FREQUENTLY ASKED QUESTIONS (JAVA) IN INTERVIEWS


1) What is OOPs?

Ans: Object oriented programming organizes a program around its data, i.e., objects and a set of well defined

interfaces to that data. An object-oriented program can be characterized as data controlling access to code.

2) What is the difference between Procedural and OOPs?

Ans: a) In procedural program, programming logic follows certain procedures and the instructions are executed

one after another. In OOPs program, unit of program is object, which is nothing but combination of data

and code.

b) In procedural program, data is exposed to the whole program whereas in OOPs program, it is accessible

with in the object and which in turn assures the security of the code.

3) What are Encapsulation, Inheritance and Polymorphism?

Ans: Encapsulation is the mechanism that binds together code and data it manipulates and keeps both safe from

outside interference and misuse.

Inheritance is the process by which one object acquires the properties of another object.

Polymorphism is the feature that allows one interface to be used for general class actions.

4) What is the difference between Assignment and Initialization?

Ans: Assignment can be done as many times as desired whereas initialization can be done only once.

5) What are Class, Constructor and Primitive data types?

Ans: Class is a template for multiple objects with similar features and it is a blue print for objects. It defines a

type of object according to the data the object can hold and the operations the object can perform.

Constructor is a special kind of method that determines how an object is initialized when created.

Primitive data types are 8 types and they are:

byte, short, int, long

float, double

boolean

char

6) What is an Object and how do you allocate memory to it?

Ans: Object is an instance of a class and it is a software unit that combines a structured set of data with a set of

operations for inspecting and manipulating that data. When an object is created using new operator, memory

is allocated to it.

7) What is the difference between constructor and method?

Ans: Constructor will be automatically invoked when an objected is created whereas method has to be called

explicitly.

8) What are methods and how are they defined?

Ans: Methods are functions that operate on instances of classes in which they are defined. Objects can

communicate with each other using methods and can call methods in other classes.

Method definition has four parts. They are name of the method, type of object or primitive type the method

returns, a list of parameters and the body of the method. A method’s signature is a combination of the first

three parts mentioned above.

9) What is the use of bin and lib in JDK?

Ans: Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.

10) What is casting?

Ans: Casting is used to convert the value of one type to another.

11) How many ways can an argument be passed to a subroutine and explain them?

Ans: An argument can be passed in two ways. They are passing by value and passing by reference.

Passing by value: This method copies the value of an argument into the formal parameter of the subroutine.

Passing by reference: In this method, a reference to an argument (not the value of the argument) is passed to

the parameter.

12) What is the difference between an argument and a parameter?

Ans: While defining method, variables passed in the method are called parameters. While using those methods,

values passed to those variables are called arguments.

13) What are different types of access modifiers?

Ans: public: Any thing declared as public can be accessed from anywhere.

private: Any thing declared as private can’t be seen outside of its class.

protected: Any thing declared as protected can be accessed by classes in the same package and subclasses in

the other packages.

default modifier : Can be accessed only to classes in the same package.

14) What is final, finalize() and finally?

Ans: final : final keyword can be used for class, method and variables.

· A final class cannot be subclassed and it prevents other programmers from subclassing a secure class to invoke insecure methods.

· A final method can’ t be overridden

· A final variable can’t change from its initialized value.

    
          finalize( ) : finalize( ) method is used just before an object is destroyed and can be called just prior to 
          garbage collection.
          finally : finally, a key word used in exception handling, creates a block of code that will be executed after a 
          try/catch block has completed and before the code following the try/catch block. The finally block will 
          execute whether or not an exception is thrown. 
                        For example, if a method opens a file upon exit, then you will not want the code that closes the file 
          to be bypassed by the exception-handling mechanism. This finally keyword is designed to address this 
          contingency.       

15) What is UNICODE?

Ans: Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each

other.

16) What is Garbage Collection and how to call it explicitly?

Ans: When an object is no longer referred to by any variable, java automatically reclaims memory used by that

object. This is known as garbage collection.

System.gc() method may be used to call it explicitly.

17)                  What is  finalize() method ?
Ans: finalize( ) method is used just before an object is destroyed and can be called just prior to garbage collection.
 

18) What are Transient and Volatile Modifiers?

Ans: Transient : The transient modifier applies to variables only and it is not stored as part of its object’s

persistent state. Transient variables are not serialized.

Volatile : Volatile modifier applies to variables only and it tells the compiler that the variable modified by

volatile can be changed unexpectedly by other parts of the program.

19) What is method overloading and method overriding?

Ans: Method overloading : When a method in a class having the same method name with different arguments is

said to be method overloading.

Method overriding : When a method in a class having the same method name with

same arguments is said to be method overriding.

20) What is difference between overloading and overriding?

Ans: a) In overloading, there is a relationship between methods available in the same class whereas in overriding,

there is relationship between a superclass method and subclass method.

b) Overloading does not block inheritance from the superclass whereas overriding blocks inheritance from

the superclass.

c) In overloading, separate methods share the same name whereas in overriding, subclass method replaces

the superclass.

d) Overlaoding must have different method signatures whereas overriding must have same signature.

21) What is meant by Inheritance and what are its advantages?

Ans: Inheritance is the process of inheriting all the features from a class. The advantages of inheritance are

reusability of code and accessibility of variables and methods of the superclass by subclasses.

22) What is the difference between this and super?

Ans: this can be used to invoke a constructor of the same class whereas super can be used to invoke a super class

constructor.

23) What is the difference between superclasss and subclass?

Ans: A super class is a class that is inherited whereas sub class is a class that does the inheriting.

24) What modifiers may be used with top-level class?

Ans: public, abstract and final can be used for top-level class.

25) What are inner class and anonymous class?

Ans: Inner class : classes defined in other classes, including those defined in methods are called inner classes.

An inner class can have any accessibility including private.

Anonymous class : Anonymous class is a class defined inside a method without a name and is instantiated

and declared in the same place and cannot have explicit constructors.

26) What is a package?

Ans: A package is a collection of classes and interfaces that provides a high-level layer of access protection and

name space management.

27) What is a reflection package?  
Ans: java.lang.reflect package has the ability to analyze itself in runtime.

28) What is interface and its use?

Ans: Interface is similar to a class which may contain method’s signature only but not bodies and it is a formal

set of method and constant declarations that must be defined by the class that implements it.

Interfaces are useful for:

a) Declaring methods that one or more classes are expected to implement

b) Capturing similarities between unrelated classes without forcing a class relationship.

c) Determining an object’s programming interface without revealing the actual body of the class.

29) What is an abstract class?

Ans: An abstract class is a class designed with implementation gaps for subclasses to fill in and is deliberately

incomplete.

30) What is the difference between Integer and int?

Ans: a) Integer is a class defined in the java.lang package, whereas int is a primitive data type defined in the Java

language itself. Java does not automatically convert from one to the other.

b) Integer can be used as an argument for a method that requires an object, whereas int can be used for

calculations.

 
31) What is a cloneable interface and how many methods does it contain ?
Ans- It is not having any method because it is a TAGGED or MARKER interface.
 

32) What is the difference between abstract class and interface?

Ans: a) All the methods declared inside an interface are abstract whereas abstract class must have at least one

abstract method and others may be concrete or abstