lu s te r in g
M o n ito r in g , A u d itin g , L o g g in g e tc
m o r e ...............
W e b C o n ta in e r
S e rvle ts JS P
T a g
lib ra ry
E J B C o n ta in e r
S e s s io n B e a n s E n tity B e a n s M e s s a g e d riv e n b e a n s
A J2EE component is a self-contained functional software unit that is assembled into a J2EE application with its
related classes and files and communicates with other components. The J2EE specification defines the following
J2EE components:
Component type Components Packaged as
Applet applets JAR (Java ARchive)
Application client Client side Java codes. JAR (Java ARchive)
Web component JSP, Servlet WAR (Web ARchive)
Enterprise JavaBeans Session beans, Entity beans, Message driven beans JAR (EJB Archive)
Enterprise application WAR, JAR, etc EAR (Enterprise ARchive)
Resource adapters Resource adapters RAR (Resource Adapter ARchive)
So what is the difference between a component and a service you may ask? A component is an application
level software unit as shown in the table above. All the J2EE components depend on the container for the system
level support like transactions, security, pooling, life cycle management, threading etc. A service is a component
Enterprise Java
that can be used remotely through a remote interface either synchronously or asynchronously (e.g. Web service,
messaging system, sockets, RPC etc).
Containers (Web & EJB containers) are the interface between a J2EE component and the low level platform
specific functionality that supports J2EE components. Before a Web, enterprise bean (EJB), or application client
component can be executed, it must be assembled into a J2EE module (jar, war, and/or ear) and deployed into its
A J2EE server provides system level support services such us security, transaction management, JNDI (Java
Naming and Directory Interface) lookups, remote access etc. J2EE architecture provides configurable and nonconfigurable
services. The configurable service enables the J2EE components within the same J2EE application
to behave differently based on where they are deployed. For example the security settings can be different for the
same J2EE application in two different production environments. The non-configurable services include enterprise
bean (EJB) and servlet life cycle management, resource pooling etc.
Protocols are used for access to Internet services. J2EE platform supports HTTP (HyperText Transfer Protocol),
TCP/IP (Transmission Control Protocol / Internet Protocol), RMI (Remote Method Invocation), SOAP (Simple
Object Access Protocol) and SSL (Secured Socket Layer) protocol.
The J2EE API can be summarised as follows:
J2EE technology category API (Application Program Interface)
Component model technology Java Servlet, JavaServer Pages(JSP), Enterprise JavaBeans(EJB).
Web services technology
JAXP (Java API for XML Processing), JAXR (Java API for XML Registries), SAAJ (SOAP
with attachment API for Java), JAX-RPC (Java API for XML-based RPC), JAX-WS (Java
API for XML-based Web Services).
JDBC (Java Database Connectivity), JNDI (Java Naming and Directory Interface), JMS
(Java Messaging Service), JCA (J2EE Connector Architecture), JTA (Java Transaction
API), JavaMail, JAF (JavaBeans Activation Framework – used by JavaMail), JAAS (Java
Authentication and Authorization Service), JMX (Java Management eXtenstions).
Q 02: Explain the J2EE 3-tier or n-tier architecture? SF DC
A 02: This is a very commonly asked question. Be prepared to draw some diagrams on the board. The J2EE platform is
a multi-tiered system. A tier is a logical or functional partitioning of a system.
2 – tier system 3 – tier system
2-Tier (Client/Server)
UserIn te rface
/display Logic
Bus iness
UserIn te rface
/display Logic
Bus iness
Client M /C 1 Client M /C 2
Bus iness Logic
Database logic
When the developers are not disciplined, The
display logic, business logic and database
logic are muddled up and/or duplicated in a 2-
tier client server system.
3-Tier (or n-tier)
/display logic
/display logic
Client M /C 1 Client M /C 2
Middle-tier server
Business Logic
Database Logic
The advantages of the multi-tier architecture are:
􀂃 Forced separation of user interface logic and business logic.
􀂃 Business logic sits on small number of centralized machines (may be
just one).
􀂃 Easy to maintain, to manage, to scale, loosely coupled etc.
Each tier is assigned a unique responsibility in a 3-tier system. Each tier is logically separated and loosely coupled
from each other, and may be distributed.
Enterprise Java
Client tier represents Web browser, a Java or other application, Applet, WAP phone etc. The client tier makes
requests to the Web server who will be serving the request by either returning static content if it is present in the
Web server or forwards the request to either Servlet or JSP in the application server for either static or dynamic
Presentation tier encapsulates the presentation logic required to serve clients. A Servlet or JSP in the
presentation tier intercepts client requests, manages logons, sessions, accesses the business services, and finally
constructs a response, which gets delivered to client.
Business tier provides the business services. This tier contains the business logic and the business data. All the
business logic is centralised into this tier as opposed to 2-tier systems where the business logic is scattered
between the front end and the backend. The benefit of having a centralised business tier is that same business
logic can support different types of clients like browser, WAP, other stand-alone applications etc.
Integration tier is responsible for communicating with external resources such as databases, legacy systems,
ERP systems, messaging systems like MQSeries etc. The components in this tier use JDBC, JMS, J2EE
Connector Architecture (JCA) and some proprietary middleware to access the resource tier.
Resource tier is the external resource such as a database, ERP system, Mainframe system etc responsible for
storing the data. This tier is also known as Data Tier or EIS (Enterprise Information System) Tier.
L o g ic a l o r
F u n c t io n a l T ie rs
H ig h L e v e l
T ie rs
C lie n t
C l i e n t T ie r
W e b S e rv e r
A p p lic a tio n S e rv e r
M id d le T ie r
D a ta T ie r
C l ie n t T ie r
A p p le ts , H T M L ,W M L , J a v a S c r ip t ,
A p p li c a t io n C l i e n t s e t c
P r e s e n ta t io n T ie r
R e s o u rc e T ie r
D a ta b a s e s , E R P & C R M s y s te m s ,
L e g a c y S y s te m s e tc
B u s in e s s T ie r
E J B , J a v a C la s s e s , B u s in e s s
O b je c ts e tc
H T M L , C S S , G IF F ile s e tc
(s ta t ic c o n te n t )
J S P , S e rv le ts , T a g lib ra ry a n d
o th e r U I e le m e n ts (C S S , G IF ,
e tc )
In te g ra t io n T ie r
J M S , J D B C , C o n n e c to r s ( J C A ) ,
a n d L e g a c y
J2EE patterns apply
J 2 E E T ie r s
Note: On a high level J2EE can be construed as a 3-tier system consisting of Client Tier, Middle Tier (or
Application Tier) and Data Tier. But logically or functionally J2EE is a multi-tier (or n-tier) platform.
The advantages of a 3-tiered or n-tiered application: 3-tier or multi-tier architectures force separation among
presentation logic, business logic and database logic. Let us look at some of the key benefits:
􀂃 Manageability: Each tier can be monitored, tuned and upgraded independently and different people can have
clearly defined responsibilities.
Enterprise Java
􀂃 Scalability: More hardware can be added and allows clustering (i.e. horizontal scaling).
􀂃 Maintainability: Changes and upgrades can be performed without affecting other components.
􀂃 Availability: Clustering and load balancing can provide availability.
􀂃 Extensibility: Additional features can be easily added.
Q 03: Explain MVC architecture relating to J2EE? DC DP
A 03: This is also a very popular interview question. MVC stands for Model-View-Controller architecture. It divides the
functionality of displaying and maintaining of the data to minimise the degree of coupling (i.e. promotes loose
coupling) between components.
J2EE MVC (Model-View-Controller)
(Entitiy B eans (EJB), Session B eans
(EJB), P lain Java Classes )
􀂾 Encapsulates business logic and
application state.
(JSP, JavaBeans, SWING,
Custom Tags, etc )
􀂾 Renders the m odel & has
only display logic.
􀂾 Sends user actions to the
􀂾 Allows controller to select a
(Servlet, S truts Action etc)
􀂾 controls application behaviour
􀂾 Maps user actions to m odel.
􀂾 selects view for response.
􀂾 usually one for each
get Data to display
User Action(eg: submitting a
form, clicking a button etc)
View selection (eg: selecting the next
JSP page to display as a response)
state change
Note: Typical MVC architecture is shown above. V ariations are possible (eg: Model 1 vs Model 2
EJB or Plain Java
1 . R e q u e s t
2. instantiates
3. re ad/u p date
d ata
6. Response
Client Tier MiddleTier (Application Tier) Data Tier
A model represents the core business logic and state. A model commonly maps to data in the database and will
also contain core business logic.
A View renders the contents of a model. A view accesses the data from the model and adds display logic to
present the data.
A Controller acts as the glue between a model and a view. A controller delegates the request to the model for
application logic and state and also centralises the logic for dispatching the request to the next view based on the
input parameters from the client and the application state. A controller also decouples JSP pages and the Servlet
by handling the view selection.
Enterprise Java
Q 04: How to package a module, which is, shared by both the WEB and the EJB modules? SF
A 04: Package the modules shared by both WEB and EJB modules as dependency jar files. Define the Class-Path:
property in the MANIFEST.MF file in the EJB jar and the Web war files to refer to the shared modules. [Refer Q7
in Enterprise section for diagram: J2EE deployment structure].
The MANIFEST.MF files in the EJB jar and WEB war modules should look like:
Manifest-Version: 1.0
Created-By: Apache Ant 1.5
Class-Path: myAppsUtil.jar
Q 05: Why use design patterns in a J2EE application? DP
A 05:
􀂃 They have been proven. Patterns reflect the experience and knowledge of developers who have successfully
used these patterns in their own work. It lets you leverage the collective experience of the development
Example Session facade and value object patterns evolved from performance problems experienced due to
multiple network calls to the EJB tier from the WEB tier. Fast lane reader and Data Access Object patterns
exist for improving database access performance. The flyweight pattern improves application performance
through object reuse (which minimises the overhead such as memory allocation, garbage collection etc).
􀂃 They provide common vocabulary. Patterns provide software designers with a common vocabulary. Ideas
can be conveyed to developers using this common vocabulary and format.
Example Should we use a Data Access Object (DAO)? How about using a Business Delegate? Should we
use Value Objects to reduce network overhead? Etc.
Q 06: What is the difference between a Web server and an application server? SF
A 06:
Web Server Application Server
Supports HTTP protocol. When the Web server receives
an HTTP request, it responds with an HTTP response,
such as sending back an HTML page (static content) or
delegates the dynamic response generation to some
other program such as CGI scripts or Servlets or JSPs in
the application server.
Exposes business logic and dynamic content to the client
through various protocols such as HTTP, TCP/IP, IIOP, JRMP etc.
Uses various scalability and fault-tolerance techniques. Uses various scalability and fault-tolerance techniques. In addition
provides resource pooling, component life cycle management,
transaction management, messaging, security etc.
Provides services for components like Web container for servlet
components and EJB container for EJB components.
With the advent of XML Web services the line between application servers and Web servers is not clear-cut. By passing XML
documents between request and response the Web server can behave like an application server.
Q 07: What are ear, war and jar files? What are J2EE Deployment Descriptors? SF
A 07: ear, war and jar are standard application deployment archive files. Since they are a standard, any application
server (at least in theory) will know how to unpack and deploy them.
An EAR file is a standard JAR file with an “.ear” extension, named from Enterprise ARchive file. A J2EE
application with all of its modules is delivered in EAR file. JAR files can’t have other JAR files. But EAR and WAR
(Web ARchive) files can have JAR files.
An EAR file contains all the JARs and WARs belonging to an application. JAR files contain the EJB classes and
WAR files contain the Web components (JSPs, static content (HTML, CSS, GIF etc), Servlets etc.). The J2EE
application client's class files are also stored in a JAR file. EARs, JARs, and WARs all contain an XML-based
deployment descriptor.
Enterprise Java
log4j.jar (3rd party jars)
deployment descriptor
Manifest-Version: 1.0
Craeted-By: Apache Ant
MyAppsCommon.jar , MyAppsUtil.jar
(shared by both EJB and Web modules)
class-path: log4j.jar MyAppsCommon.jar MyAppsUtil.jar
deployment descriptor
class files, properties files,configuration files etc
class files, properties files,configuration files etc
ejb classes , non-ejb class etc
JSP, HTML, CSS, GIF (can have
class-path: log4j.jar MyAppsCommon.jar MyAppsUtil.jar
deployment descriptor
struts.jar, crimson.jar
3rd party jar files
class files
J2EE deployment structure (ear, war, jar )
Deployment Descriptors
A deployment descriptor is an XML based text file with a “.xml” extension that describes a component's
deployment settings. A J2EE application and each of its modules has its own deployment descriptor. Pay attention
to elements marked in bold in the sample deployment descriptor files shown below.
􀂃 application.xml: is a standard J2EE deployment descriptor, which includes the following structural
information: EJB jar modules, WEB war modules, etc. Also since EJB jar modules are
packaged as jars the same way dependency libraries like log4j.jar, commonUtil.jar etc are packaged, the
application.xml descriptor will distinguish between these two jar files by explicitly specifying the EJB jar




Enterprise Java

Management position

􀂃 ejb-jar.xml: is a standard deployment descriptor for an EJB module.





Employee is allowed to ...






Enterprise Java



􀂃 web.xml: is a standard deployment descriptor for a WEB module.














Enterprise Java


Q 08: Explain J2EE class loaders? SF
A 08: J2EE application server sample class loader hierarchy is shown below. (Also refer to Q4 in Java section). As per
the diagram the J2EE application specific class loaders are children of the “System –classpath” class loader.
When the parent class loader is above the “System –Classpath” class loader in the hierarchy as shown in the
diagram (i.e. bootstrap class loader or extensions class loader) then child class loaders implicitly have visibility to
the classes loaded by its parents. When a parent class loader is below a “System -Classpath” class loader then
the child class loaders will only have visibility into the classes loaded by its parents only if they are explicitly
specified in a manifest file (MANIFEST.MF) of the child class loader.
Example As per the diagram, if the EJB module MyAppsEJB.jar wants to refer to MyAppsCommon.jar and
MyAppsUtil.jar we need to add the following entry in the MyAppsEJB.jar’s manifest file MANIFEST.MF.
class-path: MyAppsCommon.jar MyAppsUtil.jar
Boo tstrap (JVM)
(rt.jar, i1 8 .ja r)
Exten sions(JVM )
(lib/ e xt)
System(JVM )
(-clas spath)
Applica tion c la ss
lo ad e r (EA R)
Applicatio n clas s
loade r (EAR )
EJB c la ss loade r EJB c la ss load er
WAR c lass
lo ad e r
W AR class
loade r
WAR c lass
lo ad e r
Each E A R gets its
own instance of c la ss
loa d e r
All the E JB ja rs in a e ar
file s h are th e s am e EJB
cla ss loade r.
Ea ch W AR g e ts its own
ins tance o f cla ss loader.
Th e W E B -INF /lib lib ra ries
a re spe cific to e ach W AR
Note: Application vendor's Serve r c lass loader h ie ra rchy m ig h t s lig h tly vary
J2EE application server sam p le class loader h ierarchy
M yApps.ear
M yAppsU til.ja r
M yAppsC om mo n.jar
M yAppsE JB.jar
M yAppsW eb .war
This is because the application (EAR) class loader loads the MyAppsCommon.jar and MyAppsUtil.jar. The EJB
class loader loads the MyAppsEJB.jar, which is the child class loader of the application class loader. The WAR
class loader loads the MyAppsWeb.war.
Every J2EE application or EAR gets its own instance of the application class loader. This class loader is
responsible for loading all the dependency jar files, which are shared by both WEB and EJB modules. For
example third party libraries like log4j, utility classes, shared classes or common classes (Exception thrown by an
EJB module should be caught by a WEB module) etc.
The key difference between the EJB and WAR class loader is that all the EJB jars in the application share the
same EJB class loader whereas WAR files get their own class loader. This is because the EJBs have inherent
relationship between one another (ie EJB-EJB communication between EJBs in different applications but hosted
on the same JVM) but the Web modules do not. Every WAR file should be able to have its own WEB-INF/lib third
party libraries and need to be able to load its own version of converted logon.jsp Servlet so each WEB module is
isolated in its own class loader.
Enterprise Java
So if two different WEB modules want to use two different versions of the same EJB then we need to have two
different ear files. As was discussed in the Q4 in Java section the class loaders use a delegation model where
the child class loaders delegate the loading up the hierarchy to their parent before trying to load it itself only if the
parent can’t load it. But with regards to WAR class loaders, some application servers provide a setting to turn this
behaviour off (DelegationMode=false). This delegation mode is recommended in the Servlet 2.3 specification.
As a general rule classes should not be deployed higher in the hierarchy than they are supposed to exist. This is because
if you move one class up the hierarchy then you will have to move other classes up the hierarchy as well. This is because
classes loaded by the parent class loader can’t see the classes loaded by its child class loaders (uni-directional bottom-up
Enterprise - Servlet
Q 09: What is the difference between CGI and Servlet? SF
Q 09:
Traditional CGI
(Common Gateway Interface)
Java Servlet
Traditional CGI creates a heavy weight process to handle each
http request. N number of copies of the same traditional CGI
programs is copied into memory to serve N number of
Spawns a lightweight Java thread to handle each http
request. Single copy of a type of servlet but N number of
threads (thread sizes can be configured in an application
In the Model 2 MVC architecture, servlets process requests and select JSP views. So servlets act as controller.
Servlets intercept the incoming HTTP requests from the client (browser) and then dispatch the request to the
business logic model (e.g. EJB, POJO - Plain Old Java Object, JavaBeans etc). Then select the next JSP view for
display and deliver the view to client as the presentation (response). It is the best practice to use Web tier UI
frameworks like Struts, JavaServer Faces etc, which uses proven and tested design patterns.
Q 10: HTTP is a stateless protocol, so how do you maintain state? How do you store user data between requests? SF
A 10: This is a commonly asked question as well. You can retain the state information between different page requests
as follows:
HTTP Sessions are the recommended approach. A session identifies the requests that originate from the same
browser during the period of conversation. All the servlets can share the same session. The JSESSIONID is
generated by the server and can be passed to client through cookies, URL re-writing (if cookies are turned off) or
built-in SSL mechanism. Care should be taken to minimize size of objects stored in session and objects
stored in session should be serializable. In a Java servlet the session can be obtained as follows: CO
HttpSession session = request.getSession(); //returns current session or a new session
Sessions can be timed out (configured in web.xml) or manually invalidated.
1. Initial Request[No session] JSESSIONID Name Value
xsder12345 Firstname Peter
xsder12345 LastName Smith
A new session is created on the Server
side with JSESSIONID where
state can be maintained as
name/value pair.
2. JSESSIONID is passed to client with
the response through
cookies or URL re-writing
3. Client uses the JSESSIONID
for subsequent requests
retrieve stored state information for the
Session Management
Enterprise Java
Hidden Fields on the pages can maintain state and they are not visible on the browser. The server treats both
hidden and non-hidden fields the same way.

The disadvantage of hidden fields is that they may expose sensitive or private information to others.
URL re-writing will append the state information as a query string to the URL. This should not be used to maintain
private or sensitive information.
Cookies: A cookie is a piece of text that a Web server can store on a user’s hard disk. Cookies allow a website to
store information on a user’s machine and later retrieve it. These pieces of information are stored as name-value
pairs. The cookie data moves in the following manner:
􀂙 If you type the URL of a website into your browser, your browser sends the request to the Web server. When
the browser does this it looks on your machine for a cookie file that URL has set. If it finds it, your browser
will send all of the name-value pairs along with the URL. If it does not find a cookie file, it sends no cookie
􀂙 The URL’s Web server receives the cookie data and requests for a page. If name-value pairs are received,
the server can use them. If no name-value pairs are received, the server can create a new ID and then sends
name-value pairs to your machine in the header for the Web page it sends. Your machine stores the name
value pairs on your hard disk.
Cookies can be used to determine how many visitors visit your site. It can also determine how many are new
versus repeated visitors. The way it does this is by using a database. The first time a visitor arrives, the site
creates a new ID in the database and sends the ID as a cookie. The next time the same user comes back, the site
can increment a counter associated with that ID in the database and know how many times that visitor returns.
The sites can also store user preferences so that site can look different for each visitor.
Which mechanism to choose?
HttpSession 􀂃 There is no limit on the size of the session data kept.
􀂃 The performance is good.
􀂃 This is the preferred way of maintaining state. If we use the HTTP session with the application server’s
persistence mechanism (server converts the session object into BLOB type and stores it in the
Database) then the performance will be moderate to poor.
Note: When using HttpSession mechanism you need to take care of the following points:
􀂃 Remove session explicitly when you no longer require it.
􀂃 Set the session timeout value.
􀂃 Your application server may serialize session objects after crossing a certain memory limit. This is
expensive and affects performance. So decide carefully what you want to store in a session.
Hidden fields 􀂃 There is no limit on size of the session data.
􀂃 May expose sensitive or private information to others (So not good for sensitive information).
􀂃 The performance is moderate.
URL rewriting 􀂃 There is a limit on the size of the session data.
􀂃 Should not be used for sensitive or private information.
􀂃 The performance is moderate.
Cookies 􀂃 There is a limit for cookie size.
􀂃 The browser may turn off cookies.
􀂃 The performance is moderate.
The benefit of the cookies is that state information can be stored regardless of which server the client talks to
and even if all servers go down. Also, if required, state information can be retained across sessions.
Q 11: Explain the life cycle methods of a servlet? SF
A 11: The Web container is responsible for managing the servlet’s life cycle. The Web container creates an instance of
the servlet and then the container calls the init() method. At the completion of the init() method the servlet is in
Enterprise Java
ready state to service requests from clients. The container calls the servlet’s service() method for handling each
request by spawning a new thread for each request from the Web container’s thread pool [It is also possible to
have a single threaded Servlet, refer Q16 in Enterprise section]. Before destroying the instance the container will
call the destroy() method. After destroy() the servlet becomes the potential candidate for garbage collection.
Note on servlet reloading:
Most servers can reload a servlet after its class file has been modified provided the servlets are deployed to
$server_root/servlets directory. This is achieved with the help of a custom class loader. This feature is handy for development
and test phases. This is not recommended for production since it can degrade performance because of timestamp comparison
for each request to determine if a class file has changed. So for production it is recommended to move the servlet to server’s
class path ie $server_root/classes.
When a server dispatches a request to a servlet, the server first checks if the servlet's class file has changed on disk. If it has
changed, the server abandons the class loader used to load the old version and creates a new instance of the custom class
loader to load the new version. Old servlet versions can stay in memory indefinitely (so the effect is the other classes can still
hold references to the old servlet instances, causing odd side effects, but the old versions are not used to handle any more
requests. Servlet reloading is not performed for classes found in the server's classpath because the core, primordial class loader,
loads those classes. These classes are loaded once and retained in memory even when their class files change.
Servlet Life Cycle
called once
called once
& call init()
ready to serve requests
thread 1 : client request
thread 2 : client request
thread 3 : client request
handle m ultiple
requests and send
Q 12: Explain the directory structure of a WEB application? SF SE
A 12: Refer Q7 in Enterprise section for diagram: J2EE deployment structure and explanation in this section where
MyAppsWeb.war is depicting the Web application directory structure. The directory structure of a Web application
consists of two parts:
􀂃 A public resource directory (document root): The document root is where JSP pages, client-side classes
and archives, and static Web resources are stored.
􀂃 A private directory called WEB-INF: which contains following files and directories:
􀂃 web.xml : Web application deployment descriptor.
􀂃 *.tld : Tag library descriptor files.
􀂃 classes : A directory that contains server side classes like servlets, utility classes, JavaBeans etc.
􀂃 lib : A directory where JAR (archive files of tag libraries, utility libraries used by the server side classes)
files are stored.
Note: JSP resources usually reside directly or under subdirectories of the document root, which are directly
accessible to the user through the URL. If you want to protect your Web resources then hiding the JSP files
behind the WEB-INF directory can protect the JSP files from direct access. Refer Q35 in Enterprise section.
Q 13: What is the difference between doGet () and doPost () or GET and POST? SF SE
Enterprise Java
A 13:
GET or doGet() POST or doPost()
The request parameters are transmitted as a query string
appended to the request. Allows browser bookmarks but not
appropriate for transmitting private or sensitive information.
This is a security risk.
The request parameters are passed with the body of the
More secured.
GET was originally intended for static resource retrieval.
POST was intended for input data submits where the results
are expected to change.
GET is not appropriate when large amounts of input data are
being transferred.
Q 14: What are the ServletContext and ServletConfig objects? What are Servlet environment objects? SF
A 14: The Servlet Engine uses both objects.
ServletConfig ServletContext
The ServletConfig parameters are for a particular Servlet.
The parameters are specified in the web.xml (ie
deployment descriptor).
The ServletContext parameters are specified for the entire Web
application. The parameters are specified in the web.xml (ie
deployment descriptor).
Web App1
Servlet Engine
ServletConfig objects init
params set for eachSevlet
init param for
the web app
web.xml (Deployment Descriptor) snippets





Servlet Environment Objects
Web App2
Q 15: What is the difference between HttpServlet and GenericServlet? SF
A 15:
GenericServlet HttpServlet
A GenericServlet has a service() method to
handle requests.
The HttpServlet extends GenericServlet and adds support for HTTP
protocol based methods like doGet(), doPost(), doHead() etc.
Protocol independent. Protocol dependent.
Q 16: How do you make a Servlet thread safe? What do you need to be concerned about with storing data in Servlet
instance fields? CI PI BP
A 16: As shown in the figure Servlet Life Cycle in Q11 in Enterprise section, a typical (or default) Servlet life cycle
creates a single instance of each servlet and creates multiple threads to handle the service() method. The multithreading
aids efficiency but the servlet code must be coded in a thread safe manner. The shared resources
(e.g. instance variables, utility or helper objects etc) should be appropriately synchronized or should only use
variables in a read-only manner. Having large chunks of code in synchronized blocks in your service methods can
adversely affect performance and makes the code more complex.
Enterprise Java
Alternatively it is possible to have a single threaded model of a servlet by implementing the marker or null
interface javax.servlet.SingleThreadedModel. The container will use one of the following approaches to ensure
thread safety:
􀂃 Instance pooling where container maintains a pool of servlets.
􀂃 Sequential processing where new requests will wait while the current request is being processed.
Best practice: It is best practice to use multi-threading and stay away from the single threaded model of the
servlet unless otherwise there is a compelling reason for it. Shared resources can be synchronized or used in
read-only manner or shared values can be stored in a database table. The single threaded model can adversely
affect performance.
Q 17: What is pre-initialization of a Servlet? LF
A 17: By default the container does not initialize the servlets as soon as it starts up. It initializes a servlet when it
receives a request for the first time for that servlet. This is called lazy loading. The servlet deployment descriptor
(web.xml) defines the element, which can be configured to make the servlet container load and
initialize the servlet as soon as it starts up. The process of loading a servlet before any request comes in is called
pre-loading or pre-initializing a servlet. We can also specify the order in which the servlets are initialized.
Q 18: What is a RequestDispatcher? What object do you use to forward a request? LF CO
A 18: A Servlet can obtain its RequestDispatcher object from its ServletContext.
//…inside the doGet() method
ServletContext sc = getServletContext();
RequestDispatcher rd = sc.getRequestDispatcher(url);
// forwards the control to another servlet or JSP to generate response. This method allows one servlet to do preliminary
//processing of a request and another resource to generate the response
// includes the content of the resource such as Servlet, JSP, HTML, Images etc into the calling Servlet’s response.
rd.include(request, response);
Q 19: What is the difference between forwarding a request and redirecting a request? LF DC
A 19: Both methods redirect you to a new resource like Servlet, JSP etc. But
redirecting - sendRedirect() forward
Sends a header back to the browser, which contains the name of
the resource to be redirected to. The browser will make a fresh
request from this header information. Need to provide absolute
URL path.
Forward action takes place within the server without
the knowledge of the browser.
Has an overhead of extra remote trip but has the advantage of
being able to refer to any resource on the same or different domain
and also allows book marking of the page.
No extra network trip.
Q 20: What are the considerations for servlet clustering? DC SI
A 20: The clustering promotes high availability and scalability. The considerations for servlet clustering are:
􀂃 Objects stored in a session should be serializable to support in-memory replication of sessions. Also
consider the overhead of serializing very large objects. Test the performance to make sure it is acceptable.
􀂃 Design for idempotence. Failure of a request or impatient users clicking again can result in duplicate
requests being submitted. So the Servlets should be able to tolerate duplicate requests.
􀂃 Avoid using instance and static variables in read and write mode because different instances may exist
on different JVMs. Any state should be held in an external resource such as a database.
􀂃 Avoid storing values in a ServletContext. A ServletContext is not serializable and also the different
instances may exist in different JVMs.
􀂃 Avoid using* because the files may not exist on all backend machines. Instead use
Enterprise Java
Q 21: If an object is stored in a session and subsequently you change the state of the object, will this state change
replicated to all the other distributed sessions in the cluster? DC SI
A 21: No. Session replication is the term that is used when your current service state is being replicated across multiple
application instances. Session replication occurs when we replicate the information (ie session attributes) that
are stored in your HttpSession. The container propagates the changes only when you call the setAttribute(……)
method. So mutating the objects in a session and then by-passing the setAttribute(………..) will not replicate the
state change. CO
Example If you have an ArrayList in the session representing shopping cart objects and if you just call
getAttribute() to retrieve the ArrayList and then add or change something without calling the
setAttribute(……………) then the container may not know that you have added or changed something in the
ArrayList. So the session will not be replicated.
Q 22: What is a filter, and how does it work? LF DP
A 22: A filter dynamically intercepts requests and responses to transform or use the information contained in the
requests or responses but typically do not themselves create responses. Filters can also be used to transform the
response from the Servlet or JSP before sending it back to client. Filters improve reusability by placing recurring
tasks in the filter as a reusable unit.
A good way to think of Servlet filters is as a chain of steps that a request and response must go through before
reaching a Servlet, JSP, or static resource such as an HTML page in a Web application.
W e b C o n ta in e r
S e rv le t , J S P , H T M L
F i l t e r 2
F i l t e r 3
F i l t e r 1
C lie n t
F i l te r
The filters can be used for caching and compressing content, logging and auditing, image conversions (scaling up
or down etc), authenticating incoming requests, XSL transformation of XML content, localization of the request and
the response, site hit count etc. The filters are configured through the web.xml file as follows:




The HitCounterFilter will intercept the requests from the URL pattern /usersection followed by any resource name.
Design Pattern: Servlet filters use the slightly modified version of the chain of responsibility design pattern.
Unlike the classic (only one object in the chain handle the request) chain of responsibility where filters allow
multiple objects (filters) in a chain to handle the request. If you want to modify the request or the response in the
chain you can use the decorator pattern (Refer Q11 in How would you go about… section).
Q 23: Explain declarative security for WEB applications? SE
Enterprise Java
A 23: Servlet containers implement declarative security. The administration is done through the deployment descriptor
web.xml file. With declarative security the Servlets and JSP pages will be free from any security aware code.
You can protect your URLs through web.xml as shown below:





The user will be prompted for the configured login.jsp when restricted resources are accessed. The container also
keeps track of which users have been previously authenticated.
Benefits: Very little coding is required and developers can concentrate on the application they are building and
system administrators can administer the security settings without or with minimal developer intervention. Let’s
look at a sample programmatic security in a Web module like a servlet: CO
User user = new User();
Principal principal = request.getUserPrincipal();
if (request.isUserInRole("boss"))
Q 24: Explain the Front Controller design pattern or explain J2EE design patterns? DP
A 24: Problem: A J2EE system requires a centralized access point for HTTP request handling to support the integration
of system services like security, data validation etc, content retrieval, view management, and dispatching. When
the user accesses the view directly without going through a centralized mechanism, two problems may occur:
􀂃 Each view is required to provide its own system services often resulting in duplicate code.
􀂃 View navigation is left to the views. This may result in shared code for view content and view navigation.
􀂃 Distributed control is more difficult to maintain, since changes will often need to be made in numerous
Solution: Generally you write specific servlets for specific request handling. These servlets are responsible for
data validation, error handling, invoking business services and finally forwarding the request to a specific JSP view
to display the results to the user.
J2EE F ront C o n troller p a ttern
C lien t F ro n tCo n troller Ap p licatio n F low Co n troller
V iew
Com m and
(eg : S TRUTS Actio n )
F ro n tCo n trollerServlet
F ro n tCo n trollerJSP
c lien t
reque st delegates
d ispatche s
in vo k e s
Enterprise Java
The Front Controller suggests that we only have one Servlet (instead of having specific Servlet for each specific
request) centralising the handling of all the requests and delegating the functions like validation, invoking business
services etc to a command or a helper component. For example Struts framework uses the command design
pattern to delegate the business services to an action class.
􀂃 Avoid duplicating the control logic like security check, flow control etc.
􀂃 Apply the common logic, which is shared by multiple requests in the Front controller.
􀂃 Separate the system processing logic from the view processing logic.
􀂃 Provides a controlled and centralized access point for your system.
Q 25: Briefly discuss the following patterns Composite view, View helper, Dispatcher view and Service to worker? Or
explain J2EE design patterns? DP
A 25:
􀂃 Composite View: Creates an aggregate view from atomic sub-views. The Composite View entirely focuses
on the View. The View is typically a JSP page, which has the HTML, JSP Tags etc. The JSP display pages
mostly have a side bar, header, footer and main content area. These are the sub-views of the view. The subviews
can be either static or dynamic. The best practice is to have these sub-views as separate JSP pages
and include them in the whole view. This will enable reuse of JSP sub-views and improves maintainability
by having to change them at one place only.
Composite View
View CompositeView
􀂃 View Helper: When processing logic is embedded inside the controller or view it causes code duplication in
all the pages. This causes maintenance problems, as any change to piece of logic has to be done in all the
views. In the view helper pattern the view delegates its processing responsibilities to its helper classes. The
helper classes JavaBeans: used to compute and store the presentation data and Custom Tags: used for
computation of logic and displaying them iteratively complement each other.
Benefits Avoids embedding programming logic in the views and facilitates division of labour between Java
developers and Web page designers.
􀂃 Service to Worker and Dispatcher View: These two patterns are a combination of Front Controller and View
Helper patterns with a dispatcher component. One of the responsibilities of a Front Controller is choosing a
view and dispatching the request to an appropriate view. This behaviour can be partitioned into a separate
component known as a dispatcher. But these two patterns differ in the way they suggest different division of
responsibility among the components.
Service to Worker Dispatcher View
Combines the front controller (Refer Q24 in Enterprise
section) and dispatcher, with views and view helpers (refer
Q25 in Enterprise section) to handle client requests and
dynamically prepares the response.
􀂃 Controllers delegate the content retrieval to the view
helpers, which populates the intermediate model
content for the view.
􀂃 Dispatcher is responsible for the view management
and view navigation.
This pattern is structurally similar to the service to worker
but the emphasis is on a different usage pattern. This
combines the Front controller and the dispatcher with the
view helpers but
􀂃 Controller does not delegate content retrieval to
view helpers because this activity is deferred to
view processing.
􀂃 Dispatcher is responsible for the view management
and view navigation.
Enterprise Java
Promotes more up-front work by the front controller
and dispatcher for the authentication, authorization,
content retrieval, validation, view management and
Relatively has a lightweight front controller and
dispatcher with minimum functionality and most of the
work is done by the view.
Q 26: Explain Servlet URL mapping? SF
Q 26:
Servlet U R L m apping
h ttp:/// /servlet //
http://loca lhos t:8080/myApps/servlet/myPath/MyServlet
URL eg
SERVER_HOME\WebApps\myApps\WEB-INF\File Classes\myPa th \MyServlet
Server Root
D o c u m e n t r o o t
We can define the servlet m apping in the web.xml de p loymnet descriptor file a s show n below:

MyServlet< /servlet-name>
myPath.MyServ let

MyServlet< /servlet-name>

URL after m apping h ttp://loca lhost:8080/myApps/mine/
Note: Which m eans every request w hich has a p a ttern o f http ://localhost:8080/myApps/ m ine/*.do will be handled by
the myPath.MyServlet class. (* d enotes w ild character for any a lphanum eric name). Also possible to m ap MyServlet to
the patte rn of /m ine/* , the * indicate s any resource name fo llowed by /m ine.
The w ebapp name is defined in the application.xm l dep loyment descriptor file. The denotes the w eb
app nameas show n below


m yAppsWeb.wa r< /web-u ri>
myApps< /con text-root>

< /module>

< /module>

How d o we g et the w ebapp name "myApps"
Without M apping in w eb .xml
With M apping in w eb.xml d eployment descrip tor file
Enterprise - JSP
Q 27: What is a JSP? What is it used for? What do you understand by the term JSP translation phase or compilation
phase? SF
A 27: JSP (Java ServerPages) is an extension of the Java Servlet technology. JSP is commonly used as the
presentation layer for combining HTML and Java code. While Java Servlet technology is capable of generating
HTML with out.println(“….. ”) statements, where out is a PrintWriter. This process of embedding
HTML code with escape characters is cumbersome and hard to maintain. The JSP technology solves this by
providing a level of abstraction so that the developer can use custom tags and action elements, which can speed
up Web development and are easier to maintain.
Enterprise Java
As shown in the figure the JSPs have a translation or a compilation process where the JSP engine translates
and compiles a JSP file into a JSP Servlet. The translated and compiled JSP Servlet moves to the execution
phase (run time) where they can handle requests and send response.
Unless explicitly compiled ahead of time, JSP files are compiled the first time they are accessed. On large
production sites, or in situations involving complicated JSP files, compilation may cause unacceptable delays to
users first accessing the JSP page. The JSPs can be compiled ahead of time (ie precompiled) using application
server tools/settings or by writing your own script.
Ap p lication Server
CLIENT (Browser)
Web Server
Web Container
s ta tic docs
(HTML ,CSS e tc)
Servlet Engine
JSP Engine
JSP S ervlets
re q u e st
JSP m ov in g parts
Q 28: Explain the life cycle methods of a JSP? SF
A 28:
􀂃 Pre-translated: Before the JSP file has been translated and compiled into the Servlet.
􀂃 Translated: The JSP file has been translated and compiled as a Servlet.
􀂃 Initialized: Prior to handling the requests in the service method the container calls the jspInit() to initialize the
Servlet. Called only once per Servlet instance.
􀂃 Servicing: Services the client requests. Container calls this method for each request.
􀂃 Out of service: The Servlet instance is out of service. The container calls the jspDestroy() method.
Q 29: What are the main elements of JSP? What are scriplets? What are expressions? SF
A 29: There are two types of data in a JSP page.
􀂃 Static part (ie HTML, CSS etc), which gets copied directly to the response by the JSP Engine.
􀂃 Dynamic part, which contains anything that can be translated and compiled by the JSP Engine.
There are three types of dynamic elements. (TIP: remember SAD as an abbreviation for Scripting, Action and
Directive elements).
Scripting Elements: A JSP element that provides embedded Java statements. There are three types of scripting
􀂃 Declaration Element: is the embedded Java declaration statement, which gets inserted at the Servlet class
<%! Calendar c = Calendar.getInstance(); %>
Important: declaring variables via this element is not thread-safe, because this variable ends up in the generated Servlet
as an instance variable, not within the body of the _jspservice() method. Ensure their access is either read-only or
􀂃 Expression Element: is the embedded Java expression, which gets evaluated by the service method.
<%= new Date()>
Enterprise Java
􀂃 Scriptlet Elements: are the embedded Java statements, which get executed as part of the service method.
(Note: Not recommended to use Scriptlet elements because they don’t provide reusability and maintainability.
Use custom tags (like JSTL, JSF tags, etc) or beans instead).
//Java codes
String userName=null;
Action Elements: A JSP element that provides information for execution phase.

Directive Elements: A JSP element that provides global information for the translation phase.
<%@ page import=”java.util.Date” %>
<%@ include file=”myJSP” %>
<%@ taglib uri=”tagliburi” prefix=”myTag”%>
Q 30: What are the different scope values or what are the different scope values for ? SF
A 30:
Scope Object Comment
Page PageContext Available to the handling JSP page only.
Request Request Available to the handling JSP page or Servlet and forwarded JSP page or Servlet.
Session Session Available to any JSP Page or Servlet within the same session.
Application Application Available to all the JSP pages and Servlets within the same Web Application.
Q 31: What are the differences between static and a dynamic include? SF DC
A 31:
Static include <%@ include %> Dynamic include
During the translation or compilation phase all the
included JSP pages are compiled into a single Servlet.
The dynamically included JSP is compiled into a separate Servlet.
It is a separate resource, which gets to process the request, and
the content generated by this resource is included in the JSP
No run time performance overhead. Has run time performance overhead.
Which one to use: Use “static includes” when a JSP page does not change very often. For the pages, which change frequently,
use dynamic includes. JVM has a 64kb limit on the size of the method and the entire JSP page is rendered as a single method. If
a JSP page is greater than 64kb, this probably indicates poor implementation. When this method reaches its limit of 64kb it
throws an error. This error can be overcome by splitting the JSP files and including them dynamically (i.e. using
) because the dynamic includes generate separate JSP Servlet for each included file.
Note: The “dynamic include” (jsp:include) has a flush attribute. This attribute indicates whether the buffer should be flushed
before including the new content. In JSP 1.1 you will get an error if you omit this attribute. In JSP 1.2 you can omit this attribute
because the flush attribute defaults to false.
Q 32: What are implicit objects and list them? SF
A 32: Implicit objects are the objects that are available for the use in JSP documents without being declared first. These
objects are parsed by the JSP engine and inserted into the generated Servlet. The implicit objects are:
Implicit object Scope comment
request Request request
response Page response
pageContext Page page environment
session Session session
application Application same as ServletContext
out Page writing to the outputstream
config Page same as ServletConfig
page Page this page’s Servlet
exception Page exception created on this page.
Enterprise Java
Note: Care should be taken not to name your objects the same name as the implicit objects. If you have your own object with the
same name, then the implicit objects take precedence over your own object.
Q 33: Explain hidden and output comments? SF
A 33: An output comment is a comment that is sent to the client where it is viewable in the browser’s source. CO

A hidden comment documents a JSP page but does not get sent to the client. The JSP engine ignores a hidden
comment, and does not process any code within hidden comment tags.
<%-- This comment will not be visible to the client --%>
Q 34: Is JSP variable declaration thread safe? CI
A 34: No. The declaration of variables in JSP is not thread-safe, because the declared variables end up in the generated
Servlet as an instance variable, not within the body of the _jspservice() method.
The following declaration is not thread safe: because these are declarations, and will only be evaluated once
when the page is loaded
<%! int a = 5 %>
The following declaration is thread safe: because the variables declared inside the scriplets have the local
scope and not shared.
<% int a = 5 %>
Q 35: Explain JSP URL mapping? What is URL hiding or protecting the JSP page? SF SE
A 35: As shown in the figure, the JSP resources usually reside directly or under subdirectories (e.g. myPath) of the
document root, which are directly accessible to the user through the URL. If you want to protect your Web
resources then hiding the JSP files behind the WEB-INF directory can protect the JSP files, css (cascading style
sheets) files, Java Script files, pdf files, image files, html files etc from direct access. The request should be made
to a servlet who is responsible for authenticating and authorising the user before returning the protected JSP page
or its resources.
URL eg
SERVER_HOME\WebApps\File myApps\myPath\myPage.jsp
Server Root
Doc ume nt roo t
JSP URL Mapping
myPage.jsp is hidden or protected. cannot be
directly accessed through URL
Hidden URL
myPage.jsp is dircetly accessible through URL
myPath myPage.jsp
Unhidden URL
Enterprise Java
Q 36: What are custom tags? Explain how to build custom tags? SF
A 36: Custom JSP tag is a tag you define. You define how a tag, its attributes and its body are interpreted, and then
group your tags into collections called tag libraries that can be used in any number of JSP files. So basically it is a
reusable and extensible JSP only solution. The pre-built tags also can speed up Web development. CO
Construct the Tag handler class that defines the behaviour.
Call setXXX() m ethods on the Tag
evaluate body o f the Tag
Tag Evaluation Process
Sample code using only doSatartTag()
package m yTagPkg;
public c lass MyTag extends TagSupport
int a ttr = null;
public int setA ttr(int a ttr){this.attr = a ttr}
public int getAttr(){return attr;}
p ublic int doStartTag() throws JspException {
retu rn SKIP_BODY;
public void release(){.....}
The Tag library descriptor file (*.tld) maps the XML element names to the tag implementations. The code sample
MyTagDesc.tld is shown below:



The web.xml deployment descriptor maps the URI to the location of the *.tld (Tag Library Descriptor) file. The code
sample web.xml file is shown below:


The JSP file declares and then uses the tag library as shown below:
<%@ taglib uri="/WEB-INF/ MyTagURI" prefix="myTag" %>
< attr="”abc”"> or < attr="”abc”">
Enterprise Java
Q 37: What is a TagExtraInfo class? SF
A 37: A TagExtraInfo class provides extra information about tag attributes to the JSP container at translation time.
􀂃 Returns information about the scripting variables that the tag makes available to the rest of the JSP page
to use. The method used is:
VariableInfo[] getVariableInfo(TagData td)

Without the use of TagExtraInfo, if you want to manipulate the attribute myArray in the above code in a
scriptlet it will not be possible. This is because it does not place the myArray object on the page. You can still
use pageContext.getAttribute() but that may not be a cleaner approach because it relies on the page
designer to correctly cast to object type. The TagExtraInfo can be used to make items stored in the
pageContext via setAttribute() method available to the scriptlet as shown below.

<%-- scriplet code %>
<% for(int i=0; ihtml +=
  • + myArray[i] +
  • ;

    􀂃 Validates the attributes passed to the Tag at translation time.
    Example It can validate the myArray array list to have not more than 100 objects. The method used is:
    boolean isValid(TagData data)
    Q 38: What is the difference between custom JSP tags and JavaBeans? SF
    A 38: In the context of a JSP page, both accomplish similar goals but the differences are:
    Custom Tags JavaBeans
    Can manipulate JSP content. Can’t manipulate JSP content.
    Custom tags can simplify the complex operations much
    better than the bean can. But require a bit more work to
    set up.
    Easier to set up.
    Used only in JSPs in a relatively self-contained manner. Can be used in both Servlets and JSPs. You can define a bean in
    one Servlet and use them in another Servlet or a JSP page.
    JavaBeans declaration and usage example: CO

    <%=identifier.getclassField() %>
    Q 39: Tell me about JSP best practices? BP
    A 39:
    􀂃 Separate HTML code from the Java code: Combining HTML and Java code in the same source code can
    make the code less readable. Mixing HTML and scriplet will make the code extremely difficult to read and
    maintain. The display or behaviour logic can be implemented as a custom tags by the Java developers and
    Web designers can use these Tags as the ordinary XHTML tags.
    􀂃 Place data access logic in JavaBeans: The code within the JavaBean is readily accessible to other JSPs
    and Servlets.
    Enterprise Java
    􀂃 Factor shared behaviour out of Custom Tags into common JavaBeans classes: The custom tags are not
    used outside JSPs. To avoid duplication of behaviour or business logic, move the logic into JavaBeans and
    get the custom tags to utilize the beans.
    􀂃 Choose the right “include” mechanism: What are the differences between static and a dynamic include?
    Using includes will improve code reuse and maintenance through modular design. Which one to use? Refer
    Q31 in Enterprise section.
    􀂃 Use style sheets (e.g. css), template mechanism (e.g. struts tiles etc) and appropriate comments (both
    hidden and output comments).
    Q 40: How will you avoid scriptlet code in JSP? BP
    A 40: Use JavaBeans or Custom Tags instead.
    Enterprise - JDBC
    Q 41: What is JDBC? How do you connect to a database? SF
    A 41: JDBC stands for Java Database Connectivity. It is an API which provides easy connection to a wide range of
    databases. To connect to a database we need to load the appropriate driver and then request for a connection
    object. The Class.forName(….) will load the driver and register it with the DriverManager (Refer Q4 in Java section
    for dynamic class loading).
    String url = jdbc:oracle:thin:@hostname:1526:myDB;
    Connection myConnection = DriverManager.getConnection(url, “username”, “password”);
    The DataSource interface provides an alternative to the DriverManager for making a connection. DataSource
    makes the code more portable than DriverManager because it work with JNDI and it is created, deployed and
    managed separately from the application that uses it. If the DataSource location changes, then there is no need to
    change the code but change the configuration properties in the server. This makes your application code easier to
    maintain. DataSource allows the use of connection pooling and support for distributed transactions. A DataSource
    is not only a database but also can be a file or a spreadsheet. A DataSource object can be bound to JNDI and an
    application can retrieve and use it to make a connection to the database. J2EE application servers provide tools to
    define your DataSource with a JNDI name. When the server starts it loads all the DataSources into the Application
    Server’s JNDI service.
    DataSource configuration properties are shown below:
    􀂃 JNDI Name 􀃆 jdbc/myDataSource
    􀂃 URL 􀃆 jdbc:oracle:thin:@hostname:1526:myDB
    􀂃 UserName, Password
    􀂃 Implementation classname 􀃆 oracle.jdbc.pool.OracleConnectionPoolDataSource
    􀂃 Classpath 􀃆 ora_jdbc.jar
    􀂃 Connection pooling settings like 􀃆 minimum pool size, maximum pool size, connection timeout, statement cache size etc.
    Once the DataSource has been set up, then you can get the connection object as follows:
    Context ctx = new InitialContext();
    DataSource ds = (DataSource)ctx.lookup("jdbc/myDataSource");
    Connection myConnection = ds.getConnection(“username”,”password”);
    In a basic implementation a Connection obtained from a DataSource and a DriverManager are identical. But,
    DataSource is recommended because of its better portability.
    Design Pattern: JDBC architecture decouples an abstraction from its implementation so that the implementation
    can vary independent of the abstraction. This is an example of the bridge design pattern. The JDBC API
    provides the abstraction and the JDBC drivers provide the implementation. New drivers can be plugged-in to the
    JDBC API without changing the client code.
    Q 42: What are JDBC Statements? What are different types of statements? How can you create them? SF
    Enterprise Java
    A 42: A statement object is responsible for sending the SQL statements to the Database. Statement objects are created
    from the connection object and then executed. CO
    Statement stmt = myConnection.createStatement();
    ResultSet rs = stmt.executeQuery(“SELECT id, name FROM myTable where id =1245”);// to read
    stmt.executeUpdate(“INSERT INTO (field1,field2) values (1,3)”);// to insert/update/delete/create table
    The types of statements are:
    􀂃 Statement (regular statement as shown above)
    􀂃 PreparedStatement (more efficient than statement due to pre-compilation of SQL)
    􀂃 CallableStatement (to call stored procedures on the database)
    To use prepared statement:
    PreparedStatement prepStmt =
    myConnection.prepareStatement("SELECT id, name FROM myTable where id = ? ");
    prepStmt.setInt(1, 1245);
    Callable statements are used for calling stored procedures.
    CallableStatement calStmt = myConnection.prepareCall("{call PROC_SHOWMYBOOKS}");
    ResultSet rs = cs.executeQuery();
    Q 43: What is a Transaction? What does setAutoCommit do? TI PI
    A 43: A transaction is a set of operations that should be completed as a unit. If one operation fails then all the other
    operations fail as well. For example if you transfer funds between two accounts there will be two operations in the
    1. Withdraw money from one account.
    2. Deposit money into other account.
    These two operations should be completed as a single unit. Otherwise your money will get lost if the withdrawal is
    successful and the deposit fails. There are four characteristics (ACID properties) for a Transaction.
    Atomicity Consistency Isolation Durability
    All the individual
    operations should
    either complete or fail.
    The design of the
    transaction should
    update the database
    Prevents data being corrupted by concurrent
    access by two different sources. It keeps
    transactions isolated or separated from each
    other until they are finished.
    Ensures that the database
    is definitely updated once
    the Transaction is
    Transactions maintain data integrity. A transaction has a beginning and an end like everything else in life. The
    setAutocommit(….), commit() and rollback() are used for marking the transactions (known as transaction
    demarcation). When a connection is created, it is in auto-commit mode. This means that each individual SQL
    statement is treated as a transaction and will be automatically committed immediately after it is executed. The way
    to allow two or more statements to be grouped into a transaction is to disable auto-commit mode: CO
    Connection myConnection = dataSource.getConnection();
    // set autoCommit to false
    myConnection .setAutoCommit(false);
    withdrawMoneyFromFirstAccount(.............); //operation 1
    depositMoneyIntoSecondAccount(.............); //operation 2
    myConnection .commit();
    catch(Exception sqle){
    myConnection .rollback();
    }catch( Exception e){}
    try{if( conn != null) {conn.close();}} catch( Exception e) {}
    Enterprise Java
    The above code ensures that both operation 1 and operation 2 succeed or fail as an atomic unit and consequently
    leaves the database in a consistent state. Also turning auto-commit off will provide better performance.
    Q 44: What is the difference between JDBC-1.0 and JDBC-2.0? What are Scrollable ResultSets, Updateable ResultSets,
    RowSets, and Batch updates? SF
    A 44: JDBC2.0 has the following additional features or functionality:
    JDBC 1.0 JDBC 2.0
    With JDBC-1.0 the
    ResultSet functionality
    was limited. There was no
    support for updates of any
    kind and scrolling through
    the ResultSets was
    forward only (no going
    With JDBC 2.0 ResultSets are updateable and also you can move forward and backward.
    Example This example creates an updateable and scroll-sensitive ResultSet
    Statement stmt = myConnection.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,
    With JDBC-1.0 the
    statement objects submits
    updates to the database
    individually within same or
    separate transactions.
    This is very inefficient
    large amounts of data
    need to be updated.
    With JDBC-2.0 statement objects can be grouped into a batch and executed at once. We call
    addBatch() multiple times to create our batch and then we call executeBatch() to send the SQL
    statements off to database to be executed as a batch (this minimises the network overhead).
    Statement stmt = myConnection.createStatement();
    stmt.addBatch(“INSERT INTO myTable1 VALUES (1,”ABC”)”);
    stmt.addBatch(“INSERT INTO myTable1 VALUES (2,”DEF”)”);
    stmt.addBatch(“INSERT INTO myTable1 VALUES (3,”XYZ”)”);

    int[] countInserts = stmt.executeBatch();
    - The JDBC-2.0 optional package provides a RowSet interface, which extends the ResultSet. One
    of the implementations of the RowSet is the CachedRowSet, which can be considered as a
    disconnected ResultSet.
    Q 45: How to avoid the “running out of cursors” problem? DC PI MI
    A 45: A database can run out of cursors if the connection is not closed properly or the DBA has not allocated enough
    cursors. In a Java code it is essential that we close all the valuable resources in a try{} and finally{} block. The
    finally{} block is always executed even if there is an exception thrown from the catch {} block. So the resources like
    connections and statements should be closed in a finally {} block. CO
    Wrong Approach -
    Connections and statements will not be closed if there
    is an exception:
    public void executeSQL() throws SQLException{
    Connection con = DriverManager.getConnection(........);
    Statement stmt = con.createStatement();
    //line 20 where exception is thrown
    ResultSet rs = stmt.executeQuery("SELECT * from myTable");
    Note: if an exception is thrown at line 20 then the
    close() statements are never reached.
    Right Approach -
    public void executeSQL() throws SQLException{
    Connection con = DriverManager.getConnection(........);
    Statement stmt = con.createStatement();
    //line 20 where exception is thrown
    ResultSet rs = stmt.executeQuery("SELECT * from myTable");
    try {
    if(rs != null) rs.close();
    if(stmt != null) stmt.close();
    if(con != null) con.close();
    catch(Exception e){}
    Note: if an exception is thrown at line 20 then the
    finally clause is called before the exception is thrown to
    the method.
    Try{} Finally {} blocks to close Exceptions
    Enterprise Java
    Q 46: What is the difference between statements and prepared statements? SF PI SE BP
    A 46:
    􀂃 Prepared statements offer better performance, as they are pre-compiled. Prepared statements reuse the
    same execution plan for different arguments rather than creating a new execution plan every time. Prepared
    statements use bind arguments, which are sent to the database engine. This allows mapping different
    requests with same prepared statement but different arguments to execute the same execution plan.
    􀂃 Prepared statements are more secure because they use bind variables, which can prevent SQL injection
    The most common type of SQL injection attack is SQL manipulation. The attacker attempts to modify the
    SQL statement by adding elements to the WHERE clause or extending the SQL with the set operators like
    Example Let us look at the following SQL:
    SELECT * FROM users where username=’bob’ AND password=’xyfdsw’ ;
    The attacker can manipulate the SQL as follows
    SELECT * FROM users where username=’bob’ AND password=’xyfdsw’ OR ‘a’ = ‘a’ ;
    The above “WHERE” clause is always true because of the operator precedence. The PreparedStatement
    can prevent this by using bind variables:
    String strSQL = SELECT * FROM users where username=? AND password=?);
    PreparedStatement pstmt = myConnection.prepareStatement(strSQL);
    pstmt.setString(2, “xyfdsw”);
    Q 47: Explain differences among java.util.Date, java.sql.Date, java.sql.Time, and java.sql.Timestamp? SF
    A 47: As shown below all the sql Date classes extend the util Date class.
    java.sql.Date java.sql.Time java.sql.TimeStamp
    Java Date classes
    java.util.Date - class supports both the Date (ie year/month/date etc) and the Time (hour, minute, second, and
    millisecond) components.
    java.sql.Date - class supports only the Date (ie year/month/date etc) component. The hours, minutes, seconds
    and milliseconds of the Time component will be set to zero in the particular time zone with which the instance is
    java.sql.Time - class supports only Time (ie hour, minute, second, and millisecond) component. The date
    components should be set to the "zero epoch" value of January 1, 1970 and should not be accessed.
    java.sql.TimeStamp – class supports both Date (ie year/month/date etc) and the Time (hour, minute, second,
    millisecond and nanosecond) components.
    Note: the subtle difference between java.util.Date and java.sql.Date.
    Enterprise Java
    To keep track of time Java counts the number of milliseconds from January 1, 1970 and stores it as a long value in
    java.util.Date class. The GregorianCalendar class provides us a way to represent an arbitrary date. The
    GregorianCalendar class also provides methods for manipulating dates.
    Enterprise – JNDI & LDAP
    Q 48: What is JNDI? And what are the typical uses within a J2EE application? SF
    A 48: JNDI stands for Java Naming and Directory Interface. It provides a generic interface to LDAP (Lightweight
    Directory Access Protocol) and other directory services like NDS, DNS (Domain Name System) etc. It provides a
    means for an application to locate components that exist in a name space according to certain attributes. A J2EE
    application component uses JNDI interfaces to look up and reference system-provided and user-defined objects in
    a component environment. JNDI is not specific to a particular naming or directory service. It can be used to access
    many different kinds of systems including file systems.
    The JNDI API enables applications to look up objects such as DataSources, EJBs, MailSessions and JMS by
    name. The Objects can be loaded into the JNDI tree using a J2EE application server’s administration console. To
    load an object in a JNDI tree, choose a name under which you want the object to appear in a JNDI tree. J2EE
    deployment descriptors indicate the placement of J2EE components in a JNDI tree.
    O bjec ts a n d /or S ervice
    In itia lC o n te x t
    su b -con text
    nam ed object/serv ice
    re fe ren c e
    nam ed object/serv ice
    re fe ren c e
    JND I T ree
    N ote: JND I tree lives in the serve r a s a collection o f nam ed o b ject o r service references.
    The parameters you have to define for JNDI service are as follows:
    􀂃 The name service provider class name (WsnInitialContext for Websphere).
    Hashtable env = new Hashtable();
    env.put(Context.INITIAL_CONTEXT_FACTORY, "");
    􀂃 The provider URL :
    􀂃 The name service hostname.
    􀂃 The name service port number.
    env.put(Context. PROVIDER_URL, " iiop://localhost:1050");
    Context ctx = new InitialContext(env);
    JNDI is like a file system or a Database.
    File System JNDI Database
    File system starts with a mounted
    drive like c:\
    JNDI starts with an InitialContext. i.e. new
    Database instance
    Enterprise Java
    Uses a subdirectory. C:\subdir1 Navigate to a sub-context. e.g. Subcontext1 Tablespace
    Access a subdirectory
    Drill down through other sub-contexts. e.g.
    Access a file.
    Access an object or a service.
    New InitialContext().lookup(“objectName”);
    Select * from demo.myTable
    Q 49: Explain the difference between the look up of “java:comp/env/ejb/MyBean” and “ejb/MyBean”? SF
    A 49:
    java:comp/env/ejb/MyBean ejb/MyBean
    This is a logical reference, which will be used in your code.
    This is a physical reference where an object will be mapped to in
    a JNDI tree.
    The logical reference (or alias) java:comp/env/ejb/MyBean is the recommended approach because you cannot
    guarantee that the physical JNDI location (ejb/MyBean) you specify in your code will be available. Your code will
    break if the physical location is changed. The deployer will not be able to modify your code. Logical references
    solve this problem by binding the logical name to the physical name in the application server. The logical names
    will be declared in the deployment descriptors (web.xml and/or ejb-jar.xml) as follows and these will be mapped to
    physical JNDI locations in the application server specific deployment descriptors.
    To look up a JDBC resource from either WEB (web.xml) or EJB (ejb-jar.xml) tier, the deployment descriptor should
    have the following entry:

    The DataSource

    To use it:
    Context ctx = new InitialContext();
    Object ref = ctx.lookup(java:comp/env/jdbc/MyDataSource);
    To look up EJBs from another EJB or a WEB module, the deployment descriptor should have the following entry:


    To use it:
    Context ctx = new InitialContext();
    Object ref = ctx.lookup(java:comp/env/ejb/MyBean);
    Q 50: What is a JNDI InitialContext? SF
    A 50: All naming operations are relative to a context. The InitalContext implements the Context interface and provides
    an entry point for the resolution of names.
    Q 51: What is an LDAP server? And what is it used for in an enterprise environment? SF SE
    A 51: LDAP stands for Lightweight Directory Access Protocol. This is an extensible open network protocol standard that
    provides access to distributed directory services. LDAP is an Internet standard for directory services that run on
    TCP/IP. Under OpenLDAP and related servers, there are two servers – slapd, the LDAP daemon where the
    queries are sent to and slurpd, the replication daemon where data from one server is pushed to one or more
    This will make full logical path to the bean
    This will make full logical path to the bean
    Enterprise Java
    slave servers. By having multiple servers hosting the same data, you can increase reliability, scalability, and
    􀂃 It defines the operations one may perform like search, add, delete, modify, change name
    􀂃 It defines how operations and data are conveyed.
    LDAP has the potential to consolidate all the existing application specific information like user, company phone
    and e-mail lists. This means that the change made on an LDAP server will take effect on every directory service
    based application that uses this piece of user information. The variety of information about a new user can be
    added through a single interface which will be made available to Unix account, NT account, e-mail server, Web
    Server, Job specific news groups etc. When the user leaves his account can be disabled to all the services in a
    single operation.
    So LDAP is most useful to provide “white pages” (e.g. names, phone numbers, roles etc) and “yellow pages” (e.g.
    location of printers, application servers etc) like services. Typically in a J2EE application environment it will be
    used to authenticate and authorise users.
    Why use LDAP when you can do the same with relational database (RDBMS)?
    In general LDAP servers and RDBMS are designed to provide different types of services. LDAP is an open
    standard access mechanism, so an RDBMS can talk LDAP. However the servers, which are built on LDAP, are
    optimized for read access so likely to be much faster than RDBMS in providing read access. So in a nutshell,
    LDAP is more useful when the information is often searched but rarely modified. (Another difference is that
    RDBMS systems store information in rows of tables whereas LDAP uses object oriented hierarchies of entries.) .
    Key LDAP Terms:
    DIT: Directory Information Tree. Hierarchical structure of entries, those make up a directory.
    DN: Distinguished Name. This uniquely identifies an entry in the directory. A DN is made up of relative DNs of
    the entry and each of entry’s parent entries up to the root of the tree. DN is read from right to left and commas
    separate these names. For example ‘cn=Peter Smith, o=ACME, c=AUS’.
    objectClass: An objectClass is a formal definition of a specific kind of objects that can be stored in the directory.
    An ObjectClass is a distinct, named set of attributes that represent something concrete such as a user, a
    computer, or an application.
    LDAP URL: This is a string that specifies the location of an LDAP resource. An LDAP URL consists of a server
    host and a port, search scope, baseDN, filter, attributes and extensions. Refer to diagram below:
    LDAP D irectory structure
    c=AUS c=UK
    o=ACME o=XYZRetail o=QuickCorp
    cn=Peter Smith
    Enterprise Java
    So the complete distinguished name for bottom left entry (ie Peter Smith) is cn=Peter Smith, o=ACME, c=AUS.
    Each entry must have at least one attribute that is used to name the entry. To manage the part of the LDAP
    directory we should specify the highest level parent distinguished names in the server configuration. These
    distinguished names are called suffixes. The server can access all the objects that are below the specified suffix
    in the hierarchy. For example in the above diagram to answer queries about ‘Peter Smith’ the server should have
    the suffix of ‘o=ACME, c=AUS’. So we can look for “Peter Smith” by using the following distinguished name:
    cn=Peter Smith, o=ACME, c=AUS //where o=ACME, c=AUS is the suffix
    LDAP schema: defines rules that specify the types of objects that a directory may contain and the required
    optional attributes that entries of different types should have.
    Filters: In LDAP the basic way to retrieve data is done with filters. There is a wide variety of operators that can be
    used as follows: & (and), | (or), ! (not), ~= (approx equal), >= (greater than or equal), <= (less than or equal), *
    (any) etc.
    (& (uid=a*) (uid=*l) )
    So where does JNDI fit into this LDAP? JNDI provides a standard API for interacting with naming and directory
    services using a service provider interface (SPI), which is analogous to JDBC driver. To connect to an LDAP
    server, you must obtain a reference to an object that implements the DirContext. In most applications, this is done
    by using an IntialDirContext object that takes a Hashtable as an argument:
    Hashtable env = new Hashtable();
    env.put(Context.INITIAL_CONTEXT_FACTORY, “com.sun.jndi.ldap.LdapCtxFactory”);
    env.put(Context.PROVIDER_URL, “ldap://localhost:387”);
    env.put(Context.SECURITY_AUTHENTICATION, “simple”);
    env.put(Context.SECURITY_PRINCIPAL, “cn=Directory Manager”);
    env.put(Context.SECURITY_CREDENTIALS, “myPassword”);
    DirContext ctx = new InitialDirContext(env);
    Enterprise - RMI
    Q 52: Explain the RMI architecture? SF
    A 52: Java Remote Method Invocation (RMI) provides a way for a Java program on one machine to communicate with
    objects residing in different JVMs (or processes or address spaces). The important parts of the RMI architecture
    are the stub class, object serialization and the skeleton class. RMI uses a layered architecture where each of the
    layers can be enhanced without affecting the other layers. The layers can be summarised as follows:
    􀂃 Application Layer: The client and server program
    􀂃 Stub & Skeleton Layer: Intercepts method calls made by the client. Redirects these calls to a remote RMI
    􀂃 Remote Reference Layer: Sets up connections to remote address spaces, manages connections, and
    understands how to interpret and manage references made from clients to the remote service objects.
    􀂃 Transport layer: Based on TCP/IP connections between machines in a network. It provides basic connectivity,
    as well as some firewall penetration strategies.
    Design pattern: RMI stub classes provide a reference to a skeleton object located in a different address space on
    the same or different machine. This is a typical example of a proxy design pattern (i.e. remote proxy), which
    makes an object executing in another JVM appear like a local object. In JDK 5.0 and later, the RMI facility uses
    dynamic proxies instead of generated stubs, which makes RMI easier to use. Refer Q11 in “How would you
    about…” section for a more detailed discussion on proxy design pattern and dynamic proxies.
    Enterprise Java
    Client Server
    RMI Transport Layer
    Client Process Server Process
    R M I Trans port L a ye r
    Remote Reference
    Remote Reference
    RMI Registry
    (or JNDI S erver)
    2 . l ook u p Stu b
    4 . M eth o d ca l l o n rem o te s er v er O b jec ts
    5. S en d re s ults o r Exceptio n
    3. Return Stub Stub
    1. Load Stubs
    Note: Steps 4 & 5 are logical explanation only. Neither the Stubs nor Skeletons
    use sockets directly. The actual calls are made through the Remote Reference
    Manager. The Remote Reference M anager handles the actual details of
    communicating with the remote process. This extra layer m anages network
    communication and conserves scarce resources like sockets.
    RMI A rchitecture
    Program 1 stub
    stub Program 2
    RMI runtime steps (as shown in the diagram above) involved are:
    Step 1: Start RMI registry and then the RMI server. Bind the remote objects to the RMI registry.
    Step 2: The client process will look up the remote object from the RMI registry.
    Step 3: The lookup will return the stub to the client process from the server process.
    Step 4: The client process will invoke method calls on the stub. The stub calls the skeleton on the server process
    through the RMI reference manager.
    Step 5: The skeleton will execute the actual method call on the remote object and return the result or an exception
    to the client process via the RMI reference manager and the stub.
    Q 53: What is a remote object? Why should we extend UnicastRemoteObject? SF
    A 53: A remote object is one whose methods can be invoked from another JVM (or process). A remote object class must
    implement the Remote interface. A RMI Server is an application that creates a number of remote objects.
    An RMI Server is responsible for
    􀂃 Creating an instance of the remote object (e.g. CarImpl instance = new CarImpl()).
    􀂃 Exporting the remote object.
    􀂃 Binding the instance of the remote object to the RMI registry.
    By exporting a remote object you make it available to accept incoming calls from the client. You can export the
    remote object by either extending the java.rmi.server.UnicastRemoteObject or if your class is already extending
    another class then you can use the static method
    Enterprise Java
    UnicastRemoteObject.exportObject (this);
    If the UnicastRemoteObject is not extended (ie if you use UnicastRemoteObject.exportObject(…) then the
    implementation class is responsible for the correct implementations of the hashCode(), equals() and toString()
    methods. A remote object is registered in the RMI registry using:
    Naming.rebind(String serviceName, Remote remoteObj);
    Remote interface
    eg: public interface Car extends Remote{}
    Implementation of Remote interface
    eg: public class CarImpl extends UnicastRemoteObject implements Car{}
    Compile Car &
    use rmic to generate stubs & skeletons
    rmic -d /classes CarImpl
    stub class skeleton class
    Client Object
    stub Object
    skeleton Object
    remote Object
    Remote Objects
    Q 54: What is the difference between RMI and CORBA? SF
    A 54:
    Java only solution. The interfaces,
    implementations and the clients are all written
    in Java.
    CORBA was made specifically for interoperability among various
    languages. For example the server could be written in C++ and the
    business logic can be in Java and the client can be written in COBOL.
    RMI allows dynamic loading of classes at
    In a CORBA environment with multi-language support it is not possible to
    have dynamic loading.
    Q 55: What are the services provided by the RMI Object? SF
    A 55: In addition to its remote object architecture RMI provides some basic object services, which can be used in a
    distributed application. These services are
    􀂃 Object naming/registry service: RMI servers can provide services to clients by registering one or more
    remote objects with its local RMI registry.
    􀂃 Object activation service: It provides a way for server (or remote) objects to be started on an as-needed
    basis. Without the remote activation service, a server object has to be registered with the RMI registry service.
    􀂃 Distributed garbage collection: It is an automatic process where an object, which has no further remote
    references, becomes a candidate for garbage collection.
    Q 56: What are the differences between RMI and a socket? SF
    A 56:
    Enterprise Java
    Socket RMI
    A socket is a transport mechanism. Sockets are like
    applying procedural networking to object oriented
    RMI uses sockets. RMI is object oriented. Methods can be
    invoked on the remote objects running on a separate JVM.
    Sockets-based network programming can be laborious. RMI provides a convenient abstraction over raw sockets. Can
    send and receive any valid Java object utilizing underlying
    object serialization without having to worry about using data
    Q 57: How will you pass parameters in RMI? SF
    A 57:
    􀂃 Primitive types are passed by value (e.g. int, char, boolean etc).
    􀂃 References to remote objects (i.e. objects which implements the Remote interface) are passed as remote
    references that allows the client process to invoke methods on the remote objects.
    􀂃 Non-remote objects are passed by value using object serialization. These objects should allow them to be
    serialized by implementing the interface.
    Note: The client process initiates the invocation of the remote method by calling the method on the stub. The stub
    (client side proxy of the remote object) has a reference to the remote object and forwards the call to the skeleton
    (server side proxy of the remote object) through the reference manager by marshalling the method arguments.
    During marshalling each object is checked to determine whether it implements java.rmi.Remote interface. If it does
    then the remote reference is used as the marshalled data otherwise the object is serialized into byte streams and
    sent to the remote process where it is deserialized into a copy of the local object. The skeleton converts this
    request from the stub into the appropriate method call on the actual remote object by unmarshalling the method
    arguments into local stubs on the server (if they are remote reference) or into local copy (if they are sent as
    serialized objects).
    Q 58: What is HTTP tunnelling or how do you make RMI calls across firewalls? SF SE
    A 58: RMI transport layer generally opens direct sockets to the server. Many Intranets have firewalls that do not allow
    this. To get through the firewall an RMI call can be embedded within the firewall-trusted HTTP protocol. To get
    across firewalls, RMI makes use of HTTP tunnelling by encapsulating RMI calls within an HTTP POST request.
    Proxy Server
    RMI Client
    Web Server
    on port 80
    RMI Server
    RMI call
    forwarded by
    CGI script
    Firewall Firewall
    HTTP tunnelling
    When a firewall proxy server can forward HTTP requests only to a well-known HTTP port: The firewall proxy
    server will forward the request to a HTTP server listening on port 80, and a CGI script will be executed to forward
    the call to the target RMI server port on the same machine.
    When a firewall proxy server can forward HTTP requests to any arbitrary port: The firewall proxy will forward
    to any arbitrary port on the host machine and then it is forwarded directly to the port on which RMI Server is
    The disadvantages of HTTP tunnelling are performance degradation, prevents RMI applications from using callbacks,
    CGI script will redirect any incoming request to any port, which is a security loophole, RMI calls cannot be
    multiplexed through a single connection since HTTP tunnelling follows a request/response protocol etc.
    Enterprise Java
    Q 59: Why use RMI when we can achieve the same benefits from EJB? SF
    A 59: EJBs are distributed components, which use the RMI framework for object distribution. An EJB application server
    provides more services like transactions, object pooling, database connection-pooling etc, which RMI does not
    provide. These extra services that are provided by the EJB server simplify the programming effort at the cost of
    performance overhead compared to plain RMI. So if performance is important then pure RMI may be a better
    solution (or under extreme situations Sockets can offer better performance than RMI).
    Note: The decision to go for RMI or EJB or Sockets should be based on requirements such as maintainability, ease of coding,
    extensibility, performance, scalability, availability of application servers, business requirements etc.
    Enterprise – EJB 2.x
    There are various persistence mechanisms available like EJB 2.x, Object-to-Relational (O/R) mapping tools like Hibernate, JDBC and
    EJB 3.0 (new kid on the block) etc. You will have to evaluate the products based on the application you are building because each
    product has its strengths and weaknesses. You will find yourself trading ease of use for scalability, standards with support for special
    features like stored procedures, etc. Some factors will be more important to you than for others. There is no one size fits all solution.
    Let’s compare some of the persistence products:
    EJB 2.x EJB 3.0 Hibernate JDBC
    􀂃 Security is provided for free
    for accessing the EJB.
    􀂃 Provides declarative
    􀂃 EJBs are pooled and
    cached. EJB life cycles are
    managed by the container.
    􀂃 Has remote access
    capabilities and can be
    clustered for scalability.
    􀂃 A lot less artefacts than EJB
    2.x. Make use of annotations
    or attributes based
    􀂃 Narrows the gap between EJB
    2.x and O/R mapping.
    􀂃 Do support OO concepts like
    􀂃 Simple to write CRUD
    (create, retrieve, update,
    delete) operations.
    􀂃 No container or application
    server is required and can be
    plugged into an existing
    􀂃 Tools are available to simplify
    mapping relational data to
    objects and quick to develop.
    􀂃 You have complete control
    over the persistence
    because this is the building
    blocks of nearly all other
    persistence technologies in
    􀂃 Can call Stored Procedures.
    􀂃 Can manipulate relatively
    large data sets.
    􀂃 Need to understand the
    intricacies like rolling back
    a transaction, granularity
    etc, infrastructures like
    session facades, business
    delegates, value objects etc
    and strategies like lazy
    loading, dirty marker etc.
    􀂃 EJBs use lots of resources
    and have lots of artifacts.
    􀂃 Does not support OO
    concepts like inheritance.
    􀂃 Since it is new, might be too
    early to use in commercial
    􀂃 It is still evolving.
    􀂃 Little or no capabilities for
    remote access and
    􀂃 Mapping schemas can be
    tedious and O/R mapping
    has its tricks like using lazy
    initialization, eager loading
    etc. What works for one may
    not work for another.
    􀂃 Limited clustering
    􀂃 Large data sets can still
    cause memory issues.
    􀂃 Support for security at a
    database level only and no
    support for role based
    security without any add on
    APIs like Aspect Oriented
    Programming etc.
    􀂃 You will have to write a lot
    of code to perform a little.
    Easy to make mistakes in
    properly managing
    connections and can cause
    out of cursors issues.
    􀂃 Harder to maintain because
    changes in schemas can
    cause lot of changes to your
    􀂃 Records need to be locked
    manually (e.g. select for
    As a rule of thumb, suitable
    for distributed and clustered
    applications, which is heavily
    transaction based. Records
    in use say between 1 and 50.
    As a rule of thumb, suitable for
    distributed and clustered
    applications, which is heavily
    transaction based. Records in
    use say between 1 and 100.
    Suitable for records in use
    between 100 and 5000. Watch
    out for memory issues, when
    using large data sets.
    Where possible stay away
    from using JDBC unless you
    have compelling reason to
    use it for batch jobs where
    large amount of data need to
    be transferred, records in use
    greater than 5000, required
    to use Stored Procedures
    Enterprise Java
    The stateless session beans and message driven beans have wider acceptance in EJB 2.x compared to stateful session
    beans and entity beans. Refer Emerging Technologies/Frameworks section for Hibernate and EJB 3.0.
    Q 60: What is the role of EJB 2.x in J2EE? SF
    A 60: EJB 2.x (Enterprise JavaBeans) is a widely adopted server side component architecture for J2EE.
    􀂃 EJB is a remote, distributed multi-tier system and supports protocols like JRMP, IIOP, and HTTP etc.
    􀂃 It enables rapid development of reusable, versatile, portable business components across middleware,
    transactional and scalable applications.
    􀂃 EJB is a specification for J2EE servers. EJB components contain only business logic and system level
    programming and services like transactions, security, instance pooling, threading, persistence etc are
    managed by the EJB Container and hence simplify the programming effort.
    􀂃 Message driven EJBs have support for asynchronous communication.
    Note: Having said that EJB 2.x is a widely adopted server side component, EJB 3.0 is taking ease of
    development very seriously and has adjusted its model to offer the POJO (Plain Old Java Object) persistence and
    the new O/R mapping model based on Hibernate. In EJB 3.0, all kinds of enterprise beans are just POJOs.
    EJB 3.0 extensively uses Java annotations, which replaces excessive XML, based configuration files and
    eliminates the need for the rigid component model used in EJB 1.x, 2.x. Annotations can be used to define the
    bean’s business interface, O/R mapping information, resource references etc. Refer Q18 in Emerging
    Technologies/Frameworks section. So, for future developments look out for EJB 3.0 and/or Hibernate framework.
    Refer Q14 – Q16 in Emerging Technologies/Frameworks section for discussion on Hibernate framework.
    J2EE Server
    C++ application
    HTTP Client
    (eg: Browser, Wireless etc)
    Java Applet,
    Java stand-alone application
    Other J2EE
    (use JavaBeans)
    (use JavaBeans)
    EJB Container (Enterprise Java Beans are deployed)
    Connectors (JCA)
    Legacy System,
    ERP System etc
    (use JavaBeans)
    Web Services
    proprietary protocol
    EJB Session Bean EJB Message Driven Bean EJB Session Bean
    EJB Entity Bean EJB Session Bean EJB Session Bean
    Message Oriented
    Middleware Topic
    SQL (fast Lane Reader)
    EJB - Big Picture
    Business Logic provided by
    the developer through EJB
    System Level Services like
    transaction, Security etc
    are provided by the
    Business Delegate
    (use JavaBeans)
    Other J2EE
    Web Services
    Q 61: What is the difference between EJB and JavaBeans? SF
    A 61: Both EJB and JavaBeans have very similar names but this is where the similarities end.
    JavaBeans Enterprise JavaBeans (EJB)
    The components built based on JavaBeans live in a single
    local JVM (or address space) and can be either visual (e.g.
    GUI components like Button, List etc) or non-visual at
    The Enterprise JavaBeans are non-visual distributable
    components, which can live across multiple JVMs (or address
    Enterprise Java
    No explicit support exists for services like transactions etc. EJBs can be transactional and the EJB servers provide
    transactional support.
    JavaBeans are fine-grained components, which can be
    used to assemble coarse-grained components or an
    EJBs are coarse-grained components that can be deployed as
    is or assembled with other components into larger
    applications. EJBs must be deployed in a container that
    provides services like instance pooling, multi-threading,
    security, life-cycle management, transactions etc
    Must conform to JavaBeans specification. Must conform to EJB specification.
    Q 62: Explain EJB architecture? SF
    A 62:
    EJB Client
    (eg Servlet, JSP, Stand alone
    application, Applet etc)
    E JB S e rv e r
    E JB C o n ta in er
    E n te rp r is e J a v a B e a n s
    S e s s io n B e a n s
    s ta te fu l / s ta te le s s
    E n tity B e an s
    C M P / BM P
    H om e /L o c a lH om e
    In te r fa c e
    R em o te /L o c a l
    In te r fa c e
    H om e O b je c t /
    L o c a l H om e O b je c t
    E JB O b je c t /
    E JB L o c a lO b je c t
    s y n c h ro n o u s
    s y n c h ro n o u s
    E n te rp r is e S e rv ic e s a n d A P I
    JN D I JM S T ra n s a c tio n s S e cu rity
    D a ta b a s e S e rv e r
    JM S
    M e s s a g e
    P ro d u c e r
    (e g p u b lis h /
    s u b s c rib e
    T o p ic )
    JM S M e s s a g e
    A s yn c h ro n o u s L is te n e r In te r fa c e
    E n te rp r is e J a v a B e a n s
    M e s s a g e -D r iv e n E IS S ys tem
    B ean s
    E JB C o n ta in e r
    C lie n t
    JN D I
    H om e O b je c t
    E JB O B je c t
    D e p lo ym e n t d e s c rip to r
    - B e a n d e fin itio n
    - T ra n s a c tio n
    - S e c u r ity e tc
    E JB C on te xt
    b e a n in s ta n c e
    1 . Loo kup
    2. ge t S T UB
    H om e In te r fa c e
    R em o te In te r fa c e
    3. In v ok e
    cre a te ( )
    find ()
    re m o ve ()
    o n th e stub
    4. in terce p t & app ly
    5. new
    7. R e fe r
    6. intercept & apply services
    8 . be an life - c y c l e m e th o d s
    e jb C re a te ()o r
    e jb F in d ()
    9 . in v o k e
    b e a n m e t h o ds
    1 0 . b e a n bus in e s s m e th o d s
    g e t H o rs e P o w e r() e tc
    S am p le C o d e :
    C o n te x t in it ia lC tx = n e w In it ia lC o n te x t ( ) ; //In itia liz e th e J N D I c o n te x t . ie e n t ry p o in t .
    C a rH om e h om e O b je c t = (C a rH om e ) in itia lC tx .lo o k u p (e jb /M yE jb ) ; / / S te p s 1 & 2 in th e a b o ve d ia g ram
    C a r c a rO b je c t = h om eO b je c t.c re a te ( ) ; // S te p s 3 - 8
    c a rO b je c t .g e tH o rs e P ow e r ( ) ; // S te p s 9 - 1 0
    N o te : A n E JB c lie n t s h o u ld n e ve r a c c e s s a n E n te rp r is e J a va B e a n d ire c tly . A n y a c c e s s is d o n e th ro u g h th e
    c o n ta in e r g e n e ra te d c la s s e s w h ic h in tu rn in vo k e b e a n m e th o d s . T h e c o n ta in e r g e n e ra te d c la s s e s in te rc e p t th e
    re q u e s t a n d a p p ly s e rv ic e s lik e tra n s a c tio n , s e c u rity e tc p rio r to in v o k in g th e a c tu a l m e th o d o n th e E n e te rp ris e J a va
    B e a n s .
    E JB A rch ite c tu re
    P ers is te n c e
    EJB Container: EJBs are software components, which run in an environment called an EJB container. An EJB
    cannot function outside an EJB Container. The EJB container hosts and manages an Enterprise JavaBean in a
    similar manner that a Web container hosts a servlet or a Web browser hosts a Java Applet. The EJB container
    manages the following services so that the developer can concentrate on writing the business logic:
    􀂃 Transactions (refer Q71 – Q75 in Enterprise section)
    􀂃 Persistence
    􀂃 EJB instance pooling
    􀂃 Security (refer Q81 in Enterprise section)
    􀂃 Concurrent access (or multi-threading)
    􀂃 Remote access
    Design pattern: EJBs use the proxy design pattern to make remote invocation (i.e. remote proxy) and to add
    container managed services like security and transaction demarcation. Refer Q11 in “How would you about…”
    section for a more detailed discussion on proxy design pattern and dynamic proxies.
    Enterprise Java
    EJBContext: Every bean obtains an EJBContext object, which is a reference directly to the container. The EJB
    can request information about its environment like the status of a transaction, a remote reference to itself (an EJB
    cannot use ‘this’ to reference itself) etc.
    Deployment Descriptor: The container handles all the above mentioned services declaratively for an EJB based
    on the XML deployment descriptor (ejb-jar.xml). When an EJB is deployed into a container the deployment
    descriptor is read to find out how these services are handled. Refer to the J2EE deployment structure diagram in
    Q6 in Enterprise section.
    EJB: The EJB architecture defines 3 distinct types of Enterprise JavaBeans.
    􀂃 Session beans.
    􀂃 Entity beans.
    􀂃 Message-driven beans.
    The session and entity beans are invoked synchronously by the client and message driven beans are invoked
    asynchronously by a message container such as a publish/subscribe topic. Let’s look at some of the EJB
    container services in a bit more detail:
    Instance pooling
    EJB instance pooling
    EJB Se rve r
    Note :
    1 The client looks up the stub f rom the jndi and invokes the create() method on the EJBHome object.
    CarHome hom eObject = (CarHome) initialCtx.lookup(ejb/MyEjb);
    Car carObject = homeObject.create()
    2-3 The EJbHome creates an EJBObject by invoking new Instance() and assigns a bean instance f rom the pool to the
    EJBObject. Now the assigned bean instance becomes in ready state f rom the pooled state.
    4 Now the EJBObject can service c lient requests and reference is returned to the client.
    carObject .getHorsePow er();
    Finally once the client is f inshed w ith EJBObject reference the bean instance is returned back to the pool to serve other clients
    Clie nt Application
    bean instance pool
    stub 1 . create()
    2. ne wI nstanc e()
    3. assign an instance
    to EJB Object
    4 . return EJB Object reference
    to client
    The above diagram shows how the EJB instances are pooled and assigned to EJB Object and then returned to
    the pool. Let’s look at in detail for different types of EJBs.
    EJB S erver
    stateless session & en tity bean pooling
    The diagram on th e left shows that s ince the
    stateless sess ion beans and en tity beans
    do not m aintain any c lien t sta te the bean
    instance A was firstly a llocated to c lien t s tub
    1 a n d later on allocated to c lient s tub 2. So if
    there a re 10 00 concurrent c lients then 30
    instances o f bean can serve them b y taking
    tu rns.
    This behaviour is not possib le w ith regards to
    stateful ses s ion beans which maintain the
    client s tate. S o there w ill be a dedicated
    instance o f the bean for each client s tub. So
    if there are 1000 clients the n there w ill be
    1000 instances o f beans. So how do we
    conserve m emory. T h is is done by a ctivation
    and passiva tion. Passiva tion is the p rocess
    where the bea n instance is serialize d into a
    persistent s tore when not u sed to conserve
    memory an d Activation is process where the
    serilized bea n instance is de -serialized back
    into memory to serve c lient request. This
    process a ffects p e rformance.
    EJB Server
    bean instance pool
    C D
    Client s tub 1
    Client s tub 2
    Client s tub 1
    bean instance pool
    C D
    Client s tub 2 Object
    Enterprise Java
    From the diagrams it is clear that bean instances can be reused for all the bean types except for the stateful
    session bean where the client state is maintained. So we need a dedicated stateful session bean for each client.
    E JB S erver
    M DB-2 b ean instan c e pool
    fo r Q 2
    E JB
    O bject
    E JB
    O bject
    JM S C lie n t 1
    JM S C lie n t 2
    M DB-1 b ean instan c e pool
    fo r Q 1
    A B C
    JM S C lie n t 3 A
    m s g X f o r Q 1
    m s g y fo r Q 2
    m s g Z fo r Q 2
    E JB
    O bject B
    Note: M DB a re lik e s ta te le s s s es s ion b eans ,
    Th e ins tance p o o ls a re c re a te d for e a c h M DB a n d w ithin e a c h p o o l m ultiple ins ta n c e s a re c re a ted. In term s o f
    num b e r o f ins ta n c e s cre a te d in eac h p o o l a re very s im ilar to s ta tele s s s e s s ion b e a n s o r e n tity b e a n s (ie 3 instances
    of M DB -1 for Q 1 ins tanc e p o o l c a n s e rve 1 0 JM S c lients for Q 1).
    M essag e D riven B ean (M D B ) po o lin g
    Concurrent access
    The session beans do not support concurrent access. The stateful session beans are exclusively for a client so
    there is no concurrent access. The stateless session beans do not maintain any state. It does not make any sense
    to have concurrent access. The Entity beans represent data that is in the database table, which is shared between
    the clients. So to make concurrent access possible the EJB container need to protect the data while allowing many
    clients simultaneous access. When you try to share distributed objects you may have the following problem:
    If 2 clients are using the same EJBObject, how do you keep one client from writing over the changes of the other?
    Say for example
    Client-1 reads a value x= 5
    Client-2 modifies the value to x=7
    Now the client-1’s value is invalid.
    The entity bean addresses this by prohibiting concurrent access to bean instances. Which means several clients
    can be connected to one EJBObject but only one client can access the EJB instance at a time.
    Entity beans basically represent the data in a relational database. An Entity Bean is responsible for keeping its
    state in sync with the database.
    Entity beans representing data in the database
    id = 1001 (primary-key)
    bsb = 1234
    account_number = 98765432
    instance for id = 1001
    id = 1002 (primary-key)
    bsb = 1234
    account_number = 12345678
    instance for id = 1002
    id bsb account_num
    1001 1234 98765432
    1002 1234 12345678
    Account Table
    􀂃 Container-managed persistence (CMP) - The container is responsible for saving the bean’s state with the help
    of object-relational mapping tools.
    􀂃 Bean-managed persistence (BMP) – The Entity Bean is responsible for saving its own state.
    Enterprise Java
    If entity beans performance is of concern then there are other persistence technologies and frameworks like
    JDBC, JDO, Hibernate, OJB and Oracle TopLink (commercial product).
    Q 63: What are the different kinds of enterprise beans? SF
    A 63:
    Session Bean: is a non-persistent object that implements some business logic running on the server. Session
    beans do not survive system shut down. There are two types of session beans
    􀂃 Stateless session beans (each session bean can be reused by multiple EJB clients)
    􀂃 Stateful session beans (each session bean is associated with one EJB client)
    Entity Bean: is a persistent object that represents object views of the data, usually a row in a database. They
    have the primary key as a unique identifier. Multiple EJB clients can share each entity bean. Entity beans can
    survive system shut shutdowns. Entity beans can have two types of persistence
    􀂃 Container-managed persistence (CMP) - The container is responsible for saving the bean’s state.
    􀂃 Bean-managed persistence (BMP) – The Entity Bean is responsible for saving its own state.
    Message-driven Bean: is integrated with the Java Message Service (JMS) to provide the ability to act as a
    message consumer and perform asynchronous processing between the server and the message producer.
    Q 64: What is the difference between session and entity beans? SF
    A 64:
    Session Beans Entity Beans
    Use session beans for application logic. Use entity beans to develop persistent object model.
    Expect little reuse of session beans. Insist on reuse of entity beans.
    Session beans control the workflow and transactions of a
    group of entity beans.
    Domain objects with a unique identity (ie-primary key) shared
    by multiple clients.
    Life is limited to the life of a particular client. Handle
    database access for a particular client.
    Persist across multiple invocations. Handles database access
    for multiple clients.
    Do not survive system shut downs or server crashes.
    Do survive system shut downs or server crashes.
    Q 65: What is the difference between stateful and stateless session beans? SF
    A 65:
    Stateless Session Beans Stateful Session Bean
    Do not have an internal state. Can be reused by different
    Do have an internal state. Reused by the same client.
    Need not be activated or passivated since the beans are
    pooled and reused.
    Need to handle activation and passivation to conserve system
    memory since one session bean object per client.
    Q 66: What is the difference between Container Managed Persistence (CMP) and Bean Managed Persistence (BMP)?
    A 66:
    Container Managed Persistence (CMP) Bean Managed Persistence (BMP)
    The container is responsible for persisting state of the bean. The bean is itself responsible for persisting its own state.
    Container needs to generate database (SQL) calls. The bean needs to code its own database (SQL) calls.
    The bean persistence is independent of its database (e.g.
    DB2, Oracle, Sybase etc). So it is portable from one data
    source to another.
    The bean persistence is hard coded and hence may not be
    portable between different databases (e.g. DB2, Oracle etc).
    Q 67: Can an EJB client invoke a method on a bean directly? SF
    A 67: An EJB client should never access an EJB directly. Any access is done through the container. The container will
    intercept the client call and apply services like transaction, security etc prior to invoking the actual EJB. This
    relationship between the EJB and the container is like “don’t call us, we will call you”.
    Q 68: How does an EJB interact with its container and what are the call-back methods in entity beans? SF
    A 68: EJB interacts with its container through the following mechanisms
    Enterprise Java
    􀂃 Call-back Methods: Every EJB implements an interface (extends EnterpriseBean) which defines several
    methods which alert the bean to various events in its lifecycle. A container is responsible for invoking these
    methods. These methods notify the bean when it is about to be activated, to be persisted to the database, to
    end a transaction, to remove the bean from the memory, etc. For example the entity bean has the following
    call-back methods:
    public interface javax.ejb.EntityBean {
    public void setEntityContext(javax.ejb.EntityContext c);
    public void unsetEntityContext();
    public void ejbLoad();
    public void ejbStore();
    public void ejbActivate();
    public void ejbPassivate();
    public void ejbRemove();
    􀂃 EJBContext: provides methods for interacting with the container so that the bean can request information
    about its environment like the identity of the caller, security, status of a transaction, obtains remote reference
    to itself etc. e.g. isUserInRole(), getUserPrincipal(), isRollbackOnly(), etc
    􀂃 JNDI (Java Naming and Directory Interface): allows EJB to access resources like JDBC connections, JMS
    topics and queues, other EJBs etc.
    Q 69: What is the difference between EJB 1.1 and EJB 2.0? What is the difference between EJB 2.x and EJB 3.0? SF
    A 69: EJB 2.0 has the following additional advantages over the EJB 1.1
    􀂃 Local interfaces: These are beans that can be used locally, that means by the same Java Virtual Machines,
    so they do not required to be wrapped like remote beans, and arguments between those interfaces are
    passed directly by reference instead of by value. This improves performance.
    􀂃 ejbHome methods: Entity beans can declare ejbHome methods that perform operations related to the EJB
    component but that are not specific to a bean instance.
    􀂃 Message Driven Beans (MDB): is a completely new enterprise bean type, which is designed specifically to
    handle incoming JMS messages.
    􀂃 New CMP Model. It is based on a new contract called the abstract persistence schema, which will allow to
    the container to handle the persistence automatically at runtime.
    􀂃 EJB Query Language: It is a sql-based language that will allow the new persistence schema to implement
    and execute finder methods.
    Let’s look at some of the new features on EJB 2.1
    􀂃 Container-managed timer service: The timer service provides coarse-grained, transactional, time-based
    event notifications to enable enterprise beans to model and manage higher-level business processes.
    􀂃 Web service support: EJB 2.1 adds the ability of stateless session beans to implement a Web service
    endpoint via a Web service endpoint interface.
    􀂃 EJB-QL: Enhanced EJB-QL includes support for aggregate functions and ordering of results.
    Current EJB 2.x model is complex for a variety of reasons:
    􀂃 You need to create several component interfaces and implement several unnecessary call-back methods.
    􀂃 EJB deployment descriptors are complex and error prone.
    􀂃 EJB components are not truly object oriented, as they have restrictions for using inheritance and
    􀂃 EJB modules cannot be tested outside an EJB container and debugging an EJB inside a container is very
    Enterprise Java
    Note: EJB 3.0 is taking ease of development very seriously and has adjusted its model to offer the POJO (Plain Old Java
    Object) persistence and the new O/R mapping model based on Hibernate. In EJB 3.0, all kinds of enterprise beans are just
    POJOs. EJB 3.0 extensively uses Java annotations, which replaces excessive XML based configuration files and eliminate
    the need for rigid component model used in EJB 1.x, 2.x. Annotations can be used to define the bean’s business interface, O/R
    mapping information, resource references etc. Refer Q18 in Emerging Technologies/Frameworks section.
    Q 70: What are the implicit services provide by an EJB container? SF
    A 70:
    􀂃 Lifecycle Management: Individual enterprise beans do not need to explicitly manage process allocation,
    thread management, object activation, or object destruction. The EJB container automatically manages the
    object lifecycle on behalf of the enterprise bean.
    􀂃 State Management: Individual enterprise beans do not need to explicitly save or restore conversational
    object state between method calls. The EJB container automatically manages object state on behalf of the
    enterprise bean.
    􀂃 Security: Individual enterprise beans do not need to explicitly authenticate users or check authorisation
    levels. The EJB container automatically performs all security checking on behalf of the enterprise bean.
    􀂃 Transactions: Individual enterprise beans do not need to explicitly specify transaction demarcation code to
    participate in distributed transactions. The EJB container can automatically manage the start, enrolment,
    commitment, and rollback of transactions on behalf of the enterprise bean.
    􀂃 Persistence: Individual enterprise beans do not need to explicitly retrieve or store persistent object data from
    a database. The EJB container can automatically manage persistent data on behalf of the enterprise bean.
    Q 71: What are transactional attributes? SF TI
    A 71: EJB transactions are a set of mechanisms and concepts, which insures the integrity and consistency of the
    database when multiple clients try to read/update the database simultaneously.
    Transaction attributes are defined at different levels like EJB (or class), a method within a class or segment of a
    code within a method. The attributes specified for a particular method take precedence over the attributes
    specified for a particular EJB (or class). Transaction attributes are specified declaratively through EJB deployment
    descriptors. Unless there is any compelling reason, the declarative approach is recommended over programmatic
    approach where all the transactions are handled programmatically. With the declarative approach, the EJB
    container will handle the transactions.
    Required Methods executed within a transaction. If client provides a transaction, it is used. If not, a new transaction is
    generated. Commit at end of method that started the transaction. Which means a method that has Required
    attribute set, but was called when the transaction has already started will not commit at the method
    completion. Well suited for EJB session beans.
    Mandatory Client of this EJB must create a transaction in which this method operates, otherwise an error will be
    reported. Well-suited for entity beans.
    RequiresNew Methods executed within a transaction. If client provides a transaction, it is suspended. If not a new
    transaction is generated, regardless. Commit at end of method.
    Supports Transactions are optional.
    NotSupported Transactions are not supported. If provided, ignored.
    Never Code in the EJB responsible for explicit transaction control.
    Q 72: What are isolation levels? SF TI PI
    A 72: Isolation levels provide a degree of control of the effects one transaction can have on another concurrent
    transaction. Since concurrent effects are determined by the precise ways in which, a particular relational database
    handles locks and its drivers may handle these locks differently. The semantics of isolation mechanisms based on
    these are not well defined. Nevertheless, certain defined or approximate properties can be specified as follows:
    Isolation level Description
    TRANSACTION_SERIALIZABLE Strongest level of isolation. Places a range lock on the data set, preventing other
    users from updating or inserting rows into the data set until the transaction is
    complete. Can produce deadlocks.
    Enterprise Java
    TRANSACTION_REPEATABLE_READ Locks are placed on all data that is used in a query, preventing other users from
    updating the data, but new phantom records can be inserted into the data set
    by another user and are included in later reads in the current transaction.
    TRANSACTION_READ_COMMITTED Can't read uncommitted data by another transaction. Shared locks are held while
    the data is being read to avoid dirty reads, but the data can be changed before
    the end of the transaction resulting in non-repeatable reads and phantom
    TRANSACTION_READ_UNCOMMITTED Can read uncommitted data (dirty read) by another transaction, and nonrepeatable
    reads and phantom records are possible. Least restrictive of all
    isolation levels. No shared locks are issued and no exclusive locks are
    Isolation levels are not part of the EJB specification. They can only be set on the resource manager either
    explicitly on the Connection (for bean managed persistence) or via the application server specific configuration.
    The EJB specification indicates that isolation level is part of the Resource Manager.
    As the transaction isolation level increases, likely performance degradation follows, as additional locks are
    required to protect data integrity. If the underlying data does not require such a high degree of integrity, the
    isolation level can be lowered to improve performance.
    Q 73: What is a distributed transaction? What is a 2-phase commit? SF TI
    A 73: A Transaction (Refer Q43 in Enterprise section) is a series of actions performed as a single unit of work in which
    either all of the actions performed as a logical unit of work in which, either all of the actions are performed or none
    of the actions. A transaction is often described by ACID properties (Atomic, Consistent, Isolated and Durable). A
    distributed transaction is an ACID transaction between two or more independent transactional resources like
    two separate databases. For the transaction to commit successfully, all of the individual resources must commit
    successfully. If any of them are unsuccessful, the transaction must rollback in all of the resources. A 2-phase
    commit is an approach for committing a distributed transaction in 2 phases.
    Phase 1 is prepare: Each of the resources votes on whether it’s ready to commit – usually by going ahead and
    persisting the new data but not yet deleting the old data.
    Phase 2 is committing: If all the resources are ready, they all commit – after which old data is deleted and
    transaction can no longer roll back. 2-phase commit ensures that a distributed transaction can always be
    committed or always rolled back if one of the databases crashes. The XA specification defines how an application
    program uses a transaction manager to coordinate distributed transactions across multiple resource managers.
    Any resource manager that adheres to XA specification can participate in a transaction coordinated by an XAcompliant
    transaction manager.
    Q 74: What is dooming a transaction? TI
    A 74: A transaction can be doomed by the following method call CO
    The above call will force transaction to rollback. The doomed transactions decrease scalability and if a transaction
    is doomed why perform compute intensive operations? So we can detect a doomed transaction as shown below:
    public void doComputeIntensiveOperation() throws Exception {
    If ( ejbContext.getRollbackOnly() ) {
    return; //transaction is doomed so return (why unnecessarily perform compute intensive operation )
    else {
    Q 75: How to design transactional conversations with session beans? SF TI
    A 75: A stateful session bean is a resource which has an in memory state which can be rolled back in case of any
    failure. It can participate in transactions by implementing SessionSynchronization. CO
    Enterprise Java
    The uses of SessionSynchronization are:
    􀂃 Enables the bean to act as a transactional resource and undo state changes on failure.
    􀂃 Enables you to cache database data to improve performance.
    Q 76: Explain exception handling in EJB? SF EH CO
    A 76: Java has two types of exceptions:
    􀂃 Checked exception: derived from java.lang.Exception but not java.lang.RuntimeException.
    􀂃 Unchecked exception: derived from java.lang.RuntimeException thrown by JVM.
    public void depositAmount() throws InsufficientFundException {
    if(this.amount <= 0) {
    throw new InsufficientFundException ("Balance is <= 0");
    try {
    } catch (SQLException e) {
    throw new EJBException(e);
    } catch (Exception e) {
    throw new EJBException(e);
    Application Exception
    System Exception
    System vs Application Exception
    EJB has two types of exceptions:
    􀂃 System Exception: is an unchecked exception derived from java.lang.RuntimeException.
    􀂃 Application Exception: is specific to an application and thrown because of violation of business rules.
    A System Exception is thrown by the system and is not recoverable. For example EJB container losing
    connection to the database server, failed remote method objects call etc. Because the System Exceptions are
    unpredictable, the EJB container is the only one responsible for trapping the System Exceptions. The container
    automatically wraps any RuntimeException in RemoteException, which subsequently gets thrown to the caller (or
    client). In addition to intercepting System Exception the container may log the errors.
    An Application Exception is specific to an application and is thrown because of violation of business rules. The
    client should be able to determine how to handle an Application Exception. If the account balance is zero then an
    Application Exception like InsufficientFundException can be thrown. If an Application Exception should be
    treated as a System Exception (e.g. SQLException) it needs to be wrapped in an EJBException so that it can be
    managed properly and propagated to the client.
    Q 77: How do you rollback a container managed transaction in EJB? SF TI EH
    public class MyBean implements SessionBean, SessionSynchronization{
    public int oldVal ; public int val ;
    public void ejbCreate(int val) throws CreateException {
    public void afterBegin() { this.oldVal = this.val ;}
    public void beforeCompletion(){};
    public void afterCompletion(boolean b) { if (b == false) this.val = this.oldVal ; }
    public interface javax.ejb.SessionSynchronization {
    public void afterBegin();
    public void beforeCompletion();
    public void afterCompletion(boolean b);
    Enterprise Java
    A 77: The way the exceptions are handled affects the way the transactions are managed. CO
    When the container manages the transaction, it is automatically rolled back when a System Exception occurs.
    This is possible because the container can intercept System Exception. However when an Application Exception
    occurs, the container does not intercept it and therefore leaves it to the code to roll back using
    Be aware that handling exceptions in EJB is different from handling exceptions in Java. The Exception handling
    best practice tips are:
    􀂃 If you cannot recover from System Exception let the container handle it.
    􀂃 If a business rule is violated then throw an application exception.
    􀂃 Catch the Exceptions in a proper order.
    􀂃 It is a poor practice to catch java.lang.Exception because this is a big basket, which will catch all the
    unhandled exceptions. It is shown in the above diagrams for illustration purpose only. You should avoid this
    because if you add a new piece of code, which throws a new, checked exception, then the compiler won’t pick
    it up.
    Q 78: What is the difference between optimistic and pessimistic concurrency control? TI
    A 78:
    Pessimistic Concurrency Optimistic Concurrency
    A pessimistic design assumes conflicts will occur in the
    database tables and avoids them through exclusive
    locks etc.
    An optimistic approach assumes conflicts won’t occur, and deal with
    them when they do occur.
    EJB (also non-EJB) locks the source data until it
    completes its transaction.
    􀂃 Provides reliable access to data.
    􀂃 Suitable for short transactions.
    􀂃 Suitable for systems where concurrent access is
    EJB (also non-EJB) implements a strategy to detect whether a
    change has occurred. Locks are placed on the database only for a
    small portion of the time.
    􀂃 Suitable for long transactions.
    􀂃 Suitable for systems requiring frequent concurrent accesses.
    The pessimistic locking imposes high locking
    overheads on the server and lower concurrency.
    The optimistic locking is used in the context of cursors. The
    optimistic locking works as follows:
    􀂃 No locks are acquired as rows are read.
    􀂃 No locks are acquired while values in the current row are
    􀂃 When changes are saved, a copy of the row in the database is
    read in the locked mode.
    􀂃 If the data was changed after it was read into the cursor, an error
    is raised so that the transaction can be rolled back and retried.
    Note: The testing for changes can be done by comparing the
    values, timestamp or version numbers.
    Q 79: How can we determine if the data is stale (for example when using optimistic locking)? TI
    A 79: We can use the following strategy to determine if the data is stale:
    􀂃 Adding version numbers
    Rolling back Container Managed Transactions
    public void depositAmount() throws InsufficientFundExceptiion {
    try {
    }catch (InsufficientFundException e)
    throw new InsufficientFundExceptiion(e.getMessage());
    } catch (SQLException e) {
    throw new EJBException(e);
    } catch (Exception e) {
    throw new EJBException(e);
    Application Exception is thrown so
    the transaction should be rolled back
    in the code ctx.setRollbackOnly().
    EJBException is a System
    Exception so the container will
    automatically roll back the
    Enterprise Java
    1. Add a version number (Integer) to the underlying table.
    2. Carry the version number along with any data read into memory (through value object, entity bean etc).
    3. Before performing any update compare the current version number with the database version number.
    4. If the version numbers are equal update the data and increment the version number.
    5. If the value object or entity bean is carrying an older version number, reject the update and throw an
    Note: You can also do the version number check as part of the update by including the version column in the
    where clause of the update without doing a prior select.
    􀂃 Adding a timestamp to the underlying database table.
    􀂃 Comparing the data values.
    These techniques are also quite useful when implementing data caching to improve performance. Data caches
    should regularly keep track of stale data to refresh the cache. These strategies are valid whether you use EJB or
    other persistence mechanisms like JDBC, Hibernate etc.
    Q 80: What are not allowed within the EJB container? SF
    A 80: In order to develop reliable and portable EJB components, the following restrictions apply to EJB code
    􀂃 Avoid using static non-final fields. Declaring all static fields in EJB component as final is recommended. This
    enables the EJB container to distribute instances across multiple JVMs.
    􀂃 Avoid starting a new thread (conflicts with EJB container) or using thread synchronization (allow the EJB
    container to distribute instances across multiple JVMs).
    􀂃 Avoid using AWT or Swing functionality. EJBs are server side business components.
    􀂃 Avoid using file access/ operations. EJB business components are meant to use resource managers
    such as JDBC to store and retrieve application data. Also deployment descriptors can be used to store .
    􀂃 Avoid accepting or listening to socket connections. EJB components are not meant to provide network socket
    functionality. However the specification lets EJB components act as socket clients or RMI clients.
    􀂃 Avoid using the reflection API. This restriction enforces Java security.
    􀂃 Can’t use custom class loaders.
    Q 81: Discuss EJB container security? SF SE
    A 81: EJB components operate inside a container environment and rely heavily on the container to provide security. The
    four key services required for the security are:
    􀂃 Identification: In Java security APIs this identifier is known as a principal.
    􀂃 Authentication: To prove the identity one must present the credentials in the form of password, swipe card,
    digital certificate, finger prints etc.
    􀂃 Authorisation (Access Control): Every secure system should limit access to particular users. The common
    way to enforce access control is by maintaining security roles and privileges.
    􀂃 Data Confidentiality: This is maintained by encryption of some sort. It is no good to protect your data by
    authentication if someone can read the password.
    The EJB specification concerns itself exclusively with authorisation (access control). An application using EJB
    can specify in an abstract (declarative) and portable way that is allowed to access business methods. The EJB
    container handles the following actions:
    􀂃 Find out the Identity of the caller of a business method.
    Enterprise Java
    􀂃 Check the EJB deployment descriptor to see if the identity is a member of a security role that has been
    granted the right to call this business method.
    􀂃 Throw java.rmi.RemoteException if the access is illegal.
    􀂃 Make the identity and the security role information available for a fine grained programmatic security check.
    public void closeAccount() {
    if (ejbContext.getCallerPrincipal().getName() = “SMITH”) {
    if (!ejbContext .isCallerInRole(CORPORATE_ACCOUNT_MANAGER)) {
    throw new SecurityException(“Not authorized to close this account”);
    􀂃 Optionally log any illegal access.
    There are two types of information the EJB developer has to provide through the deployment descriptor.
    􀂃 Security roles
    􀂃 Method permissions

    Allowed to open and close accounts


    Allowed to read only


    There is a many-to-many relationship between the security roles and the method permissions.



    Just as we must declare the resources accessed in our code for other EJBs that we reference in our code we
    should also declare the security role we access programmatically to have a fine grained control as shown below.

    Allowed to open and close accounts


    There is also many-to-many relationship between the EJB specific security roles that are in the deployment
    descriptor and the application based target security system like LDAP etc. For example there might be more than
    one group users and individual users that need to be mapped to a particular EJB security role ‘account_manager’.
    Q 82: What are EJB best practices? BP
    A 82:
    􀂃 Use local interfaces that are available in EJB2.0 if you deploy both the EJB client and the EJB in the same
    server. Use vendor specific pass-by-reference implementation to make EJB1.1 remote EJBs operate as local.
    Enterprise Java
    [Extreme care should be taken not to affect the functionality by switching the application, which was written
    and tested in pass-by-reference mode to pass-by-value without analysing the implications and re-testing the
    􀂃 Wrap entity beans with session beans to reduce network calls (refer Q84 in Enterprise section) and promote
    declarative transactions. Where possible use local entity beans and session beans can be either local or
    remote. Apply the appropriate EJB design patterns as described in Q83 – Q87 in Enterprise section.
    􀂃 Cache ejbHome references to avoid JNDI look-up overhead using service locator pattern.
    􀂃 Handle exceptions appropriately (refer Q76, Q77 in Enterprise section).
    􀂃 Avoid transaction overhead for non-transactional methods of session beans by declaring transactional
    attribute as ‘Supports’.
    􀂃 Choose plain Java object over EJB if you do not want services like RMI/IIOP, transactions, security,
    persistence, thread safety etc. There are alternative frameworks such as Hibernate, Spring etc.
    􀂃 Choose Servlet’s HttpSession object rather than stateful session bean to maintain client state if you do not
    require component architecture of a stateful bean.
    􀂃 Apply Lazy loading and Dirty marker strategies as described in Q88 in Enterprise section.
    Session Bean
    Session Bean (stateful) Entity Bean
    􀂃 Tune the pool size to
    avoid overhead of
    creation and destruction.
    􀂃 Use setSessionContext(..)
    or ejbCreate(..) method to
    cache any bean specific
    􀂃 Release any acquired
    resources like Database
    connection etc in
    ejbRemove() method
    􀂃 Tune the pool size to avoid
    overhead of creation and
    􀂃 Set proper time out to avoid
    resource congestion.
    􀂃 Remove it explicitly from
    client using remove()
    􀂃 Use ‘transient’ variable
    where possible to avoid
    serialization overhead.
    􀂃 Tune the pool size to avoid overhead of creation and
    􀂃 Use setEntityContext(..) method to cache any bean
    specific resources and unsetEntityContext() method to
    release acquired resources.
    􀂃 Use lazy-loading to avoid any unnecessary loading of
    dependent data. Use dirty marker to avoid unchanged
    data update.
    􀂃 Commit the data after a transaction completes to reduce
    any database calls in between.
    􀂃 Where possible perform bulk updates, use CMP rather
    than BMP, Use direct JDBC (Fast-lane-reader) instead
    of entity beans, use of read-only entity beans etc.
    Q 83: What is a business delegate? Why should you use a business delegate? DP PI
    A 83: Questions Q83 – Q88 are very popular EJB questions.
    Problem: When presentation tier components interact directly with the business services components like EJB,
    the presentation components are vulnerable to changes in the implementation of business services components.
    Solution: Use a Business Delegate to reduce the coupling between the presentation tier components and the
    business services tier components. Business Delegate hides the underlying implementation details of the business
    service, such as look-up and access details of the EJB architecture.
    Business delegate is responsible for:
    􀂃 Invoking session beans in Session Facade.
    􀂃 Acting as a service locator and cache home stubs to improve performance.
    􀂃 Handling exceptions from the server side. (Unchecked exceptions get wrapped into the remote exception,
    checked exceptions can be thrown as an application exception or wrapped in the remote exception.
    unchecked exceptions do not have to be caught but can be caught and should not be used in the method
    􀂃 Re-trying services for the client (For example when using optimistic locking business delegate will retry the
    method call when there is a concurrent access.).
    Enterprise Java
    B u s in e s s D e le g a te
    C lie n t B u s in e s s D e le g a te
    E J B L o o k u p S e rv ic e
    B u s in e s s S e rv ic e E J B
    3 . lo o k u p /c re a te
    1 .u s e s
    2 .u s e s
    4 .u s e s
    Q 84: What is a session façade? DP PI
    A 84: Problem: Too many method invocations between the client and the server will lead to network overhead, tight
    coupling due to dependencies between the client and the server, misuse of server business methods due to fine
    grained access etc.
    Solution: Use a session bean as a façade to encapsulate the complexities between the client and the server
    interactions. The Session Facade manages the business objects, and provides a uniform coarse-grained service
    access layer to clients.
    Session façade is responsible for
    􀂃 Improving performance by minimising fine-grained method calls over the network.
    􀂃 Improving manageability by reducing coupling, exposing uniform interface and exposing fewer methods to
    􀂃 Managing transaction and security in a centralised manner.
    Q 85: What is a value object pattern? DP PI
    A 85: Problem: When a client makes a remote call to the server, there will be a process of network call and serialization
    of data involved for the remote invocation. If you make fine grained calls there will be performance degradation.
    Solution: Avoid fine-grained method calls by creating a value object, which will help the client, make a coarsegrained
    V alue O bje c t p attern
    W ith o u t V alue O bject
    W ith o u t valu e o b je c t 4 rem ote ca lls a re
    m a d e to g e t a ll th e rele va n t info
    S ervlet
    (c lie n t)
    Sess io n B ean
    g e tF irstN a m e( )
    g e tS urnam e ()
    ge tG e nd e r()
    ge tAg e()
    W ith V alue O bje c t
    W ith valu e o b je c t 1 rem ote call a n d 4 lo cal
    c a lls a re m a d e to g e t a ll th e releva n t info.
    S e s s io n
    B ean
    g e tP e r s o n Info ( )
    P erson
    V alue
    O bject
    g e tS urn a m e()
    getF irstName ()
    Session Facade
    Without Session Facade With Session Facade
    Entitity Bean 1
    Entitity Bean 2
    Entitity Bean 3
    remote call 1
    remote call 2
    remote call 3
    Entitity Bean 1
    Entitity Bean 2
    Entitity Bean 3
    remote call 1
    Session Bean
    local call 1
    local call 2
    local call 3
    Enterprise Java
    Q 86: What is a fast-lane reader? DP PI
    A 86: Problem: Using Entity beans to represent persistent, read only tabular data incurs performance cost at no benefit
    (especially when large amount of data to be read).
    Solution: Access the persistent data directly from the database using the DAO (Data Access Object) pattern
    instead of using Entity beans. The Fast lane readers commonly use JDBC, Connectors etc to access the read-only
    data from the data source. The main benefit of this pattern is the faster data retrieval.
    Q 87: What is a Service Locator? DP PI
    A 87: Problem: J2EE makes use of the JNDI interface to access different resources like JDBC, JMS, EJB etc. The client
    looks up for these resources through the JNDI look-up. The JNDI look-up is expensive because the client needs to
    get a network connection to the server first. So this look-up process is expensive and redundant.
    Solution: To avoid this expensive and redundant process, service objects can be cached when a client performs
    the JNDI look-up for the first time and reuse that service object from the cache for the subsequent look-ups. The
    service locator pattern implements this technique. Refer to diagram below:
    Q 88: Explain lazy loading and dirty marker strategies? DP PI
    A 88: Lazy Loading: Lazy loading means not creating an object until the first time it is accessed. This technique is
    useful when you have large hierarchies of objects. You can lazy load some of the dependent objects. You only
    create the dependent (subordinate) objects only when you need them.
    If ( = null) {
    //lazy load data
    For a CMP bean the default scenario is set to no lazy loading and the finder method will execute a single SQL
    select statement against the database. So, for example, with the findAllCustomers() method will retrieve all
    customer objects with all the CMP fields in each customer object.
    Servic e Loc a tor
    W ithout S ervic e Loc a tor
    (clie n t -1)
    (clie n t - 2 )
    (clie n t - 3 )
    lo o k u p
    loo kup
    W ithout service locator loo k up e ve ry tim e
    from th e JN D I
    W ith S ervic e Loc a tor
    (c lie n t -1 )
    (clien t - 2 )
    (clien t - 3 )
    Loc a tor
    lo ok up
    loo k u p
    lo oku p firs t tim e o nly
    W ith s ervic e lo ca to r loo k up firs t tim e from
    the JNDI a n d secon d tim e o nwards lookup
    from the cach e in the service locator.
    Fast Lane Reader
    J2EE Server
    EJB Container
    Web Container
    Data Access Object
    Session Bean Entity Bean
    (client) normal lane
    normal lane
    Fast Lane Reader
    Fast Lane
    Use Fast Lane Reader for read only access and the normal lane for read/write access to the DataSource.
    Enterprise Java
    If you turn on lazy loading then only the primary keys of the objects within the finder are returned. Only when you
    access the object, the container uploads the actual object based on the primary key. You may want to turn on the
    lazy loading feature if the number of objects that you are retrieving is so large that loading them all into local cache
    would adversely affect the performance. (Note: The implementation of lazy loading strategy may vary from
    container vendor to vendor).
    Dirty Marker (Store optimisation): This strategy allows us to persist only the entity beans that have been
    modified. The dependent objects need not be persisted if they have not been modified. This is achieved by using a
    dirty flag to mark an object whose contents have been modified. The container will check every dependent object
    and will persist only those objects that are dirty. Once it is persisted its dirty flag will be cleared. (Note: The
    implementation of dirty marker strategy may vary from container vendor to vendor).
    Note: If your job requires a very good understanding of EJB 2.x then following books are recommended:
    􀂃 Mastering Enterprise JavaBeans – by Ed Roman
    􀂃 EJB Design Patterns – by Floyd Marinescu
    Enterprise - JMS
    Q 89: What is Message Oriented Middleware? What is JMS? SF
    A 89: Message Oriented Middleware (MOM) is generally defined as a software infrastructure that asynchronously
    communicates with other disparate systems through the production and consumption of messages. A message
    may be a request, a report, or an event sent from one part of an enterprise application to another.
    Messaging enables loosely coupled distributed communication. A component sends a message to a destination,
    and the recipient can retrieve the message from the destination. However, the sender and the receiver do not
    have to be available at the same time in order to communicate and also they are not aware of each other. In fact,
    the sender does not need to know anything about the receiver; nor does the receiver need to know anything about
    the sender. The sender and the receiver need to know only what message format and what destination to use. In
    this respect, messaging differs from tightly coupled technologies, such as Remote Method Invocation (RMI), which
    requires an application to know a remote application's methods.
    Message Oriented Middleware (MOM) systems like MQSeries, MQSonic, etc are proprietary systems. Java
    Message Service (JMS) is a Java API that allows applications to create, send, receive, and read messages in a
    standard way. Designed by Sun and several partner companies, the JMS API defines a common set of interfaces
    and associated semantics that allow programs written in the Java programming language to communicate with
    other messaging implementations (e.g. MQSonic, TIBCO etc). The JMS API minimises the set of concepts a
    programmer must learn to use messaging products but provides enough features to support sophisticated
    messaging applications. It also strives to maximise the portability of JMS applications across JMS providers.
    Companies have spent decades developing their legacy systems. Rather than throwing these systems out, XML
    can be used in a non-proprietary way to move data from legacy systems to distributed systems like J2EE over the
    wire-using MOM and JMS.
    How JMS is different from RPC?
    Remote Procedure Call (e.g. RMI) JMS
    Remote Procedure Call (RPC) technologies like RMI
    attempt to mimic the behaviour of system that runs in one
    process. When a remote procedure is invoked the caller is
    blocked until the procedure completes and returns control
    to the caller. This is a synchronized model where process
    is performed sequentially ensuring that tasks are
    completed in a predefined order. The synchronized nature
    of RPC tightly couples the client (the software making the
    call) to the server (the software servicing the call). The
    client can not proceed (its blocked) until the server
    responds. The tightly coupled nature of RPC creates
    highly interdependent systems where a failure on one
    system has an immediate impact on other systems.
    With the use of Message Oriented Middleware (MOM), problems
    with the availability of subsystems are less of an issue. A
    fundamental concept of MOM is that communications between
    components is intended to be asynchronous in nature. Code that
    is written to connect the pieces together assumes that there is a
    one-way message that requires no immediate response. In other
    words, there is no blocking. Once a message is sent the sender
    can move on to other tasks; it doesn't have to wait for a
    response. This is the major difference between RPC and
    asynchronous messaging and is critical to understanding the
    advantages offered by MOM systems.
    In an asynchronous messaging system each subsystem
    (Customer, Account etc) is decoupled from the other systems.
    They communicate through the messaging server, so that a
    failure in one does not impact the operation of the others.
    Enterprise Java
    Client is blocked while it is being processed. Asynchronous messages also allows for parallel processing i.e.
    client can continue processing while the previous request is
    being satisfied.
    Are messaging applications slow? While there is some overhead in all messaging systems, but this does not
    mean that the applications that are using messaging are necessarily slow. Messaging systems can achieve a
    throughput of 100 messages per second depending on the installation, messaging modes (synchronous versus
    asynchronous, persistent versus non-persistent), and acknowledgment options such as auto mode, duplicates
    okay mode, and client mode etc. The asynchronous mode can significantly boost performance by multi-tasking.
    For example: In an Internet based shopping cart application, while a customer is adding items to his/her shopping
    cart, your application can trigger an inventory checking component, and a customer data retrieval component to
    execute concurrently.
    Are messaging applications reliable? This is basically a trade-off between performance and reliability. If
    reliability is more important then the:
    􀂃 Acknowledgment option should be set to automode where once only delivery is guaranteed
    􀂃 Message delivery mode should be set to persistent where the MOM writes the messages to a secure
    storage like a database or a file system to insure that the message is not lost in transit due to a system
    What are some of the key message characteristics defined in a message header?
    Characteristic Explanation
    JMSCorrelationID Used in request/response situations where a JMS client can use the JMSCorrelationID header to
    associate one message with another. For example: a client request can be matched with a response
    from a server based on the JMSCorrelationID.
    JMSMessageID Uniquely identifies a message in the MOM environment.
    JMSDeliveryMode This header field contains the delivery modes: PERSISTENT or NON_PERSISTENT.
    JMSExpiration This contains the time-to-live value for a message. If it is set to zero, then a message will never expire.
    JMSPriority Sets the message priority but the actual meaning of prioritization is MOM vendor dependent.
    What are the different body types (aka payload types) supported for messages? All JMS messages are
    read-only once posted to a queue or a topic.
    􀂃 Text message: body consists of java.lang.String.
    􀂃 Map message: body consists of key-value pairs.
    􀂃 Stream message: body consists of streams of Java primitive values, which are accessed sequentially.
    XML documents make use of this type.
    􀂃 Object message: body consists of a Serializable Java object.
    􀂃 Byte message: body consists of arbitrary stream of bytes.
    What is a message broker?
    A message broker acts as a server in a MOM. A message broker performs the following operations on a message
    it receives:
    􀂃 Processes message header information.
    􀂃 Performs security checks and encryption/decryption of a received message.
    􀂃 Handles errors and exceptions.
    􀂃 Routes message header and the payload (aka message body).
    􀂃 Invokes a method with the payload contained in the incoming message (e.g. calling onMessage(..) method
    on a Message Driven Bean (MDB)).
    􀂃 Transforms the message to some other format. For example XML payload can be converted to other
    formats like HTML etc with XSLT.
    Q 90: What type of messaging is provided by JMS? SF
    A 90: Point-to-Point: provides a traditional queue based mechanism where the client application sends a message
    through a queue to typically one receiving client that receives messages sequentially. A JMS message queue is
    an administered object that represents the message destination for the sender and the message source for the
    Enterprise Java
    Publish/Subscribe: is a one-to-many publishing model where client applications publish messages to topics,
    which are in turn subscribed by other interested clients. All subscribed clients will receive each message.
    Q 91: Discuss some of the design decisions you need to make regarding your message delivery? SF DC
    A 91:
    During your design phase, you should carefully consider various options or modes like message acknowledgment
    modes, transaction modes and delivery modes. For example: for a simple approach you would not be using
    transactions and instead you would be using acknowledgment modes. If you need reliability then the delivery
    mode should be set to persistent. This can adversely affect performance but reliability is increased.
    ent options or
    Acknowledgement mode and transaction modes are used to determine if a message will be lost or
    re-delivered on failure during message processing by the target application. Acknowledgment
    modes are set when creating a JMS session.
    InitialContext ic = new InitialContext(…);
    QueueConnectionFactory qcf = (QueueConnectionFactory)ic.lookup(“AccountConnectionFactory”);
    QueueConnection qc = qcf.createQueueConnection();
    QueueSession session = qc.createQueueSession(false, Session.AUTO_ACKNOWLEDGE);
    the above code sample, the transaction mode is set to false and acknowledgment mode is set to auto
    mode. Let us look at acknowledgment modes:
    AUTO_ACKNOWLEDGE: The messages sent or received from the session are automatically
    acknowledged. This mode also guarantees once only delivery. If a failure occurs while executing
    onMessage() method of the destination MDB, then the message is re-delivered. A message is
    automatically acknowledged when it successfully returns from the onMessage(…) method.
    DUPS_OK_ACKNOWLEDGE: This is just like AUTO_ACKNOWLEDGE mode, but under rare
    circumstances like during failure recovery messages might be delivered more than once. If a failure occurs
    then the message is re-delivered. This mode has fewer overheads than AUTO_ACKNOWLEDGE mode.
    CLIENT_ACKNOWLEDGE: The messages sent or received from sessions are not automatically
    acknowledged. The destination application must acknowledge the message receipt. This mode gives an
    application full control over message acknowledgment at the cost of increased complexity. This can be
    acknowledged by invoking the acknowledge() method on javax.jms.Message class.
    In JMS, a transaction organizes a message or a group of messages into an atomic processing unit. So, if a
    message delivery is failed, then the failed message may be re-delivered. Calling the commit() method
    commits all the messages the session receives and calling the rollback method rejects all the messages.
    InitialContext ic = new InitialContext(…);
    QueueConnectionFactory qcf = (QueueConnectionFactory)ic.lookup(“AccountConnectionFactory”);
    QueueConnection qc = qcf.createQueueConnection();
    QueueSession session = qc.createQueueSession(true, -1);
    In the above code sample, the transaction mode is set to true and acknowledgment mode is set to -1,
    which means acknowledgment mode has no use in this mode. Let us look at transaction modes:
    Message Driven Bean (MDB) with container managed transaction demarcation: An MDB participates
    in a container transaction by specifying the transaction attributes in its deployment descriptor. A transaction
    automatically starts when the JMS provider removes the message from the destination and delivers it to
    the MDB’s onMessage(…) method. Transaction is committed on successful completion of the onMessage()
    method. A MDB can notify the container that a transaction should be rolled back by setting the
    MessageDrivenContext to setRollBackOnly(). When a transaction is rolled back, the message is redelivered.
    public void onMessage(Message aMessage) {

    If(someCondtionIsTrue) {
    //everything is good. Transaction will be committed automatically on completion of onMessage(..)
    Message Driven Bean (MDB) with bean managed transaction demarcation: A MDB chooses not to
    Enterprise Java
    participate in a container managed transaction and the MDB programmer has to design and code
    programmatic transactions. This is achieved by creating a UserTransaction object from the MDB’s
    MessageDrivenContext as shown below and then invoking the commit() and rollback() methods on this
    UserTransaction object.
    public void onMessage(Message aMessage) {
    UserTransaction uT = mdbContext.getUserTransaction();
    If(someCondtionIsTrue) {
    Transacted session: An application completely controls the message delivery by either committing or
    rolling back the session. An application indicates successful message processing by invoking Session
    class’s commit() method. Also it can reject a message by invoking Session class’s rollback() method. This
    committing or rollback is applicable to all the messages received by the session.
    public void process(Message aMessage, QueueSession qs) {
    If(someCondtionIsTrue) {

    What happens to rolled-back messages?
    Rolled back messages are re-delivered based on the re-delivery count parameter set on the JMS
    provider. The re-delivery count parameter is very important because some messages can never be
    successful and this can eventually crash the system. When a message reaches its re-delivery count, the
    JMS provider can either log the message or forward the message to an error destination. Usually it is not
    advisable to retry delivering the message soon after it has been rolled-back because the target application
    might still not be ready. So we can specify a time to re-deliver parameter to delay the re-delivery process
    by certain amount of time. This time delay allows the JMS provider and the target application to recover to
    a stable operational condition.
    Care should be taken not to make use of a single transaction when using the JMS request/response
    paradigm where a JMS message is sent, followed by the synchronous receipt of a reply to that message.
    This is because a JMS message is not delivered to its destination until the transaction commits, and the
    receipt of the reply will never take place within the same transaction.
    Note: when you perform a JNDI lookup for administered objects like connection factories, topics and/or
    queues, you should use the logical reference java:comp/env/jms as the environment subcontext. It is also
    vital to release the JMS resources like connection factories, sessions, queues, topics etc when they are no
    longer required in a try{} and finally{} block.
    What happens, when the messages are with the JMS provider (i.e. MOM) and a catastrophic failure occurs
    prior to delivering the messages to the destination application? The messages will be lost if they are nondurable.
    The message’s state whether they are lost or not does not depend on acknowledgment modes
    or transaction modes discussed above. It depends on the delivery mode, which defines whether the
    message can be durable (aka persistent) or non-durable (aka non-persistent). If you choose the durable
    delivery mode then the message is stored into a database or a file system by the JMS server before
    delivering it to the consumer. Durable messages have an adverse effect on performance, but ensure that
    message delivery is guaranteed.
    InitialContext ic = new InitialContext(…);
    QueueConnectionFactory qcf = (QueueConnectionFactory)ic.lookup(“AccountConnectionFactory”);
    QueueConnection qc = qcf.createQueueConnection();
    QueueSession session = qc.createQueueSession(false, Session.AUTO_ACKNOWLEDGE);
    //senderQueue is an object of type javax.jms.Queue
    QueueSender sender = session.createSender(senderQueue);
    Sender.send(message, intDeliveryMode, intPriority, longTimeToLove );
    Best practices 􀂃 A JMS connection represents a TCP/IP connection from the client to the JMS server. A connection is
    Enterprise Java
    to improve
    a valuable resource, which should be opened at the appropriate time, should be used concurrently by
    creating and using pool of sessions, and close the connection in a finally{} block when finished.
    􀂃 Optimize your JMS sessions with the appropriate acknowledgment mode and transaction mode as
    discussed above and close your sessions when you are finished with them.
    􀂃 Choose your message type (text message, byte message, stream message etc) carefully because the
    size of a message depends on its type and size can affect performance. For example byte messages
    takes less space than text messages and for object messages you can reduce the serialization cost by
    marking some of the variables which need not be sent over the network as transient.
    􀂃 Optimize your destinations like queues and topics as follows:
    􀂃 Choose a non-durable (aka non-persistent) delivery mode where appropriate.
    􀂃 Set time to live parameter appropriately after which the message expires.
    􀂃 Where applicable receive messages asynchronously (non-blocking call). If you want to receive
    messages synchronously you can use one of the following methods on the message consumer:
    receive(); 􀃆 blocks the call until it receives the next message.
    receive(long timeout); 􀃆 blocks till a timeout occurs.
    receiveNoWait(); 􀃆 never blocks.
    Q 92: Give an example of a J2EE application using Message Driven Bean with JMS? SF
    A 92:
    Enterprise - XML
    What is XML? And why is XML important? XML stands for eXtensible Markup Language. XML is a grammatical
    system for constructing custom markup languages for describing business data, mathematical data, chemical data etc.
    XML loosely couples disparate applications or systems utilizing JMS, Web services etc. XML uses the same
    building blocks that HTML does: elements, attributes and values. Let’s look at why XML is important.
    Scalable: Since XML is not in a binary format you can create and edit files with anything and it’s also easy to debug. XML
    can be used to efficiently store small amounts of data like configuration files (web.xml, application.xml, struts-config.xml
    etc) to large company wide data with the help of XML stored in the database.
    Fast Access: XML documents benefit from their hierarchical structure. Hierarchical structures are generally faster to
    access because you can drill down to the section you are interested in.
    Easy to identify and use: XML not only displays the data but also tells you what kind of data you have. The mark up
    tags identifies and groups the information so that different information can be identified by different application.
    Messgage Driven Beans
    J2EE Server
    EJB Container
    Web Container
    Servlet Session Bean
    EJB Container
    (to the subscriber)
    The MessageDrivenBean is the consumer
    of the message.
    Enterprise Java
    Stylability: XML is style-free and whenever different styles of output are required the same XML can be used with
    different style-sheets (XSL) to produce output in XHTML, PDF, TEXT, another XML format etc.
    Linkability, in-line useability, universally accepted standard with free/inexpensive tools etc
    Q 93: What is the difference between a SAX parser and a DOM parser? SF PI MI
    A 93:
    SAX parser DOM parser
    A SAX (Simple API for XML) parser does not create any
    internal structure. Instead, it takes the occurrences of
    components of an input document as events (i.e., event
    driven), and tells the client what it reads as it reads through
    the input document.
    A DOM (Document Object Model) parser creates a tree
    structure in memory from an input document and then waits
    for requests from client.
    A SAX parser serves the client application always only with
    pieces of the document at any given time.
    A DOM parser always serves the client application with the
    entire document no matter how much is actually needed by the
    A SAX parser, however, is much more space efficient in
    case of a big input document (because it creates no internal
    structure). What's more, it runs faster and is easier to learn
    than DOM parser because its API is really simple. But from
    the functionality point of view, it provides a fewer functions,
    which means that the users themselves have to take care of
    more, such as creating their own data structures.
    A DOM parser is rich in functionality. It creates a DOM tree in
    memory and allows you to access any part of the document
    repeatedly and allows you to modify the DOM tree. But it is
    space inefficient when the document is huge, and it takes a
    little bit longer to learn how to work with it.
    Use SAX parser when
    􀂃 Input document is too big for available memory.
    􀂃 When only a part of the document is to be read and we
    create the data structures of our own.
    􀂃 If you use SAX, you are using much less memory and
    performing much less dynamic memory allocation.
    Use DOM when
    􀂃 Your application has to access various parts of the
    document and using your own structure is just as
    complicated as the DOM tree.
    􀂃 Your application has to change the tree very frequently
    and data has to be stored for a significant amount of time.
    SAX Parser example: Xerces, Crimson etc
    Use JAXP (Java API for XML Parsing) which enables
    applications to parse and transform XML documents
    independent of the particular XML parser. Code can be
    developed with one SAX parser in mind and later on can be
    changed to another SAX parser without changing the
    application code.
    DOM Parser example: XercesDOM, SunDOM, OracleDOM
    Use JAXP (Java API for XML Parsing) which enables
    applications to parse and transform XML documents
    independent of the particular XML parser. Code can be
    developed with one DOM parser in mind and later on can be
    changed to another DOM parser without changing the
    application code.
    Q 94: Which is better to store data as elements or as attributes? SF
    A 94: A question arising in the mind of XML/DTD designers is whether to model and encode certain information using an
    element, or alternatively, using an attribute. The answer to the above question is not clear-cut. But the general
    guideline is:
    􀂃 Using an element: Lord of the Rings...: If you consider the information in
    question to be part of the essential material that is being expressed or communicated in the XML, put it in an
    􀂃 Using an attribute: : If you consider the information to be peripheral or
    incidental to the main communication, or purely intended to help applications process the main
    communication, use attributes.
    The principle is data goes in elements and metadata goes in attributes. Elements are also useful when they
    contain special characters like “<”, “>”, etc which are harder to use in attributes.
    Q 95: What is XPATH? What is XSLT/XSL/XSL-FO/XSD/DTD etc? What is JAXB? What is JAXP? SF
    A 95:
    Enterprise Java
    Explanation Example
    XML XML stands for eXtensible Markup Language Sample.xml

    Not Much
    < language="”Spanish”">No Mucho

    DTD DTD stands for Document Type Definition. XML provides
    an application independent way of sharing data. With a
    DTD, independent groups of people can agree to use a
    common DTD for interchanging data. Your application can
    use a standard DTD to verify that data that you receive
    from the outside world is valid. You can also use a DTD to
    verify your own data. So the DTD is the building blocks or
    schema definition of the XML document.

    XSD XSD stands for Xml Schema Definition, which is a
    successor of DTD. So XSD is a building block of an XML
    If you have DTD then why use XSD you may ask?
    XSD is more powerful and extensible than DTD. XSD has:
    • Support for simple and complex data types.
    • Uses XML syntax. So XSD are extensible just like
    XML because they are written in XML.
    • Better data communication with the help of data
    types. For example a date like 03-04-2005 will be
    interpreted in some countries as 3rd of April 2005 and
    in some other countries as 04th March 2005.


    use=”Required” />

    XSL XSL stands for eXtensible Stylesheet Language. The XSL
    consists of 3 parts:
    • XSLT: Language for transforming XML documents
    from one to another.
    • XPath: Language for defining the parts of an XML
    • XSL-FO: Language for formatting XML documents.
    For example to convert an XML document to a PDF
    document etc.
    XSL can be thought of as a set of languages that can :
    • Define parts of an XML
    • Transform an XML document to XHTML (eXtensible
    Hyper Text Markup Language) document.
    • Convert an XML document to a PDF document.
    • Filter and sort XML data.
    XSLT processor example: Xalan (From Apache)
    PDF Processor example: FOP (Formatting Objects
    Processor from Apache)
    To convert the Sample.xml file to a XHTML file let us apply the
    following Sample.xsl through XALAN parser.

    <xsl:value-of <br />select="content/@language"> <br />

    You get the following output XHTML file:

    Enterprise Java

    Now to convert the Sample.xml into a PDF file apply the
    following FO (Formatting Objects) file Through the FOP

    which gives a basic Sample.pdf which has the following line
    Not Much
    XPath Xml Path Language, a language for addressing parts of an
    XML document, designed to be used by both XSLT and
    XPointer. We can write both the patterns (context-free) and
    expressions using the XPATH Syntax. XPATH is also used
    in XQuery.
    As per Sample.xsl


    JAXP Stands for Java API for XML Processing. This provides a
    common interface for creating and using SAX, DOM, and
    XSLT APIs in Java regardless of which vendor’s
    implementation is actually being used (just like the JDBC,
    JNDI interfaces). JAXP has the following packages:
    DOM example using JAXP:
    DocumentBuilderFactory dbf =
    DocumentBuilder db = dbf.newDocumentBuilder();
    Document doc =
    db.parse(new File("xml/Test.xml"));
    NodeList nl = doc.getElementsByTagName("to");
    Node n = nl.item(0);
    SAX example using JAXP:
    SAXParserFactory spf =
    SAXParser sp = spf.newSAXParser();
    SAXExample se = new SAXExample();
    sp.parse(new File("xml/Sample.xml"),se);
    where SAXExample.Java code snippet
    public class SAXExample extends DefaultHandler {
    public void startElement(
    String uri,
    String localName,
    String qName,
    Attributes attr)
    throws SAXException {
    System.out.println("--->" + qName);
    Enterprise Java
    to from
    • javax.xml.parsers 􀃆 common interface for different
    vendors of SAX, DOM parsers).
    • org.xml.sax 􀃆 Defines basic SAX API.
    • org.w3c.dom 􀃆 Defines Document Object Model and
    its componenets.
    • javax.xml.transform 􀃆 Defines the XSLT API which
    allows you to transform XML into other forms like
    PDF, XHTML etc.
    Required JAR files are jaxp.jar, dom.jar, xalan.jar,
    The DefaultHandler implements ContentHandler,
    DTDHandler, EntityResolver, ErrorHandler
    XSLT example using JAXP:
    StreamSource xml =
    new StreamSource(new File("/xml/Sample.xml"));
    StreamSource xsl = new StreamSource(
    new File("xml/Sample.xsl"));
    StreamResult result =
    new StreamResult(new File("xml/Sample.xhtml"));
    TransformerFactory tf =
    Transformer t = tf.newTransformer(xsl);
    t.transform(xml, result);
    This gives you Sample.xhtml


    JAXB Stands for Java API for XML Binding. This standard
    defines a mechanism for writing out Java objects as XML
    (marshalling) and for creating Java objects from
    XMLstructures (unmarshalling). (You compile a class
    description to create the Java classes, and use those
    classes in your application.)
    Lets look at some code:
    For binding: –p com.binding sample.xsd –d work
    -p identifies the package for the generated Java files (ie
    -d option identifies the target.
    Unmarshalling the XML document:
    JAXBContext jc = JAXBContext.newInstance(
    Unmarshaller um = jc.createUnmarshaller();
    Object o = um.unMarshall(
    new File(“/xml/”));
    Enterprise Java
    Java files
    (*.java interfaces
    class files
    Java content
    to from
    Note n = (Note) n;
    Now to modify the in memory object content:
    n. getFrom().getContent().set(0, “newValue”);
    Marshalling the change back to different XML file:
    Marshaller m = jc.createMarshaller();
    FileOutputStream fos = new FileOutputStream(
    new File(“/xml/SampleNew.xml”));
    m.marshall(n, fos);
    Enterprise – SQL, Tuning and O/R mapping
    Q 96: Explain inner and outer joins? SF
    A 96: Joins allow database users to combine data from one table with data from one or more other tables (or views, or
    synonyms). Tables are joined two at a time making a new table containing all possible combinations of rows from
    the original two tables. Lets take an example (syntax vary among RDBMS):
    Joins and Set operations in Relational Databases
    Inner join Left outer join Right outer join Full outer join
    Enterprise Java
    Employees table
    Id firstname Surname state
    1001 John Darcy NSW
    1002 Peter Smith NSW
    1003 Paul Gregor NSW
    1004 Sam Darcy VIC
    Executives table
    Id firstname Surname state
    1001 John Darcy NSW
    1002 Peter Smith NSW
    1005 John Gregor WA
    Inner joins: Chooses the join criteria using any column names that happen to match between the two tables. The
    example below displays only the employees who are executives as well.
    SELECT emp.firstname, exec.surname FROM employees emp, executives exec
    WHERE =;
    The output is:
    Firstname surname
    John Darcy
    Peter Smith
    Left Outer joins: A problem with the inner join is that only rows that match between tables are returned. The
    example below will show all the employees and fill the null data for the executives.
    SELECT emp.firstname, exec.surname FROM employees emp left join executives exec
    ON =;
    On oracle
    SELECT emp.firstname, exec.surname FROM employees emp, executives exec
    where =;
    The output is:
    Firstname surname
    John Darcy
    Peter Smith
    Right Outer join: A problem with the inner join is that only rows that match between tables are returned. The
    example below will show all the executives and fill the null data for the employees.
    SELECT emp.firstname, exec.surname FROM employees emp right join executives exec
    ON =;
    On oracle
    SELECT emp.firstname, exec.surname FROM employees emp, executives exec
    WHERE =;
    The output is:
    Firstname surname
    John Darcy
    Peter Smith
    Full outer join: To cause SQL to create both sides of the join
    SELECT emp.firstname, exec.surname FROM employees emp full join executives exec
    ON =;
    On oracle
    SELECT emp.firstname, exec.surname FROM employees emp, executives exec
    WHERE = (+)
    Enterprise Java
    SELECT emp.firstname, exec.surname FROM employees emp, executives exec
    WHERE =
    Note: Oracle9i introduced the ANSI compliant join syntax. This new join syntax uses the new keywords inner join, left outer join,
    right outer join, and full outer join, instead of the (+) operator.
    The output is:
    Firstname surname
    John Darcy
    Peter Smith
    Self join: A self-join is a join of a table to itself. If you want to find out all the employees who live in the same city
    as employees whose first name starts with “Peter”, then one way is to use a sub-query as shown below:
    SELECT emp.firstname, emp.surname FROM employees emp WHERE
    city IN (SELECT city FROM employees where firstname like ‘Peter’)
    The sub-queries can degrade performance. So alternatively we can use a self-join to achieve the same results.
    On oracle
    SELECT emp.firstname, emp.surname FROM employees emp, employees emp2
    WHERE emp.state = emp2.state
    AND emp2.firstname LIKE 'Peter'
    The output is:
    Firstname Surname
    John Darcy
    Peter Smith
    Paul Gregor
    Q 97: Explain a sub-query? How does a sub-query impact on performance? SF PI
    A 97: It is possible to embed a SQL statement within another. When this is done on the WHERE or the HAVING
    statements, we have a subquery construct. What is subquery useful for? It is used to join tables and there are
    cases where the only way to correlate two tables is through a subquery.
    SELECT emp.firstname, emp.surname FROM employees emp WHERE NOT IN (SELECT id FROM executives);
    There are performance problems with sub-queries, which may return NULL values. The above sub-query can be
    re-written as shown below by invoking a correlated sub-query:
    SELECT emp.firstname, emp.surname FROM employees emp WHERE NOT EXISTS (SELECT id FROM executives);
    The above query can be re-written as an outer join for a faster performance as shown below:
    SELECT emp.firstname, exec.surname FROM employees emp left join executives exec
    on = AND IS NULL;
    The above execution plan will be faster by eliminating the sub-query.
    Q 98: What is normalization? When to denormalize? DC PI
    A 98: Normalization is a design technique that is widely used as a guide in designing relational databases. Normalization
    is essentially a two step process that puts data into tabular form by removing repeating groups and then removes
    duplicated data from the relational tables (Additional reading recommended).
    Redundant data wastes disk space and creates maintenance problems. If data that exists in more than one place
    must be changed, the data must be changed in exactly the same way in all locations which is time consuming and
    prone to errors. A change to a customer address is much easier to do if that data is stored only in the Customers
    table and nowhere else in the database.
    Enterprise Java
    Inconsistent dependency is a database design that makes certain assumptions about the location of data. For
    example, while it is intuitive for a user to look in the Customers table for the address of a particular customer, it
    may not make sense to look there for the salary of the employee who calls on that customer. The employee's
    salary is related to, or dependent on, the employee and thus should be moved to the Employees table.
    Inconsistent dependencies can make data difficult to access because the path to find the data may not be logical,
    or may be missing or broken.
    First normal form Second Normal Form Third Normal Form
    A database is said to be in First
    Normal Form when all entities
    have a unique identifier or key,
    and when every column in every
    table contains only a single value
    and doesn't contain a repeating
    group or composite field.
    A database is in Second Normal Form
    when it is in First Normal Form plus
    every non-primary key column in the
    table must depend on the entire primary
    key, not just part of it, assuming that the
    primary key is made up of composite
    A database is in Third Normal Form when
    it is in Second Normal Form and each
    column that isn't part of the primary key
    doesn't depend on another column that
    isn't part of the primary key.
    When to denormalize? Normalize for accuracy and denormalize for performance.
    Typically, transactional databases are highly normalized. This means that redundant data is eliminated and
    replaced with keys in a one-to-many relationship. Data that is highly normalized is constrained by the primary
    key/foreign key relationship, and thus has a high degree of data integrity. Denormalized data, on the other hand,
    creates redundancies; this means that it's possible for denormalized data to lose track of some of the relationships
    between atomic data items. However, since all the data for a query is (usually) stored in a single row in the table, it
    is much faster to retrieve.
    Q 99: How do you implement one-to-one, one-to-many and many-to-many relationships while designing tables? SF
    A 99: One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign
    key relationships.
    One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key
    Many-to-Many relationships are implemented using join table with the keys from both the tables forming the
    composite primary key of the junction table.
    Q 100: How can you performance tune your database? PI
    A 100:
    􀂃 Denormalize your tables where appropriate.
    􀂃 Proper use of index columns: An index based on numeric fields is more efficient than an index based on
    character columns.
    􀂃 Reduce the number of columns that make up a composite key.
    􀂃 Proper partitioning of tablespaces and create a special tablespace for special data types like CLOB,
    BLOB etc.
    􀂃 Data access performance can be tuned by using stored procedures to crunch data in the database server
    to reduce the network overhead and also caching data within your application to reduce the number of
    Q 101: How will you map objects to a relational database? How will you map class inheritance to relational data model?
    A 101: Due to impedance mismatch between object and relational technology you need to understand the process of
    mapping classes (objects) and their relationships to tables and relationships between them in a database.
    Classes represent both behaviour and data whereas relational database tables just implement data. Database
    schemas have keys (primary keys to uniquely identify rows and foreign keys to maintain relationships between
    rows) whereas object schema does not have keys and instead use references to implement relationships to
    other objects. Let us look at some basic points on mapping:
    Enterprise Java
    -EmployeeId (PK)
    -EmployeeId (PK)
    -EmployeeId (PK)
    -EmployeeId (PK)
    -EmployeeId (PK)
    -EmployeeId (PK)
    -EmployeeId (PK)
    -EmployeeId (PK)
    -EmployeeId (PK)
    Object to Relational (O/R) mapping of class inheritance structure
    Class diagram
    Map class hierarchy to a single database table
    Map each class to its own table
    Map each concrete class to its own table
    􀂃 Classes map to tables in a way but not always directly.
    􀂃 An attribute of a class can be mapped to zero or more columns in a database. Not all attributes are
    􀂃 Some attributes of an object are objects itself. For example an Employee object has an Address object as
    an attribute. This is basically an association relationship between two objects (i.e. Employee and
    Address). This is a recursive relationship where at some point the attribute will be mapped to zero or
    more columns. In this example attributes of the Address class will be mapped zero or more columns.
    􀂃 In its simple form an attribute maps to a single column whereas each has same type (ie attribute is a
    string and column is a char, or both are dates etc). When you implement mapping with different types
    (attribute is a currency and column is a float) then you will need to be able to convert them back and forth.
    How do you map inheritance class structure to relational data model?
    Relational databases do not support inheritance. Class inheritance can be mapped to relational tables as
    Map class hierarchy to single database table: The whole class hierarchy can be stored in a single table by
    adding an additional column named “EmployeeType”. The column “EmployeeType” will hold the values
    “Permanent”, “Contract” and “SubContract”. New employee types can be added as required. Although this
    approach is straightforward it tends to break when you have combinations like an employee is of type both
    “Contractor” and “SubContractor”. So when you have combinations, you can use refactored table by replacing
    type code column “EmployeeType” with boolean values such as isPermanent, isContractor and isSubContractor.
    Map each class to its own table: You create one table per class. The data for a permanent employee is stored
    in two tables (Employee and Permanent), therefore to retrieve this data you need to join these two tables. To
    support additional employee type say a Contractor, add a new table.
    Enterprise Java
    Map each concrete class to its own table: You create one table per concrete class. There are tables
    corresponding to each class like Permanent, Contractor and SubContractor. So join is not required. To support
    additional employee type, add a new table.
    So which approach to use? Easiest approach is to have one table per hierarchy and easy to refactor. If you
    need a “pure design approach” then use one table per class approach. Try to stay away from one table per
    concrete class approach because it makes refactoring difficult by copying data back and forth between tables.
    No approach is ideal for all situations.
    Another option for mapping inheritance into relational database is to take a generic meta-data driven approach.
    This approach supports all forms of mapping. In this approach, value of a single attribute will be stored as a row
    in a table called “Value”. So, to store 5 attributes you need 5 rows in “Value” table. You will have a table called
    “Class” where class names are stored, a table called “Inheritance” where subclass and superclass information is
    stored, a table called “Attributes” where class attributes are stored and an “AttributeType” lookup table.
    Q 102: What is a view? Why will you use a view? What is an aggregate function? Etc. SF PI
    A 102:
    Question Explanation
    What is view? Why use a view? View is a precompiled SQL query, which is used to select data from one or more tables.
    A view is like a table but it doesn’t physically take any space (ie not materialised). Views
    are used for
    􀂃 Providing inherent security by exposing only the data that is needed to be shown to
    the end user.
    􀂃 Enabling re-use of SQL statements.
    􀂃 Allows changes to the underlying tables to be hidden from clients, aiding
    maintenance of the database schema (i.e. encapsulation).
    Views with multiple joins and filters can dramatically degrade performance because
    views contain no data and any retrieval needs to be processed. The solution for this is to
    use materialised views or create de-normalised tables to store data. This technique is
    quite handy in overnight batch processes where a large chunk of data needs to be
    processed. Normalised data can be read and inserted into some temporary denormalised
    table and processed with efficiency.
    Explain aggregate SQL functions? SQL provides aggregate functions to assist with the summarisation of large volumes of
    We’ll look at functions that allow us to add and average data, count records meeting
    specific criteria and find the largest and smallest values in a table.
    1001 John Darcy 25 10.5
    1002 Peter Smith 25 10.5
    1003 Sam Gregory 25 10.5
    SELECT SUM(QTY) AS Total FROM Orders;
    The output is: Total = 75
    SELECT AVG(UnitPrice * QTY) As AveragePrice FROM Orders;
    The output is: AveragePrice = 262.50
    If we inserted another row to the above table:
    1004 John Darcy 20 10.50
    The output is: John 45
    Explain INSERT, UPDATE, and
    DELETE statements?
    INSERT statements can be carried out several ways:
    INSERT INTO ORDERS values (1004, 'John', 'Darcy', 20, 10.50);
    Enterprise Java
    The above statement is fine but the one below is recommended since it is less
    ambiguous and less prone to errors.
    INSERT INTO ORDERS (orderid, firstname, surname, qty, unitprice)
    values (1005, 'John', 'Darcy', 20, 10.50);
    We can also use INSERT with the SELECT statements as shown below
    INSERT into NEW_ORDERS (orderid, firstname, surname, qty, unitprice)
    SELECT orderid, firstname, surname, qty, unitprice
    FROM orders WHERE orderid = 1004;
    UPDATE statement allows you to update a single or multiple statements.
    UPDATE ORDERS set firstname='Peter', surname='Piper'
    WHERE orderid=1004;
    Also can have more complex updates like
    UPDATE supplier
    SET supplier_name = ( SELECT
    FROM customers
    WHERE customers.customer_id = supplier.supplier_id)
    FROM customers
    WHERE customers.customer_id = supplier.supplier_id);
    DELETE statements allow you to remove records from the database.
    DELETE FROM ORDERS WHERE orderid=1004;
    We can clear the entire table using
    TRUNCATE TABLE employees;
    When running UPDATE/DELETE care should be taken to include WHERE clause
    otherwise you can inadvertently modify or delete records which you do not intend to
    How can you compare a part of
    the name rather than the entire
    You can use wild card characters like:
    • * ( % in oracle) 􀃆 Match any number of characters.
    • ? ( _ in oracle) 􀃆 Match a single character.
    To find all the employees who has “au”:
    SELECT * FROM employees emp
    WHERE emp.firstname LIKE ‘%au%’;
    How do you get distinct entries
    from a table?
    The SELECT statement in conjunction with DISTINCT lets you select a set of distinct
    values from a table in a database.
    SELECT DISTINCT empname FROM emptable
    How can you find the total number
    of records in a table?
    Use the COUNT key word:
    SELECT COUNT(*) FROM emp WHERE age>25
    What's the difference between a
    primary key and a unique key?
    Both primary key and unique key enforce uniqueness of the column on which they are
    defined. But by default primary key creates a clustered index on the column, whereas
    unique creates a non-clustered index by default. Another major difference is that,
    primary key doesn't allow NULLs, but unique key allows one NULL only.
    What are constraints? Explain
    different types of constraints.
    Constraints enable the RDBMS enforce the integrity of the database automatically,
    without needing you to create triggers, rule or defaults.
    What is an index? What are the
    types of indexes? How many
    clustered indexes can be created
    on a table? What are the
    The books you read have indexes, which help you to go to a specific key word faster.
    The database indexes are similar.
    Indexes are of two types. Clustered indexes and non-clustered indexes. When you
    Enterprise Java
    advantages and disadvantages of
    creating separate index on each
    column of a table?
    create a clustered index on a table, all the rows in the table are stored in the order of the
    clustered index key. So, there can be only one clustered index per table. Non-clustered
    indexes have their own storage separate from the table data storage. The row located
    could be the RowID or the clustered index key, depending up on the absence or
    presence of clustered index on the table.
    If you create an index on each column of a table, it improves the query performance, as
    the query optimizer can choose from all the existing indexes to come up with an efficient
    execution plan. At the same time, data modification operations (such as INSERT,
    UPDATE, and DELETE) will become slow, as every time data changes in the table, all
    the indexes need to be updated. Another disadvantage is that, indexes need disk space,
    the more indexes you have, more disk space is used.
    Enterprise - RUP & UML
    Q 103: What is RUP? SD
    A 103: Rational Unified Process (RUP) is a general framework that can be used to describe a development process.
    The software development cycle has got 4 phases in the following order Inception, Elaboration, Construction,
    and Transition.
    R a tio n a l U n ified P ro c e s s
    PH ASE S
    In c e p tio n e lab o ratio n co n struc tio n tra n s itio n
    IT E R AT IO N S
    In c e p tio n # 1 E la b #1 E la b #2 C on
    # 1
    C on
    # 2
    C on
    # 3
    T ran s itio n # 1
    B us in e ss M o d ellin g
    R e qu irem e n ts
    A na lys is & D es ig n
    Im p lem e n ta tio n T e s t
    D e plo ym en t
    C onfig & C hange m gm t
    P ro je c t m gm t
    E nvironm ent
    Disciplines (vertical axis)
    PHAS E S / ITERATIO NS (Ho rizon tal axis)
    The core of the phases is state-based, and the state is determined by what fundamental questions you are trying
    to answer:
    􀂃 Inception - do you and the customer have a shared understanding of the system?
    􀂃 Elaboration - do you have baseline architecture to be able to build the system?
    􀂃 Construction - are you developing a product?
    􀂃 Transition - are you trying to get the customer to take ownership of the system?
    RUP is based on a few important philosophies and principles:
    􀂃 A software project team should plan ahead.
    􀂃 It should know where it is going.
    􀂃 It should capture project knowledge in a storable and extensible form.
    The best practices of RUP involve the following major 5 properties:
    Best practice property Description
    Use case driven Interaction between the users and the system.
    Architecture centric Based on architecture with clear relationships between architectural components.
    Iterative The problem and the solution are divided into more manageable smaller pieces, where each
    Enterprise Java
    iteration will be addressing one of those pieces.
    Incremental Each iteration builds incrementally on the foundation built in the previous iteration.
    Controlled Control with respect to process means you always know what to do next; control with respect to
    management means that all deliverables, artifacts, and code are under configuration
    Q 104: Explain the 4 phases of RUP? SD
    A 104:
    RUP Phases
    Iter 1 Iter 2 Iter 3 Iter 4 Iter 5 Iter 6
    􀂃 Inception: During the inception phase, you work out the business case for the project. You also will be
    making a rough cost estimate and return on investment. You should also outline the scope and size of the
    The fundamental question you ask at the end of this phase: do you and the customer have a shared
    understanding of the system?
    􀂃 Elaboration: At this stage you have the go ahead of the project however only have vague requirements. So
    at this stage you need to get a better understanding of the problem. Some of the steps involved are:
    􀂃 What is it you are actually going to build?
    􀂃 How are you going to build it?
    􀂃 What technology are you going to use?
    􀂃 Analysing and dealing with requirement risks, technological risks, skill risks, political risks etc.
    􀂃 Develop a domain model, use case model and a design model. The UML techniques can be used for
    the model diagrams (e.g. class diagrams, sequence diagrams etc).
    An important result of the elaboration phase is that you have a baseline architecture. This architecture
    consists of:
    􀂃 A list of use cases depicting the requirements.
    􀂃 The domain model, which captures your understanding of the domain with the help of UML class
    􀂃 Selection of key implementation technology and how they fit together. For example: Java/J2EE with
    JSP, Struts, EJB, XML, etc.
    The fundamental question you ask at the end of this phase: do you have a baseline architecture to be
    able to build the system?
    􀂃 Construction: In this phase you will be building the system in a series of iterations. Each iteration is a mini
    project. You will be performing analysis, design, unit testing, coding, system testing, and integration testing for
    the use cases assigned to each iteration. The iterations within the construction phase are incremental and
    iterative. Each iteration builds on the use cases developed in the previous iterations. The each iteration will
    involve code rewrite, refactoring, use of design patterns etc.
    The basic documentation required during the construction phase is:
    􀂃 A class diagram and a sequence diagram.
    􀂃 Some text to pull the diagrams together.
    Enterprise Java
    􀂃 If a class has complex life cycle behaviour then a state diagram is required.
    􀂃 If a class has a complex computation then an activity diagram is required.
    The fundamental question you ask at the end of this phase: do you have a developed product?
    􀂃 Transition: During this phase you will be delivering the finished code regularly. During this phase there is no
    coding to add functionality unless it is small and essential. There will be bug fixes, code optimisation etc
    during this phase. An example of a transition phase is that the time between the beta release and the final
    release of a product.
    The fundamental question you ask at the end of this phase: are you trying to get the customer to take
    ownership of the developed product or system?
    Q 105: What are the characteristics of RUP? Where can you use RUP? SD
    A 105:
    1. RUP is based on a few important philosophies and principles like planning ahead, knowing where the process
    is heading and capturing the project in storable and extensible manner.
    2. It is largely based on OO analysis and design, and use case driven etc.
    3. Iterative and incremental development as oppose to waterfall approach, which hides problems.
    4. Architecture centric approach.
    RUP is more suited for larger teams of 50-100 people. RUP can also be used as an agile (i.e. lightweight)
    process for smaller teams of 20-30 people, or as a heavy weight process for larger teams of 50-100 people.
    Extreme Programming (XP) can be considered as a subset of RUP. At the time of writing, the agile (i.e
    lightweight) software development process is gaining popularity and momentum across organizations.
    Several methodologies fit under this agile development methodology banner. All these methodologies share
    many characteristics like iterative and incremental development, test driven development, stand up
    meetings to improve communication, automatic testing, build and continuous integration of code etc.
    Refer Q136 in Enterprise Java section.
    Q 106: Why is UML important? SD DC
    A 106: The more complicated the underlying system, the more critical the communication among everyone involved in
    developing and deploying the software. UML is a software blueprint language for analysts, designers and
    developers. UML provides a common vocabulary for the business analysts, architects, developers etc.
    UML is applicable to the Object Oriented problem solving. UML begins with a model; A model is an abstraction
    of the underlying problem. The domain is the actual world from which the problem comes. The model consists of
    objects. The objects interact with each other by sending and receiving messages. The objects are
    characterised by attributes and operations (behaviours). The values of an object’s attributes determine its
    state. The classes are the blueprints (or like templates) for objects. A class wraps attributes and methods into
    a single distinct entity. The objects are the instances of classes.
    Q 107: What are the different types of UML diagrams? SD DC
    A 107: Use case diagrams: Depicts the typical interaction between external users (actors) and the system. The
    emphasis is on what a system does rather than how it does it. A use case is a summary of scenarios for a
    single task or goal. An actor is responsible for initiating a task. The connection between actor and use case is a
    communication association.
    Capturing use cases is one of the primary tasks of the elaboration phase of RUP. In its simplest usage, you
    capture a use case by talking to your users and discussing the various things they might want to do with the
    When to use ‘use case’ diagrams?
    􀂃 Determining user requirements. New use cases often generate new requirements.
    􀂃 Communicating with clients. The simplicity of the diagram makes use case diagrams a good way for
    designers and developers to communicate with clients.
    􀂃 Generating test cases. Each scenario for the use case may suggest a suite of test cases.
    Enterprise Java
    U s e c a s e d ia g r a m
    p e r fo rm s e c u r ity
    c h e c k
    In te r n a tio n a l S tu d e n t
    R e g is tra r
    S tu d e n t
    E n r o l l i n U n i v e r s i
    t y E n r o l l i n s e m in a r
    E n r o l l f a m i l y
    m e m b e rs
    * *
    a s s o c ia t io n
    a s s o c ia t io n
    a s s o c ia tio n
    in h e r ita n c e
    < <> >
    in h e r i ta n c e
    < <> >
    N o te :
    < <> > r e la t io n s h ip is c o n d it io n a l. Y o u d o
    n o t k n o w if o r w h e n e x te n d in g u s e c a s e w i l l b e
    in v o k e d .
    < <> > r e la t io n s h ip is s im i la r to a
    p r o c e d u r e c a l l .
    i n h e r i t a n c e : e x te n d s t h e b e h a v io r o f th e
    p a r e n t u s e c a s e o r a c to r .
    Class diagrams: Class diagram technique is vital within Object Oriented methods. Class diagrams describe the
    types of objects in the system and the various static relationships among them. Class diagrams also show the
    attributes and the methods. Class diagrams have the following possible relationships:
    􀂃 Association: A relationship between instances of 2 classes.
    􀂃 Aggregation: An association in which one class belongs to a collection (does not always have to be a
    collection. You can also have cardinality of “1”). This is a part of a whole relationship where the part can
    exist without the whole. For example: A line item is whole and the products are the parts. If a line item is
    deleted then the products need not be deleted.
    􀂃 Composition: An association in which one class belongs to a collection (does not always have to be a
    collection. You can also have cardinality of “1”). This is a part of a whole relationship where the part cannot
    exist without the whole. If the whole is deleted then the parts are deleted. For example: An Order is a whole
    and the line items are the parts. If an order is deleted then all the line items should be deleted as well (ie
    cascade deletes).
    􀂃 Generalization: An inheritance link indicating that one class is a superclass of the other. The Generalization
    expresses the “is a” relationship whereas the association, aggregation and composition express the “has a”
    􀂃 Dependency: A dependency is a weak relationship where one class requires another class. The dependency
    expresses the “uses” relationship. For example: A domain model class uses a utility class like Formatter etc.
    Class Diagram
    -lineItems : L ist
    -order : O rder
    1 *
    -product : P roduct
    1 1..*
    -unitP rice
    composition dependency
    When to use class diagrams?
    􀂃 Class diagrams are the backbone of Object Oriented methods. So they are used frequently.
    Enterprise Java
    􀂃 Class diagrams can have a conceptual perspective and an implementation perspective. During the analysis
    draw the conceptual model and during implementation draw the implementation model.
    Package diagrams: To simplify complex class diagrams you can group classes into packages.
    P a c k a g e D ia g r a m
    A c c o u n tin g O rd e r in g
    C u s to m e r
    d e p e n d e n c y
    d e p e n d e n c y
    When to use package diagrams?
    􀂃 Package diagrams are vital for large projects.
    Object diagrams: Object diagrams show instances instead of classes. They are useful for explaining some
    complicated objects in detail about their recursive relationships etc.
    O b je c t D ia g ram
    D e p a r tm e n t
    0 ..*
    R e cu rs iv e c la s s
    d ia g ram d iffic u lt to fu lly
    u n d e rs ta n d
    p h y s ic sM a th s : D e p a r tm e n t
    p h y s ic s : D e p a rtm e n t m a th : D e p a r tm e n t
    p u reM a th : D e p a rtm e n t a p p lie dM a th : D e p a rtm e n t
    O b je c t D ia g ram
    C la s s D ia g ram
    im p ro ve c la rity
    S h o w s th e d e ta ils o f th e re c u rs iv e o b je c t re la tio n s h ip
    C la s s n am e
    o b je c t n am e
    When to use object diagrams?
    􀂃 Object diagrams are a vital for large projects.
    􀂃 They are useful for explaining structural relationships in detail for complex objects.
    Sequence diagrams: Sequence diagrams are interaction diagrams which detail what messages are sent and
    when. The sequence diagrams are organized according to time. The time progresses as you move from top to
    bottom of the diagram. The objects involved in the diagram are shown from left to right according to when they
    take part.
    a n O rd e r : O rd e r a n E n try : O rd e rE n try
    m a ke A nO rd e r() m a ke A nO rd e r()
    i t e r a t io n [ fo r e a c h . . . ] ( )
    c o n f irm : C o n f irm a t io n
    h a s S u ffic ie n tD e ta ils ( )
    p r in tC o n firm a tio n ()
    c h e c k if s u f f ic ie n t d e ta ils
    a re a v a ila b le
    fo r e a c h lin e I te m
    c lie n t
    S e q u e n c e D ia g ram
    N o te : E a c h v e r t ic a l d o tte d lin e is a l i fe l in e . E a c h a r ro w is a m e s s a g e . T h e re c ta n g u la r b o x e s o n th e life
    lin e a re c a lle d th e a c tiv a tio n b a r w h ic h re p re s e n ts th e d u ra tio n o f e x e c u tio n o f m e s s a g e .
    Enterprise Java
    Collaboration diagrams: Collaboration diagrams are also interaction diagrams. Collaboration diagrams convey
    the same message as the sequence diagrams. But the collaboration diagrams focus on the object roles instead
    of the times at which the messages are sent.
    The collaboration diagrams use the decimal sequence numbers as shown in the diagram below to make it clear
    which operation is calling which other operation, although it can be harder to see the overall sequence. The toplevel
    message is numbered 1. The messages at the same level have the same decimal prefix but different suffixes
    of 1, 2 etc according to when they occur.
    Collaboration Diagram
    anOrder : Order
    1.1 : makeAnOrder()
    anEntry : OrderEntry
    confirm : Confirmation
    1.1.1 : m akeAnOrd er( ) : hasSufficientDetails() for each (iteration)
    message sequence
    When to use interaction diagrams?
    􀂃 When you want to look at behaviour of several objects within a single use case. If you want to look at a single
    object across multiple use cases then use statechart diagram as described below.
    State chart diagrams: Objects have behaviour and state. The state of an object is depends on its current activity
    or condition. This diagram shows the possible states of the object and the transitions that cause a change in its
    C h e c k in g s ta te D is p a tc h in g s ta te
    w a itin g s ta te D e liv e re d s ta te
    / g e t a n item
    / s om e item s n o t in s to c k
    / a ll item s a va ila b le
    / item s re c e ive d
    / d e liv e r
    S ta te c h a rt D ia g ram
    When to use statechart diagram?
    􀂃 Statechart diagrams are good at describing the behaviour of an object across several use cases. But they are
    not good at describing the interaction or collaboration between many objects. Use interaction and/or activity
    diagrams in conjunction with a statechart diagram.
    􀂃 Use it only for classes that have complex state changes and behaviour. For example: the User Interface (UI)
    control objects, Objects shared by multi-threaded programs etc.
    Enterprise Java
    Activity diagram: This is really a fancy flow chart. The activity diagram and statechart diagrams are related in a
    sense that statechart diagram focuses on object undergoing a transition process and an activity diagram focuses
    on the flow of activities involved in a single transition process.
    Activity Diagram
    check next item get first item
    Wait for items to arrive in stock
    dispatch all items
    / some items not in stock
    / All items in stock
    place an order with the supplier receive the order and enter into stock
    Order placement department dispatch department
    In domain modelling it is imperative that the diagram conveys which object (or class) is responsible for each
    activity. Activity diagrams can be divided into object swimlanes that determine which object is responsible for
    which activity. The swimlanes are quite useful because they combine the activity diagram’s depiction of logic with
    the interaction diagram’s depiction of responsibility. A single transition comes out of each activity, connecting to
    the next activity. A transition may join or fork.
    When to use activity diagrams?
    The activity and statechart diagrams are generally useful to express complex operations. The great strength of
    activity diagrams is that they support and encourage parallel behaviour. The activity and statechart diagrams are
    beneficial for workflow modelling with multi-threaded programming.
    Component and Deployment diagrams: A component is a code module. Component diagrams are physical
    diagrams analogous to a class diagram. The deployment diagrams show the physical configuration of software
    and hardware components. The physical hardware is made up of nodes. Each component belongs to a node.
    Component and Deployment Diagram
    Ratailer J2EE Server Wholesaler J2EE application server
    Order Component Dispatch Component
    OrderIF DispathIF
    Order Component
    Enterprise Java
    Q 108: What is the difference between aggregation and composition? SD DC
    A 108:
    Aggregation Composition
    Aggregation: An association in which one class
    belongs to another class or a collection. This is a part
    of a whole relationship where the part can exist
    without the whole. For example: A line item is whole
    and the products are the parts. If a line item is deleted
    then the products need not be deleted. (no cascade
    delete in database terms)
    Composition: An association in which one class belongs to
    another class a collection. This is a part of a whole relationship
    where the part cannot exist without the whole. If the whole is
    deleted then the parts are deleted. For example: An Order is a
    whole and the line items are the parts. If an order is deleted then all
    the line items should be deleted as well (i.e. cascade deletes in
    database terms).
    Aggregations are not allowed to be circular. In a garbage-collected language like Java, The whole has the
    responsibility of preventing the garbage collector to prematurely
    collect the part by holding reference to it.
    Q 109: What is the difference between a collaboration diagram and a sequence diagram? SD DC
    A 109: You can automatically generate one from the other.
    Sequence Diagram Collaboration Diagram
    The emphasis is on the sequence. The emphasis is on the object roles
    Reference: The above section on RUP & UML is based on the book UML Distilled by Martin Fowler and Kendall Scott. If you
    would like to have a good understanding of UML & RUP, then this book is recommended.
    Enterprise - Struts
    Struts is a Web-based user interface framework, which has been around for a few years. It is a matured and proven framework, which
    has been used in many J2EE projects. While Struts has been demonstrating its popularity, there is an emerging framework called
    JavaServer Faces (JSF) gaining lots of momentum and popularity. Like Struts, JSF provides Web application life cycle management
    through a controller servlet, and like Swing, JSF provides a rich component model complete with event handling and component
    rendering. So JSF can be considered as a combination of Struts frame work and Java Swing user interface framework. Refer Q19 –
    Q20 in Emerging Technologies/Frameworks section for JSF.
    Q 110: Give an overview of Struts? SF DP
    A 110: Struts is a framework with set of cooperating classes, servlets and JSP tags that make up a reusable MVC 2
    S TR U TS O v e r v iew
    C lie n t
    (BR O W SER )
    V ie w
    (JS P )
    A ctio n
    (c a lls b u s in e s s lo g ic )
    M o d e l
    (F o rm b e a n s )
    1 . H T TP reque st
    2 . D i sp a tc h
    3. Instantiate/ Set
    5 . g e t th ro u g h Ta g
    6 . H TT P r e spo nse
    4. Foprward
    F ro n t
    C o n tro l le r
    (S e rvle t )
    s tr u ts -
    c o n f ig .xm l
    􀂃 Client (Browser): A request from the client browser creates an HTTP request. The Web container will
    respond to the request with an HTTP response, which gets displayed on the browser.
    Enterprise Java
    􀂃 Controller (ActionServlet class and Request Processor class): The controller receives the request from
    the browser, and makes the decision where to send the request based on the struts-config.xml. Design
    pattern: Struts controller uses the command design pattern by calling the Action classes based on the
    configuration file struts-config.xml and the RequestProcessor class’s process() method uses template
    method design pattern (Refer Q11 in How would you go about … section) by calling a sequence of methods
    • processPath(request, response) 􀃆 read the request URI to determine path element.
    • processMapping(request,response) 􀃆 use the path information to get the action mapping
    • processRoles(request,respose,mapping) 􀃆 Struts Web application security which provides an
    authorization scheme. By default calls request.isUserInRole(). For example allow /addCustomer action if
    the role is executive.

    • processValidate(request,response,form,mapping) 􀃆 calls the vaildate() method of the ActionForm.
    • processActionCreate(request,response,mapping)􀃆 gets the name of the action class from the “type”
    attribute of the element.
    • processActionPerform(req,res,action,form,mapping) 􀃆 This method calls the execute method of the
    Action class which is where business logic is written.
    􀂃 Business Logic (Action class): The Servlet dispatches the request to Action classes, which act as a thin
    wrapper to the business logic (The actual business logic is carried out by either EJB session beans and/or
    plain Java classes). The action class helps control the workflow of the application. (Note: The Action class
    should only control the workflow and not the business logic of the application). The Action class uses the
    Adapter design pattern (Refer Q11 in How would you go about … section).
    􀂃 ActionForm class: Java representation of HTTP input data. They can carry data over from one request to
    another, but actually represent the data submitted with the request.
    􀂃 View (JSP): The view is a JSP file. There is no business or flow logic and no state information. The JSP
    should just have tags to represent the data on the browser.
    ActionServlet class is the controller part of the MVC implementation and is the core of the framework. It
    processes user requests, determines what the user is trying to achieve according to the request, pulls data from
    the model (if necessary) to be given to the appropriate view, and selects the proper view to respond to the user.
    As discussed above ActionServlet class delegates the grunt of the work to the RequestProcessor and Action
    The ActionForm class maintains the state for the Web application. ActionForm is an abstract class, which is
    subclassed for every input form model. The struts-config.xml file controls, which HTML form request maps to
    which ActionForm.
    The Action class is a wrapper around the business logic. The purpose of the Action class is to translate the
    HttpServletRequest to the business logic. To use the Action class, subclass and overwrite the execute() method.
    The actual business logic should be in a separate package or EJB to allow reuse of business logic in protocol
    independent manner (ie the business logic should be used not only by HTTP clients but also by WAP clients,
    EJB clients, Applet clients etc).
    The ExceptionHandler can be defined to execute when the Action class’s execute() method throws an Exception.
    For example

    When an IOException is thrown then it will be handled by the execute() method of the my.ExceptionHandler class.
    The struts-config.xml configuration information is translated into ActionMapping, which are put into the
    ActionMappings collection.
    Further reading is recommended for more detailed understanding.
    Enterprise Java
    Q 111: What is a synchronizer token pattern in Struts or how will you protect your Web against multiple submissions?
    DC DP
    A 111: Web designers often face the situation where a form submission must be protected against duplicate or multiple
    submissions. This situation typically occurs when the user clicks on submit button more than once before the
    response is sent back or client access a page by returning to the previously book marked page.
    􀂃 The simplest solution that some sites use is that displaying a warning message “Wait for a response after
    submitting and do not submit twice.
    􀂃 In the client only strategy, a flag is set on the first submission and from then onwards the submit button is
    disabled based on this flag. Useful in some situations but this strategy is coupled to the browser type and
    version etc.
    􀂃 For a server-based solution the J2EE pattern synchroniser token pattern can be applied. The basic
    idea is to:
    1. Set a token in a session variable on the server side before sending the transactional page back to
    the client.
    2. The token is set on the page as a hidden field. On submission of the page first check for the
    presence of a valid token by comparing the request parameter in the hidden field to the token stored
    in the session. If the token is valid continue processing otherwise take other alternative action. After
    testing the token must be reset to null.
    The synchroniser token pattern is implemented in Struts. How do we implement the alternate course of action
    when the second clicks on submit button will cancel the response from the first click. The thread for the first click
    still runs but has no means of sending the response back to the browser. This means the transaction might have
    gone through without notifying the user. The user might get the impression that transaction has not gone
    Struts support for synchronisation comes in the form of:
    ActionServlet.saveToken(HttpRequest) and ActionServlet.isTokenValid(HttpRequest) etc
    Q 112: How do you upload a file in Struts? SF
    A 112: In JSP page set the code as shown below: CO
    Please select the file that you would like to upload:

    In the FormBean set the code as shown below:
    public class FileForm extends ActionForm {
    private FormFile file;
    public void setFile(FormFile file){
    this.file = file;
    public FormFile getFile(){
    return file;
    Q 113: Are Struts action classes thread-safe? SF CI
    A 113: No. Struts action classes are not thread-safe. Struts action classes are cached and reused for performance
    optimization at the cost of having to implement the action classes in a thread-safe manner.
    Enterprise Java
    Q 114: How do you implement internationalization in Struts? SF
    A 114: Internationalization is built into Struts framework. In the JSP page set the code as shown below: CO
    <%@ taglib uri="/WEB-INF/struts-bean.tld" prefix="bean" %>
    <%@ taglib uri="/WEB-INF/struts-html.tld" prefix="html" %>
    <%@ taglib uri="/WEB-INF/struts-logic.tld" prefix="logic" %>


    Now we need to create an application resource file named
    page.title=Thank you for visiting!
    Now in Italian, create an application resource file named
    page.title=Grazie per la vostra visita!
    Finally, add reference to the appropriate resource file in the struts-config.xml.
    Q 115: What is an action mapping in Struts? How will you extend Struts? SF
    A 115: An action mapping is a configuration file (struts-config.xml) entry that, in general, associates an action name
    with an action. An action mapping can contain a reference to a form bean that the action can use, and can
    additionally define a list of local forwards that is visible only to this action.
    How will you extend Struts?
    Struts is not only a powerful framework but also very extensible. You can extend Struts in one or more of the
    following ways:
    PlugIn: Define your own PlugIn class if you want to execute some init() and destroy() methods during the
    application startup and shutdown respectively. Some services like loading configuration files, initialising
    applications like logging, auditing, etc can be carried out in the init() method.
    RequestProcessor: You can create your own RequestProcessor by extending the Struts RequestProcessor.
    For example you can override the processRoles(req, res, mapping) in your extended class if you want to query
    the LDAP server for the security authorization etc.
    ActionServlet: You can extend the ActionServlet class if you want to execute your business logic at the
    application startup or shutdown or during individual request processing. You should take this approach only
    when the above mentioned approaches are not feasible.
    Q 116: What design patterns are used in Struts? DP
    A 116: Struts is based on model 2 MVC (Model-View-Controller) architecture. Struts controller uses the command
    design pattern (Refer Q11 in How would you go about section) and the action classes use the adapter design
    pattern. The process() method of the RequestProcessor uses the template method design pattern (Refer Q11
    in How would you go about section). Struts also implement the following J2EE design patterns
    􀂃 Service to Worker (Refer Q25 in Enterprise section).
    􀂃 Dispatcher View (Refer Q25 in Enterprise section).
    􀂃 Composite View (Struts Tiles) (Refer Q25 in Enterprise section)
    􀂃 Front Controller (Refer Q24 in Enterprise section).
    􀂃 View Helper (Refer Q25 in Enterprise section).
    􀂃 Synchronizer Token (Refer Q111 in Enterprise section).
    Enterprise Java
    Enterprise - Web and Application servers
    Q 117: What application servers, Web servers, LDAP servers, and Database servers have you used?
    A 117:
    Web Servers Apache, Microsoft IIS, Netscape, Domino etc
    Application Servers IBM Websphere, BEA Weblogic, Apache Tomcat, Borland Enterprise Server, Fujitsu
    Interstage, JBoss, ATG Dynamo etc
    LDAP Servers IPlanet’s directory server, SiemensDirX etc
    Database Servers IBM DB2, Oracle, SQL Server, Sybase, Informix
    Q 118: What is the difference between a Web server and an application server? SF
    A 118: In general, an application server prepares data for a Web server -- for example, gathering data from databases,
    applying relevant business rules, processing security checks, and/or storing the state of a user’s session. The
    term application server may be misleading since the functionality isn’t limited to applications. Its role is more as
    retriever and manager of data and processes used by anything running on a Web server. In the coming age of
    Web services, application servers will probably have an even more important role in managing service oriented
    components. One of the reasons for using an application server is to improve performance by off-loading tasks
    from a Web server. When heavy traffic has more users, more transactions, more data, and more security checks
    then more likely a Web server becomes a bottleneck.
    Web Server Application Server
    Supports HTTP protocol. When a Web server receives an
    HTTP request, it responds with an HTTP response, such
    as sending back an HTML page (static content) or
    delegates the dynamic response generation to some other
    program such as CGI scripts or Servlets or JSPs in an
    application server.
    Exposes business logic and dynamic content to a client
    through various protocols such as HTTP, TCP/IP, IIOP, JRMP
    Uses various scalability and fault-tolerance techniques. Uses various scalability and fault-tolerance techniques. In
    addition provides resource pooling, component life cycle
    management, transaction management, messaging, security
    Q 119: What is a virtual host? SF
    A 119: The term virtual host refers to the practice of maintaining more than one server on one machine. They are
    differentiated by their host names. You can have name based virtual hosts and IP address based virtual hosts.
    For example
    A name-based "virtual host" has a unique domain name, but the same IP address. For example, and can have the same IP address and share the same
    Web server. We can configure the Web server as follows:

    DocumentRoot /web/company1

    DocumentRoot /web/company2

    In this scenario, both and are registered with the standard domain
    name service (DNS) registry as having the IP address A user types in the URL in their browser. The user's computer resolves the name
    Enterprise Java
    138 to the IP address The Web server on the machine that has the IP address, so it receives the request. The Web server determines which virtual host to use by matching the
    request URL It gets from an HTTP header submitted by the browser with the “ServerName” parameter in the
    configuration file shown above.
    Name-based virtual hosting is usually easier, since you have to only configure your DNS server to map each
    hostname to a single IP address and then configure the Web server to recognize the different hostnames as
    discussed in the previous paragraph. Name-based virtual hosting also eases the demand for scarce IP
    addresses limited by physical network connections [but modern operation systems supports use of virtual
    interfaces, which are also known as IP aliases]. Therefore you should use name-based virtual hosting unless
    there is a specific reason to choose IP-based virtual hosting. Some reasons why you might consider using IPbased
    virtual hosting:
    􀂃 Name-based virtual hosting cannot be used with SSL based secure servers because of the nature of the
    SSL protocol.
    􀂃 Some operating systems and network equipment implement bandwidth management techniques that cannot
    differentiate between hosts unless they are on separate IP addresses.
    􀂃 IP based virtual hosts are useful, when you want to manage more than one site (like live, demo, staging etc)
    on the same server where hosts inherit the characteristics defined by your main host. But when using SSL
    for example, a unique IP address is necessary.
    For example in development environment when using the test client and the server on the same machine we can
    define the host file as shown below:
    UNIX user: /etc/hosts
    Windows user: C:\WINDOWS\SYSTEM32\DRIVERS\ETC\HOSTS localhost
    Q 120: What is application server clustering? SI
    A 120: An application server cluster consists of a number of application servers loosely coupled on a network. The
    server cluster or server group is generally distributed over a number of machines or nodes. The important point
    to note is that the cluster appears as a single server to its clients.
    The goals of application server clustering are:
    􀂃 Scalability: should be able to add new servers on the existing node or add new additional nodes to
    enable the server to handle increasing loads without performance degradation, and in a manner
    transparent to the end users.
    􀂃 Load balancing: Each server in the cluster should process a fair share of client load, in proportion to its
    processing power, to avoid overloading of some and under utilization of other server resources. Load
    distribution should remain balanced even as load changes with time.
    􀂃 High availability: Clients should be able to access the server at almost all times. Server usage should be
    transparent to hardware and software failures. If a server or node fails, its workload should be moved
    over to other servers, automatically as fast as possible and the application should continue to run
    uninterrupted. This method provides a fair degree of application system fault-tolerance. After failure, the
    entire load should be redistributed equally among working servers of the system.
    [Good read: Uncover the hood of J2EE clustering by Wang Yu on ]
    Q 121: Explain Java Management Extensions (JMX)? SF
    A 121: JMX framework can improve the manageability of your application by
    Enterprise Java
    􀂃 Monitoring your application for performance problems, critical events, error condition statistics, etc. For
    example you can be notified if there is a sudden increase in traffic or sudden drop in performance of your
    􀂃 Making your application more controllable and configurable at runtime by directly exposing application API
    and parameters. For example you could switch your database connection to an alternate server. You can
    also change the level of debugging and logging within the application without stopping the server.
    􀂃 By interfacing JMX to your hardware, database server and application server, health checks can be
    performed of your infrastructure.
    Q 122: Explain some of the portability issues between different application servers? SI
    A 122: Transaction isolation levels, lazy loading and dirty marker strategies for EJB, class loading visibility etc.
    Enterprise - Best practices and performance considerations
    Q 123: Give some tips on J2EE application server performance tuning? PI
    A 123:
    􀂃 Set the Web container threads, which will be used to process incoming HTTP requests. The minimum
    size should be tuned to handle the average load of the container and maximum should be tuned to
    handle the peak load. The maximum size should be less than or equal to the number of threads in your
    Web server.
    􀂃 When an EJB is called from a servlet or another EJB within the same JVM (i.e. same application server)
    then performance can be improved by running EJBs in pass-by-reference mode as oppose to pass-byvalue
    which is the default mode. Care should be taken to test the application properly before going into
    production because some valid applications may not work correctly when pass-by-reference setting is
    switched on.
    􀂃 Application servers maintain a pool of JDBC resources so that a new connection does not need to be
    created for each transaction. Application servers can also cache your prepared statements to improve
    performance. So you can tune the minimum and maximum size of these pools.
    􀂃 Tune your initial heap size for the JVM so that the garbage collector runs at a suitable interval so that it
    does not cause any unnecessary overhead. Adjust the value as required to improve performance.
    􀂃 Set the session manager settings appropriately based on following guidelines:
    􀂃 Set the appropriate value for in memory session count.
    􀂃 Reduce the session size.
    􀂃 Don’t enable session persistence unless required by your application.
    􀂃 Invalidate your sessions when you are finished with them by setting appropriate session timeout.
    􀂃 Calls to EJB from a separate JVM are handled by ORB (Object Request Broker). ORB uses a pool of
    threads to handle these requests. The thread pool size should be set appropriately to handle average and
    peak loads.
    􀂃 If a servlet or JSP file is called frequently with identical URL parameters then they can be dynamically
    cached to improve performance.
    􀂃 Turn the application server tracing off unless required for debugging.
    􀂃 Some application servers support lazy loading and dirty marker strategies with EJB to improve
    Q 124: Explain some of the J2EE best practices? BP
    A 124:
    Enterprise Java
    􀂃 Recycle your valuable resources by either pooling or caching. You should create a limited number of
    resources and share them from a common pool (e.g. pool of threads, pool of database connections, pool of
    objects etc). Caching is simply another type of pooling where instead of pooling a connection or object, you
    are pooling remote data (database data) and placing it in the memory (using Hashtable etc).
    􀂃 Avoid embedding business logic in a protocol dependent manner like in JSPs, HttpServlets, Struts
    action classes etc. This is because your business logic should be not only executed by your Web clients but
    also required to be shared by various GUI clients like Swing based stand alone application, WAP clients etc.
    􀂃 Automate the build process with tools like Ant, CruiseControl, and Maven etc. In an enterprise application
    the build process can become quite complex and confusing.
    􀂃 Build test cases first (i.e. Test Driven Development (TDD), refer section Emerging Technologies) using
    tools like JUnit. Automate the testing process and integrate it with build process.
    􀂃 Separate HTML code from the Java code: Combining HTML and Java code in the same source code can
    make the code less readable. Mixing HTML and scriplet will make the code extremely difficult to read and
    maintain. The display or behaviour logic can be implemented as a custom tags by the Java developers and
    Web designers can use these Tags as the ordinary XHTML tags.
    􀂃 It is best practice to use multi-threading and stay away from single threaded model of the servlet unless
    otherwise there is a compelling reason for it. Shared resources can be synchronized or used in read-only
    manner or shared values can be stored in a database table. Single threaded model can adversely affect
    􀂃 Apply the following JSP best practices:
    􀂃 Place data access logic in JavaBeans: The code within the JavaBean is readily accessible to other
    JSPs and Servlets.
    􀂃 Factor shared behaviour out of Custom Tags into common JavaBeans classes: The custom tags
    are not used outside JSPs. To avoid duplication of behaviour or business logic, move the logic into
    JavaBeans and get the custom tags to utilize the beans.
    􀂃 Choose the right “include” mechanism: What are the differences between static and a dynamic
    include? Using includes will improve code reuse and maintenance through modular design. Which one
    to use? Refer Q31 in Enterprise section.
    􀂃 Use style sheets (e.g. css), template mechanism (e.g. struts tiles etc) and appropriate comments
    (both hidden and output comments).
    􀂃 If you are using EJBs apply the EJB best practices as described in Q82 in Enterprise section.
    􀂃 Use the J2EE standard packaging specification to improve portability across Application Servers.
    􀂃 Use proven frameworks like Struts, Spring, Hibernate, JSF etc.
    􀂃 Apply appropriate proven J2EE design patterns to improve performance and minimise network
    communications cost (Session façade pattern, Value Object pattern etc).
    􀂃 Batch database requests to improve performance. For example
    Connection con = DriverManager.getConnection(……).
    Statement stmt = con.createStatement().
    stmt.addBatch(“INSERT INTO Address…………”);
    stmt.addBatch(“INSERT INTO Contact…………”);
    stmt.addBatch(“INSERT INTO Personal”);
    int[] countUpdates = stmt.executeBatch();
    Use “PreparedStatements” instead of ordinary “Statements” for repeated reads.
    􀂃 Avoid resource leaks by
    􀂃 Closing all database connections after you have used them.
    􀂃 Clean up objects after you have finished with them especially when an object having a long life cycle
    refers to a number of objects with short life cycles (you have the potential for memory leak).
    Enterprise Java
    􀂃 Poor exception handling where the connections do not get closed properly and clean up code that
    never gets called. You should put clean up code in a finally {} block.
    􀂃 Handle and propagate exceptions correctly. Decide between checked and unchecked (i.e RunTime
    exceptions) exceptions.
    Q 125: Explain some of the J2EE best practices to improve performance? BP PI
    A 125: In short manage valuable resources wisely and recycle them where possible, minimise network overheads and
    serialization cost, and optimise all your database operations.
    􀂃 Manage and recycle your valuable resources by either pooling or caching. You should create a limited
    number of resources and share them from a common pool (e.g. pool of threads, pool of database
    connections, pool of objects etc). Caching is simply another type of pooling where instead of pooling a
    connection or object, you are pooling remote data (database data), and placing it in memory (using
    Hashtable etc). Unused stateful session beans must be removed explicitly and appropriate idle timeout
    should be set to control stateful session bean life cycle.
    􀂃 Use effective design patterns to minimise network overheads (Session facade, Value Object etc Refer
    Q84, Q85 in Enterprise section), use of fast-lane reader pattern for database access (Refer Q86 in
    Enterprise section). Caching of retrieved JNDI InitialContexts, factory objects (e.g. EJB homes) etc. using
    the service locator design pattern, which reduces expensive JNDI access with the help of caching
    􀂃 Minimise serialization costs by marking references (like file handles, database connections etc), which do
    not required serialization by declaring them ‘transient’ (Refer Q19 in Java section). Use pass-by-reference
    where possible as opposed to pass by value.
    􀂃 Set appropriate timeouts: for the HttpSession objects, after which the session expires, set idle timeout for
    stateful session beans etc.
    􀂃 Improve the performance of database operations with the following tips:
    􀂃 Database connections should be released when not needed anymore, otherwise there will be potential
    resource leakage problems.
    􀂃 Apply least restrictive but valid transaction isolation level.
    􀂃 Use JDBC prepared statements for overall database efficiency and for batching repetitive inserts and
    updates. Also batch database requests to improve performance.
    􀂃 When you first establish a connection with a database by default it is in auto-commit mode. For better
    performance turn auto-commit off by calling the connection.setAutoCommit(false) method.
    􀂃 Where appropriate (you are loading 100 objects into memory but use only 5 objects) lazy load your
    data to avoid loading the whole database into memory using the virtual proxy pattern. Virtual proxy is
    an object, which looks like an object but actually contain no fields until when one of its methods is
    called does it load the correct object from the database.
    􀂃 Where appropriate eager load your data to avoid frequently accessing the database every time over
    the network.
    Enterprise – Logging, testing and deployment
    Q 126: Give an overview of log4J? SF
    A 126: Log4j is a logging framework for Java. Log4J is designed to be fast and flexible. Log4J has 3 main components
    which work together to enable developers to log messages:
    􀂃 Loggers [was called Category prior to version 1.2]
    􀂃 Appenders
    􀂃 Layout
    Enterprise Java
    Logger: The foremost advantage of any logging API like log4J, apache commons logging etc over plain
    System.out.println is its ability to disable certain log statements while allowing others to print unhindered.
    Loggers are hierarchical. The root logger exists at the top of the hierarchy. The root logger always exists and it
    cannot be retrieved by name. The hierarchical nature of the logger is denoted by “.” notation. For example the
    logger “java.util” is the parent of child logger “java.util.Vector” and so on. Loggers may be assigned priorities
    such as DEBUG, INFO, WARN, ERROR and FATAL. If a given logger is not assigned a priority, then it inherits
    the priority from its closest ancestor. The logging requests are made by invoking one of the following printing
    methods of the logger instance: debug(), info(), warn(), error(), fatal().
    Appenders and Layouts: In addition to selectively enabling and disabling logging requests based on the logger,
    the log4J allows logging requests to multiple destinations. In log4J terms the output destination is an appender.
    There are appenders for console, files, remote sockets, JMS, etc. One logger can have more than one
    appender. A logging request for a given logger will be forwarded to all the appenders in that logger plus the other
    appenders higher in the hierarchy. In addition to the output destination the output format can be categorised as
    well. This is accomplished by associating layout with an appender. The layout is responsible for formatting the
    logging request according to user’s settings.
    Sample configuration file:
    #set the root logger priority to DEBUG and its appender to App1
    log4j.rootLogger=DEBUG, App1
    #App1 is set to a console appender
    #appender App1 uses a pattern layout
    log4j.appender.App1.layout.ConversionPattern=%-4r [%t] %-5p %c %x -%m%n
    # Print only messages of priority WARN or above in the package com.myapp
    XML configuration for log4j is available, and is usually the best practise.
    Q 127: How do you initialize and use Log4J? SF CO
    A 127:
    public class MyApp {
    //Logger is a utility wrapper class to be written with appropriate printing methods
    static Logger log = Logger.getLogger (MyApp.class.getName());
    public void my method() {
    log.debug(“This line is reached………………………..” + var1 + “-” + var2);
    Q 128: What is the hidden cost of parameter construction when using Log4J? SF PI
    A 128:
    Do not use in frequently accessed methods or loops: CO
    log.debug (“Line number” + intVal + “ is less than ” + String.valueOf(array[i]));
    The above construction has a performance cost in frequently accessed methods and loops in constructing
    the message parameter, concatenating the String etc regardless of whether the message will be logged or not.
    Do use in frequently accessed methods or loops: CO
    If (log.isDebugEnabled()) {
    log.debug (“Line number” + intVal + “ is less than ” + String.valueOf(array[i]));
    Enterprise Java
    The above construction will avoid the parameter construction cost by only constructing the message parameter
    when you are in debug mode. But it is not a best practise to place log.isDebugEnabled() around all debug code.
    Q 129: What is the test phases and cycles? SD
    A 129:
    􀂃 Unit tests (e.g. JUnit etc, carried out by developers).
    There are two popular approaches to testing server-side classes: mock objects, which test classes by
    simulating the server container, and in-container testing, which tests classes running in the actual server
    container. If you are using Struts framework, StrutsTestCase for JUnit allows you to use either approach,
    with very minimal impact on your actual unit test code.
    􀂃 System tests or functional tests (carried out by business analysts and/or testers).
    􀂃 Integration tests (carried out by business analysts, testers, developers etc).
    􀂃 Regression tests (carried out by business analysts and testers).
    􀂃 Stress volume tests or load tests (carried out by technical staff).
    􀂃 User acceptance tests (UAT – carried out by end users).
    Each of the above test phases will be carried out in cycles. Refer Q13 in How would you go about… section for
    JUnit, which is an open source unit-testing framework.
    Q 130: Brief on deployment environments you are familiar with?
    A 130: Differ from project team to project team [Hint] :
    Application environments where “ear” files get deployed.
    Development box: can have the following instances of environments in the same machine (need not be
    􀂃 Development environment 􀃆 used by developers.
    􀂃 System testing environment 􀃆 used by business analysts.
    Staging box: can have the following instances of environments in the same machine (preferably clustered
    servers with load balancing)
    􀂃 Integration testing environment 􀃆 used for integration testing, user acceptance testing etc.
    􀂃 Pre-prod environment 􀃆 used for user acceptance testing, regression testing, and load testing or stress
    volume testing (SVT). [This environment should be exactly same as the production environment].
    Production box:
    􀂃 Production environment 􀃆 live site used by actual users.
    Data environments (Database)
    Note: Separate boxes [not the same boxes as where applications (i.e. ear files) are deployed]
    􀂃 Development box (database).
    Used by applications on development and system testing environments. Separate instances can be
    created on the same box for separate environments like development and system testing.
    􀂃 Staging Box (database)
    Used by applications on integration testing and user acceptance testing environments. Separate
    instances can be created on the same box for separate environments.
    􀂃 Production Box (database)
    Live data used by actual users of the system.
    Enterprise Java
    Enterprise - Personal
    Q 131: Tell me about yourself or about some of the recent projects you have worked with? What do you consider your
    most significant achievement? Why do you think you are qualified for this position? Why should we hire you and
    what kind of contributions will you make?
    A 131: [Hint:] Pick your recent projects and give a brief overview of it. Also it is imperative that during your briefing that
    you demonstrate how you applied your skills and knowledge in some of the following key areas and fixed any
    􀂃 Design Concepts: Refer Q02, Q03, Q19, Q20, Q21, Q91, Q98, and Q101.
    􀂃 Design Patterns: Refer Q03, Q24, Q25, Q83, Q84, Q85, Q86, Q87, Q88 and Q111.
    􀂃 Performance issues: Refer Q10, Q16, Q45, Q46, Q97, Q98, Q100, Q123, and Q125.
    􀂃 Memory issues: Refer Q45 and Q93
    􀂃 Multi-threading (Concurrency issues): Refer Q16, Q34, and Q113
    􀂃 Exception Handling: Refer Q76 and Q77
    􀂃 Transactional issues: Refer Q43, Q71, Q72, Q73, Q74, Q75 and Q77.
    􀂃 Security issues: Refer Q23, Q58, and Q81
    􀂃 Scalability issues: Refer Q20, Q21, Q120 and Q122.
    􀂃 Best practices: Refer Q10, Q16, Q39, Q40, Q46, Q82, Q124, and Q125
    Refer Q66 – Q72 in Java section for frequently asked non-technical questions.
    Q 132: Have you used any load testing tools?
    A 132: Rational Robot, JMeter, LoadRunner, etc.
    Q 133: What source control systems have you used? SD
    A 133: CVS, VSS (Visual Source Safe), Rational clear case etc. Refer Q13 in How would you go about section…. for
    Q 134: What operating systems are you comfortable with? SD
    A 134: NT, Unix, Linux, Solaris etc
    Q 135: Which on-line technical resources do you use to resolve any design and/or development issues?
    A 135:,,,, etc
    Enterprise – Software development process
    Q 136: What software development processes/principles are you familiar with? SD
    A 136: Agile (i.e. lightweight) software development process is gaining popularity and momentum across
    Agile software development manifesto 􀃆 [Good read:].
    􀂃 Highest priority is to satisfy the customer.
    􀂃 Welcome requirement changes even late in development life cycle.
    􀂃 Business people and developers should work collaboratively.
    􀂃 Form teams with motivated individuals who produce best designs and architectures.
    Enterprise Java
    􀂃 Teams should be pro-active on how to become more effective without becoming complacent.
    􀂃 Quality working software is the primary measure of progress.
    Why is iterative development with vertical slicing used in agile development? Your overall software quality
    can be improved through iterative development, which provides you with constant feedback.
    Traditional Vs Agile approach
    project time
    technical scope
    milestone 1
    milestone 2
    Data Layer
    Business Layer
    Data Layer
    Business Layer
    Data Layer
    milestone 3
    Presentation Layer
    Traditional approach
    With the tradional approach, Say for
    example w e have a fundamental flaw in
    the data layer, if this flaw gets only picked
    up after the milestone 3, then there w ill be
    lot of rew ork to be done to the business
    and the presentation layer. This is the
    major draw back w ith the traditional
    development approach w here there is no
    vertical slicing.
    Agile (light weight )approach
    technical scope
    iteration 1
    iteration 2
    Data Layer
    Business Layer
    iteration 3
    Presentation Layer
    s layesr
    project time
    Data Layer
    n Layer As you can see w ith the agileiterative
    approach, a vertical slice is built for each
    iteration. So any fundamental flaw in
    design or coding can be picked up early
    and rectified. Even deployment and testing
    w ill be carried out in vertical slices.
    Several methodologies fit under this agile development methodology banner. All these methodologies share
    many characteristics like iterative and incremental development, test driven development, stand up
    meetings to improve communication, automatic testing, build and continuous integration of code etc.
    Among all the agile methodologies XP is the one which has got the most attention. Different companies use
    different flavours of agile methodologies by using different combinations of methodologies.
    How does vertical slicing influence customer perception? With the iterative and incremental approach,
    customer will be comfortable with the progress of the development as opposed to traditional big bang approach.
    Tradional Vs Agile perceived functionality
    As far as the developer is concerned
    65% of coding has been completed but
    from the customer's view only 20% of
    the functionality has been completed
    Data Layer
    Business Layer
    Presentation Layer
    Traditional Agile
    As far as the developer is concerned
    65% of coding has been completed and
    from the customer's view 65% of the
    functionality has been completed. So
    the customer is happy.
    Data Layer
    Business Layer
    Presentation Layer
    􀂃 EXtreme Programming [XP] 􀃆 simple design, pair programming, unit testing, refactoring, collective code
    ownership, coding standards, etc. Refer Q10 in “How would you go about…” section. XP has four key
    values: Communication, Feedback, Simplicity and Courage. It then builds up some tried and tested
    practices and techniques. XP has a strong emphasis on testing where tests are integrated into continuous
    integration and build process, which yields a highly stable platform. XP is designed for smaller teams of 20
    – 30 people.
    􀂃 RUP (Rational Unified Process) 􀃆 Model driven architecture, design and development; customizable
    frameworks for scalable process; iterative development methodology; Re-use of architecture, code,
    component, framework, patterns etc. RUP can be used as an agile process for smaller teams of 20-30
    Enterprise Java
    people, or as a heavy weight process for larger teams of 50-100 people. Refer Q103 – Q105 in Enterprise
    􀂃 Feature Driven Development [FDD] 􀃆 Jeff De Luca and long time OO guru Peter Coad developed feature
    Driven Development (FDD). Like the other adaptive methodologies, it focuses on short iterations that
    deliver tangible functionality. FDD was originally designed for larger project teams of around 50 people. In
    FDD's case the iterations are two weeks long. FDD has five processes. The first three are done at the
    beginning of the project. The last two are done within each iteration.
    1. Develop an Overall Model
    2. Build a Features List
    3. Plan by Feature
    4. Design by Feature
    5. Build by Feature
    The developers come in two kinds: class owners and chief programmers. The chief programmers are the
    most experienced developers. They are assigned features to be built. However they don't build them alone.
    Instead the chief programmer identifies which classes are involved in implementing the feature and gathers
    their class owners together to form a feature team for developing that feature. The chief programmer acts
    as the coordinator, lead designer, and mentor while the class owners do much of the coding of the feature.
    􀂃 Test Driven Development [TDD] 􀃆 TDD is an iterative software development process where you first write
    the test with the idea that it must fail. Refer Q1 in Emerging Technologies/Frameworks section…
    􀂃 Scrum 􀃆 Scrum divides a project into sprints (aka iterations) of 30 days. Before you begin a sprint you
    define the functionality required for that sprint and leave the team to deliver it. But every day the team holds
    a short (10 – 15 minute) meeting, called a scrum where the team runs through what it will achieve in the
    next day. Some of the questions asked in the scrum meetings are:
    􀂃 What did you do since the last scrum meetings?
    􀂃 Do you have any obstacles?
    􀂃 What will you do before next meeting?
    This is very similar to stand-up meetings in XP and iterative development process in RUP.
    Enterprise – Key Points
    􀂃 J2EE is a 3-tier (or n-tier) system. Each tier is logically separated and loosely coupled from each other, and may be
    􀂃 J2EE applications are developed using MVC architecture, which divides the functionality of displaying and
    maintaining of the data to minimise the degree of coupling between enterprise components.
    􀂃 J2EE modules are deployed as ear, war and jar files, which are standard application deployment archive files.
    􀂃 HTTP is a stateless protocol and state can be maintained between client requests using HttpSession, URL rewriting,
    hidden fields and cookies. HttpSession is the recommended approach.
    􀂃 Servlets and JSPs are by default multi-threaded, and care should be taken in declaring instance variables and
    accessing shared resources. It is possible to have a single threaded model of a servlet or a JSP but this can
    adversely affect performance.
    􀂃 Clustering promotes high availability and scalability. The considerations for servlet clustering are:
    􀂃 Objects stored in the session should be serializable.
    􀂃 Design for idempotence.
    􀂃 Avoid using instance and static variables in read and write mode.
    􀂃 Avoid storing values in the ServletContext.
    􀂃 Avoid using* and use getResourceAsStream() instead.
    Enterprise Java
    􀂃 JSPs have a translation or a compilation process where the JSP engine translates and compiles a JSP file into a JSP
    􀂃 JSPs have 4 different scope values: page, request, session and application. JSPs can be included statically, where
    all the included JSP pages are compiled into a single servlet during the translation or compilation phase or included
    dynamically, where included JSPs are compiled into separate servlets and the content generated by these servlets
    are included at runtime in the JSP response.
    􀂃 Avoid scriptlet code in your JSPs and use JavaBeans or custom tags (e.g. Struts tags, JSTL tags, JSF tags etc)
    􀂃 Databases can run out cursors if the connections are not closed properly. The valuable resources like connections
    and statements should be enclosed in a try{} and finally{} block.
    􀂃 Prepared statements offer better performance as opposed to statements, as they are precompiled and reuse the
    same execution plan with different arguments. Prepared statements are also more secure because they use bind
    variables, which can prevent SQL injection attacks.
    􀂃 JNDI provides a generic interface to LDAP and other directory services like NDS, DNS etc.
    􀂃 In your code always make use of a logical JNDI reference (java:comp/env/ejb/MyBean) as opposed to physical
    JNDI reference (ejb/MyBean) because you cannot guarantee that the physical JNDI location you specify in your
    code will be available. Your code will break if the physical location is changed.
    􀂃 LDAP servers are typically used in J2EE applications to authenticate and authorise users. LDAP servers are
    hierarchical and are optimized for read access, so likely to be faster than database in providing read access.
    􀂃 RMI facilitates object method calls between JVMs. JVMs can be located on separate host machines, still one JVM
    can invoke methods belonging to an object residing in another JVM (i.e. address space). RMI uses object
    serialization to marshal and unmarshal parameters. The remote objects should extend the UnicastRemoteObject.
    􀂃 To go through a firewall, the RMI protocol can be embedded within the firewall trusted HTTP protocol, which is called
    HTTP tunnelling.
    􀂃 EJB (i.e. 2.x) is a remote, distributed multi-tier system, which supports protocols like JRMP, IIOP, and HTTP etc. EJB
    components contain business logic and system level supports like security, transaction, instance pooling, multithreading,
    object life-cycles etc are managed by the EJB container and hence simplify the programming effort.
    Having said this, there are emerging technologies like:
    􀂃 Hibernate, which is an open source object-to-relational (O/R) mapping framework.
    􀂃 EJB 3.0, which is taking ease of development very seriously and has adjusted its model to offer the plain old
    Java objects (i.e. POJOs) based persistence and the new O/R mapping model based on hibernate.
    Refer Q14 – Q18 in Emerging technologies / Frameworks section for brief discussion on hibernate and EJB 3.0.
    􀂃 EJB transaction attributes (like Required, Mandatory, RequiresNew, Supports etc) are specified declaratively through
    EJB deployment descriptors. Isolation levels are not part of the EJB 2.x specification. So the isolation levels can be
    set on the resource manager either explicitly on the Connection or via the application server specific configuration.
    􀂃 A transaction is often described by ACID (Atomic, Consistent, Isolated and Durable) properties. A distributed
    transaction is an ACID transaction between two or more independent transactional resources like two separate
    databases. A 2-phase commit is an approach for committing a distributed transaction in 2 phases.
    􀂃 EJB 2.x has two types of exceptions:
    􀂃 System exception: is an unchecked exception derived from java.lang.RuntimeException. It is thrown by the
    system and is not recoverable.
    􀂃 Application exception: is specific to an application and is thrown because of violation of business rules.
    􀂃 EJB container managed transactions are automatically rolled back when a system exception occurs. This is possible
    because the container can intercept system exceptions. However when an application exception occurs, the
    container does not intercept and leaves it to the code to roll back using ctx.setRollbackOnly() method.
    􀂃 EJB containers can make use of lazy loading (i.e. not creating an object until it is accessed) and dirty marker (ie
    persist only the entity beans that have bean modified) strategies to improve entity beans performance.
    Enterprise Java
    􀂃 Message Oriented Middleware (MOM) is a software infrastructure that asynchronously communicates with other
    disparate systems through the production and consumption of messages. Messaging enables loosely coupled
    distributed communication. Java Messaging Service (JMS) is a Java API that allows applications to create, send,
    receive read messages in a standard way, hence improves portability.
    􀂃 Some of the design decisions you need to make in JMS are message acknowledgement modes, transaction modes,
    delivery modes etc, synchronous vs. asynchronous paradigm, message body types, setting appropriate timeouts etc.
    􀂃 XML documents can be processed in your Java/J2EE application either using a SAX parser, which is event driven or
    a DOM parser, which creates a tree structure in memory. The other XML related technologies are DTD, XSD, XSL,
    XPath, etc and Java and XML based technologies are JAXP, JAXB etc.
    􀂃 There is an impedance mismatch between object and relational technology. Classes represent both data and
    behaviour whereas relational database tables just implement data. Inheritance class structure can be mapped to
    relational data model in one of the following ways:
    􀂃 Map class hierarchy to single database table.
    􀂃 Map each class to its own table.
    􀂃 Map each concrete class to its own table
    􀂃 Generic meta-data driven approach.
    􀂃 Normalize data in your database for accuracy and denormalize data in your database for performance.
    􀂃 RUP (Rational Unified Process) has 4 phases in the following order Inception, Elaboration, Construction, and
    Transition. Agile (i.e. lightweight) software development process is gaining popularity and momentum across
    organizations. Several methodologies like XP, RUP, Scrum, FDD, TDD etc fit under this agile development
    methodology banner. All these methodologies share many characteristics like iterative and incremental development,
    stand-up meetings to improve communication, automatic build, testing and continuous integration etc.
    􀂃 UML is applicable to the object oriented (OO) problem solving. There are different types of UML diagrams like use
    case diagrams, class diagrams, sequence diagrams, collaboration diagrams, state chart diagrams, activity diagrams,
    component diagrams, deployment diagrams etc.
    􀂃 Class diagrams are vital within OO methods. Class diagrams have the following possible relationships, association,
    aggregation, composition, generalization, and dependency.
    􀂃 Struts is an MVC framework. Struts action classes are not thread-safe and care should be taken in declaring instance
    variables or accessing other shared resources. JSF is another Web UI framework like Struts gaining popularity and
    􀂃 Log4j has three main components: loggers, appenders and layouts. Logger is a utility wrapper class. JUnit is an open
    source unit-testing framework.
    􀂃 You can improve the performance of a J2EE application as follows :
    1. Manage and recycle your valuable resources like connections, threads etc by either pooling or caching.
    2. Use effective design patterns like session façade, value object, fast lane reader etc to minimise network
    3. Set appropriate timeouts for HttpSession objects.
    4. Use JDBC prepared statements as opposed to statements.
    5. Release database connections in a finally {} block when finished.
    6. Apply least restrictive but valid transaction isolation level.
    7. Batch database requests.
    8. Minimise serialization costs by marking references like file handles, database connections, etc which do not
    require serialization by declaring them transient.
    􀂃 Some of the J2EE best practices are:
    1. Recycle your valuable resources by either pooling or caching.
    2. Automate your build process with tools like Ant, CruiseControl, and Maven etc, and continuously integrate your
    code into your build process.
    3. Build test cases first using tools like JUnit.
    4. Use standard J2EE packaging to improve portability.
    5. Apply appropriate proven design patterns.
    Enterprise Java
    6. Use proven frameworks like Struts, Spring, Hibernate, JSF, JUnit, Log4J, etc.
    7. Handle and propagate exceptions correctly.
    8. Avoid resource leaks by closing all database connections after you have used them.
    􀂃 The goals of application server clustering are to achieve scalability, load balancing, and high availability.
    􀂃 Java Management Extension (JMX) framework can improve the manageability of your application, for performance
    problems, critical events, error conditions etc and perform health checks on your hardware, database server etc.
    You can also configure and control your application at runtime.
    􀂃 Finally get familiarised with some of the key Java & J2EE design patterns like:
    1. MVC design pattern: J2EE uses this design pattern or architecture.
    2. Chain of responsibility design pattern: Servlet filters use a slightly modified version of chain of responsibility
    design pattern.
    3. Front controller J2EE design pattern: provides a centralized access point for HTTP request handling to
    support the integration system services like security, data validation etc. This is a popular J2EE design pattern.
    4. Composite view J2EE design pattern: creates an aggregate view from atomic sub-views.
    5. View helper J2EE design pattern: avoids duplication of code. The helper classes are JavaBeans and custom
    tags (e.g. Struts tags, JSF tags, JSTL tags etc).
    6. Service to worker and dispatcher view J2EE design pattern: These two patterns are a combination of front
    controller and view helper patterns with a dispatcher component. These two patterns differ in the way they
    suggest different division of responsibility among components.
    7. Bridge design pattern: Java Data Base Connectivity (JDBC) uses the bridge design pattern. The JDBC API
    provides an abstraction and the JDBC drivers provide the implementation.
    8. Proxy design pattern: RMI & EJB uses the proxy design pattern. A popular design pattern.
    9. Business delegate J2EE design pattern: used to reduce the coupling between the presentation tier and the
    business services tier components.
    10. Session façade J2EE design pattern: too many fine-grained method calls between the client and the server
    will lead to network overhead and tight coupling. Use a session bean as a façade to provide a coarse-grained
    service access layer to clients.
    11. Value object J2EE design pattern: avoid fine-grained method calls by creating a value object, which will help
    the client, make a coarse-grained call.
    12. Fast-lane reader J2EE design pattern: access the persistence layer directly using a DAO (Data Access
    Object) pattern instead of using entity beans.
    13. Service locator J2EE design pattern: expensive and redundant JNDI lookups can be avoided by caching
    and reusing the already looked up service objects.
    Recommended reading on J2EE design patterns:
    􀂃 Core J2EE Patterns: Best Practices and Design Strategies, Second Edition (Hardcover) by Deepak Alur, Dan Malks,
    John Crupi.
    Enterprise Java
    LF DC
    DP SF
    CI PI
    MI SI
    SE EH
    TI BP
    Let us put all together in
    the next section
    How would you go about…?
    How would you go about…?
    􀂃 This section basically assesses your knowledge of how to perform certain
    tasks like documenting your project, identifying any potential performance,
    memory, transactional, and/or design issues etc.
    􀂃 It also assesses if you have performed any of these tasks before. If you have
    not done a particular task, you can demonstrate that you know how to go about
    it if the task is assigned to you.
    􀂃 This section also recaps some of the key considerations discussed in the Java
    and Enterprise sections. Question numbers are used for cross-referencing
    with Java and Enterprise sections.
    􀂃 Q11 & Q13 are discussed in more detail and can be used as a quick reference
    guide in a software project. All the other questions excluding Q11 & Q13 can
    be read just before an interview.
    How would you go about…?
    Q 01: How would you go about documenting your Java/J2EE application?
    A 01: To be successful with a Java/J2EE project, proper documentation is vital.
    􀂃 Before embarking on coding get the business requirements down. Build a complete list of requested features,
    sample screen shots (if available), use case diagrams, business rules etc as a functional specification
    document. This is the phase where business analysts and developers will be asking questions about user
    interface requirements, data tier integration requirements, use cases etc. Also prioritize the features based on
    the business goals, lead-times and iterations required for implementation.
    􀂃 Prepare a technical specification document based on the functional specification. The technical
    specification document should cover:
    􀂙 Purpose of the document: This document will emphasise the customer service functionality …
    􀂙 Overview: This section basically covers background information, scope, any inclusions and/or
    exclusions, referenced documents etc.
    􀂙 Basic architecture: discusses or references baseline architecture document. Answers questions like
    Will it scale? Can this performance be improved? Is it extendable and/or maintainable? Are there any
    security issues? Describe the vertical slices to be used in the early iterations, and the concepts to be
    proved by each slice. Etc. For example which MVC [model-1, model-2 etc] paradigms (Refer Q3 in
    Enterprise section for MVC) should we use? Should we use Struts, JSF, Spring etc or build our own
    framework? Should we use a business delegate (Refer Q83 in Enterprise section for business delegate)
    to decouple middle tier with the client tier? Should we use AOP (Aspect Oriented Programming) (Refer
    Q3 in Emerging Technologies/Frameworks)? Should we use dependency injection? Should we use
    annotations? Do we require internationalization? Etc.
    􀂙 Assumptions, Dependencies, Risks and Issues: highlight all the assumptions, dependencies, risks
    and issues. For example list all the risks you can identify.
    􀂙 Design alternatives for each key functional requirement. Also discuss why a particular design
    alternative was chosen over the others. This process will encourage developers analyse the possible
    design alternatives without having to jump at the obvious solution, which might not always be the best
    􀂙 Processing logic: discuss the processing logic for the client tier, middle tier and the data tier. Where
    required add process flow diagrams. Add any pre-process conditions and/or post-process conditions.
    (Refer Q9 in Java section for design by contract).
    􀂙 UML diagrams to communicate the design to the fellow developers, solution designers, architects etc.
    Usually class diagrams and sequence diagrams are required. The other diagrams may be added for any
    special cases like (Refer Q107 in Enterprise section):
    􀂙 State chart diagram: useful to describe behaviour of an object across several usecases.
    􀂙 Activity diagram: useful to express complex operations. Supports and encourages parallel
    behaviour. Activity and statechart diagrams are beneficial for workflow modelling with multi
    threaded programming.
    􀂙 Collaboration and Sequence diagrams: Use a collaboration or sequence diagram when you
    want to look at behaviour of several objects within a single use case. If you want to look at a single
    object across multiple use cases then use statechart.
    􀂙 Object diagrams: The Object diagrams show instances instead of classes. They are useful for
    explaining some complicated objects in detail such as highlighting recursive relationships etc.
    􀂙 List the package names, class names, database names and table names with a brief description of
    their responsibility in a tabular form.
    􀂃 Prepare a coding standards document for the whole team to promote consistency and efficiency. Some
    coding practices can degrade performance for example:
    􀂙 Inappropriate use of String class. Use StringBuffer instead of String for compute intensive mutations
    (Refer Q17 in Java section).
    How would you go about…?
    􀂙 Code in terms of interface. For example you might decide the LinkedList is the best choice for some
    application, but then later decide ArrayList might be a better choice. (Refer Q15 in Java section)
    Wrong approach 􀃆 ArrayList list = new ArrayList();
    Right approach 􀃆 List list = new ArrayList(100)
    􀂙 Set the initial capacity of a collection appropriately (e.g. ArrayList, HashMap etc). (Refer Q15 in Java
    􀂙 To promote consistency define standards for variable names, method names, use of logging, curly
    bracket positions etc.
    􀂃 Prepare a code review document and templates for the whole team. Let us look at some of the elements the
    code review should cover:
    􀂙 Proper variable declaration: e.g. instance versus static variables, constants etc.
    􀂙 Performance issues: e.g. Use ArrayList, HashMap etc instead of Vector, Hashtable when there is
    no thread-safety issue.
    􀂙 Memory issues: e.g. Improper instantiation of objects instead of object reuse and object pooling, not
    closing valuable resource in a finally block etc.
    􀂙 Thread-safety issues: e.g. Java API classes like SimpleDateFormat, Calendar, DecimalFormat etc
    are not thread safe, declaring variables in JSP is not thread safe, storing state information in Struts
    action class or multi-threaded servlet is not thread safe.
    􀂙 Error handling: e.g. Re-throwing exception without nesting original exception, EJB methods not
    throwing EJB exception for system exceptions, etc.
    􀂙 Use of coding standards: e.g. not using frameworks, System.out is used instead of log4j etc.
    􀂙 Design issues: No re-use of code, no clear separation of responsibility, invalid use of inheritance to
    get method reuse, servlets performing JDBC direct access instead of using DAO (Data Access
    Objects) classes, HTML code in Struts action or servlet classes, servlets used as utility classes
    rather than as a flow controller etc.
    􀂙 Documentation of code: e.g. No comments, no header files etc
    􀂙 Bugs: e.g. Calling setAutoCommit within container-managed transaction, binary OR “|” used instead
    of logical OR “||”, relying on pass-by-reference in EJB remote calls, ResultSet not being closed on
    exceptions, EJB methods not throwing EJBException for system exceptions etc (Refer Q76 & Q77 in
    Enterprise section)
    􀂃 Prepare additional optional guideline documents as per requirements to be shared by the team. This will
    promote consistency and standards. For example:
    􀂙 Guidelines to setting up J2EE development environment.
    􀂙 Guidelines to version control system (CVS, VSS etc).
    􀂙 Guidelines to deployment steps, environment settings, ant targets etc.
    􀂙 Guidelines for the data modelling (any company standards).
    􀂙 Guidelines for error handling (Refer Q34, Q35 in Java section & Q76, Q77 in Enterprise section).
    􀂙 Guidelines for user interface design.
    􀂙 Project overview document.
    􀂙 Software development process document etc.
    Some of the above mentioned documents, which are shared by the whole team, can be published in an internal
    website like Wiki. Wiki is a piece of server software that allows users to freely create and edit Web page content
    using any Web browser.
    Q 02: How would you go about designing a Java/J2EE application?
    How would you go about…?
    A 02: Design should be specific to a problem but also should be general enough to address future requirements.
    Designing reusable object oriented software involves decomposing the business use cases into relevant objects
    and converting objects into classes.
    􀂃 Create a tiered architecture: client tier, business tier and data tier. Each tier can be further logically divided
    into layers (Refer Q2, Q3 on Enterprise section). Use MVC (Model View Controller architecture for the J2EE
    and Java based GUI applications).
    􀂃 Create a data model: A data model is a detailed specification of data oriented structures. This is different
    from the class modelling because it focuses solely on data whereas class models allow you to define both
    data and behaviour. Conceptual data models (aka domain models) are used to explore domain concepts
    with project stakeholders. Logical data models are used to explore the domain concepts, and their
    relationships. Logical data models depict entity types, data attributes end entity relationships (with Entity
    Relationship (ER) diagrams). Physical data models are used to design the internal schema of a database
    depicting the tables, columns, and the relationships between the tables. Data models can be created by
    performing the following tasks:
    􀂙 Identify entity types, attributes and relationships: use entity relationship (E-R) diagrams.
    􀂙 Apply naming conventions (e.g. for tables, attributes, indices, constraints etc): Your organization
    should have standards and guidelines applicable to data modelling.
    􀂙 Assign keys: surrogate keys (e.g. assigned by the database like Oracle sequences etc, max()+1,
    universally unique identifiers UUIDs, etc), natural keys (e.g. Tax File Numbers, Social Security Numbers
    etc), and composite keys.
    􀂙 Normalize to reduce data redundancy and denormalize to improve performance: Normalized data
    have the advantage of information being stored in one place only, reducing the possibility of inconsistent
    data. Furthermore, highly normalized data are loosely coupled. But normalization comes at a
    performance cost because to determine a piece of information you have to join multiple tables whereas
    in a denormalized approach the same piece of information can be retrieved from a single row of a table.
    Denormalization should be used only when performance testing shows that you need to improve
    database access time for some of your tables.
    Note: Creating a data model (logical, physical etc) before design model is a matter of preference, but many OO methodologies
    are based on creating the data model from the object design model (i.e. you may need to do some work to create an explicit
    data model but only after you have a complete OO domain and design model ). In many cases when using ORM tools like
    Hibernate, you do not create the data model at all.
    􀂃 Create a design model: A design model is a detailed specification of the objects and relationships between
    the objects as well as their behaviour. (Refer Q107 on Enterprise section)
    􀂙 Class diagram: contains the implementation view of the entities in the design model. The design model
    also contains core business classes and non-core business classes like persistent storage, security
    management, utility classes etc. The class diagrams also describe the structural relationships between
    the objects.
    􀂙 Use case realizations: are described in sequence and collaboration diagrams.
    􀂃 Design considerations when decomposing business use cases into relevant classes: designing
    reusable and flexible design models requires the following considerations:
    􀂙 Granularity of the objects (fine-grained, coarse-grained etc): Can we minimise the network trip by
    passing a coarse-grained value object instead of making 4 network trips with fine-grained parameters?
    (Refer Q85 in Enterprise section). Should we use method level (coarse-grained) or code level (finegrained)
    thread synchronization? (Refer Q40 in Java section). Should we use a page level access
    security or a fine-grained programmatic security?
    􀂙 Coupling between objects (loosely coupled versus strongly coupled). Should we use business
    delegate pattern to loosely couple client and business tier? (Refer Q83 in Enterprise section) or Should
    we use dependency injection? (Refer Q09 in Emerging Technologies/Frameworks).
    􀂙 Network overheads for remote objects like EJB, RMI etc: Should we use the session façade, value
    object patterns? (Refer Q84 & Q85 in Enterprise section).
    How would you go about…?
    􀂙 Definition of class interfaces and inheritance hierarchy: Should we use an abstract class or an
    interface? Is there any common functionality that we can move to the super class (or parent class)?
    Should we use interface inheritance with object composition for code reuse as opposed to
    implementation inheritance? Etc. (Refer Q8, Q10 in Java section).
    􀂙 Establishing key relationships (aggregation, composition, association etc): Should we use
    aggregation or composition? [composition may require cascade delete] (Refer Q107, Q108 in Enterprise
    section – under class diagrams). Should we use an “is a” (generalization) relationship or a “has a”
    (composition) relationship? (Refer Q7 in Java section).
    􀂙 Applying polymorphism and encapsulation: Should we hide the member variables to improve
    integrity and security? (Refer Q8 in Java section). Can we get a polymorphic behaviour so that we can
    easily add new classes in the future? (Refer Q8 in Java section).
    􀂙 Applying well-proven design patterns (like Gang of four design patterns, J2EE design patterns, EJB
    design patterns etc) help designers to base new designs on prior experience. Design patterns also help
    you to choose design alternatives (Refer Q11 in How would you go about…).
    􀂙 Scalability of the system: Vertical scaling is achieved by increasing the number of servers running on
    a single machine. Horizontal scaling is achieved by increasing the number of machines in the cluster.
    Horizontal scaling is more reliable than the vertical scaling because there are multiple machines involved
    in the cluster. In vertical scaling the number of server instances that can be run on one machine are
    determined by the CPU usage and the JVM heap memory.
    􀂙 How do we replicate the session state? Should we use stateful session beans or HTTP session?
    Should we serialize this object so that it can be replicated?
    􀂙 Internationalization requirements for multi-language support: Should we support other languages?
    Should we support multi-byte characters in the database?
    􀂃 Vertical slicing: Getting the reusable and flexible design the first time is impossible. By developing the initial
    vertical slice of your design you eliminate any nasty integration issues later in your project. Also get the
    design patterns right early on by building the vertical slice. It will give you experience with what does work and
    what does not work with Java/J2EE. Once you are happy with the initial vertical slice then you can apply it
    across the application. The initial vertical slice should be based on a typical business use case. Refer Q136
    in Enterprise section.
    􀂃 Ensure the system is configurable through property files, xml descriptor files, annotations etc. This will
    improve flexibility and maintainability. Avoid hard coding any values. Use a constant class for values, which
    rarely change and use property files, xml descriptor files, annotations etc for values, which can change more
    frequently (e.g. process flow steps etc) and/or environment related configurations(e.g. server name, server
    port, LDAP server location etc).
    􀂃 Design considerations during design, development and deployment phases: designing a fast, secured,
    reliable, robust, reusable and flexible system require considerations in the following key areas:
    􀂙 Performance issues (network overheads, quality of the code etc): Can I make a single coarse-grained
    network call to my remote object instead of 3 fine-grained calls?
    􀂙 Concurrency issues (multi-threading etc): What if two threads access my object simultaneously will it
    corrupt the state of my object?
    􀂙 Transactional issues (ACID properties): What if two clients access the same data simultaneously?
    What if one part of the transaction fails, do we rollback the whole transaction? What if the client
    resubmits the same transactional page again?
    􀂙 Security issues: Are there any potential security holes for SQL injection or URL injection by hackers?
    􀂙 Memory issues: Is there any potential memory leak problems? Have we allocated enough heap size for
    the JVM? Have we got enough perm space allocated since we are using 3rd party libraries, which
    generate classes dynamically? (e.g. JAXB, XSLT, JasperReports etc)
    􀂙 Scalability issues: Will this application scale vertically and horizontally if the load increases? Should
    this object be serializable? Does this object get stored in the HttpSession?
    How would you go about…?
    􀂙 Maintainability, reuse, extensibility etc: How can we make the software reusable, maintainable and
    extensible? What design patterns can we use? How often do we have to refactor our code?
    􀂙 Logging and auditing if something goes wrong can we look at the logs to determine the root cause of
    the problem?
    􀂙 Object life cycles: Can the objects within the server be created, destroyed, activated or passivated
    depending on the memory usage on the server? (e.g. EJB).
    􀂙 Resource pooling: Creating and destroying valuable resources like database connections, threads etc
    can be expensive. So if a client is not using a resource can it be returned to a pool to be reused when
    other clients connect? What is the optimum pool size?
    􀂙 Caching can we save network trips by storing the data in the server’s memory? How often do we have
    to clear the cache to prevent the in memory data from becoming stale?
    􀂙 Load balancing: Can we redirect the users to a server with the lightest load if the other server is
    􀂙 Transparent fail over: If one server crashes can the clients be routed to another server without any
    􀂙 Clustering: What if the server maintains a state when it crashes? Is this state replicated across the
    other servers?
    􀂙 Back-end integration: How do we connect to the databases and/or legacy systems?
    􀂙 Clean shutdown: Can we shut down the server without affecting the clients who are currently using the
    􀂙 Systems management: In the event of a catastrophic system failure who is monitoring the system? Any
    alerts or alarms? Should we use JMX? Should we use any performance monitoring tools like Tivoli etc?
    􀂙 Dynamic redeployment: How do we perform the software deployment while the site is running? (Mainly
    for mission critical applications 24hrs X 7days).
    􀂙 Portability issues: Can I port this application to a different server 2 years from now?
    Q 03: How would you go about identifying performance and/or memory issues in your Java/J2EE application?
    A 03: Profiling can be used to identify any performance issues or memory leaks. Profiling can identify what lines of code
    the program is spending the most time in? What call or invocation paths are used to reach at these lines? What
    kinds of objects are sitting in the heap? Where is the memory leak? Etc.
    􀂃 There are many tools available for the optimization of Java code like JProfiler, Borland OptimizeIt etc.
    These tools are very powerful and easy to use. They also produce various reports with graphs.
    Optimizeit™ Request Analyzer provides advanced profiling techniques that allow developers to analyse the
    performance behaviour of code across J2EE application tiers. Developers can efficiently prioritize the
    performance of Web requests, JDBC, JMS, JNDI, JSP, RMI, and EJB so that trouble spots can be
    proactively isolated earlier in the development lifecycle.
    Thread Debugger tools can be used to identify threading issues like thread starvation and contention issues
    that can lead to system crash.
    Code coverage tools can assist developers with identifying and removing any dead code from the
    􀂃 Hprof which comes with JDK for free. Simple tool.
    Java –Xprof myClass
    java -Xrunhprof:[help]|[

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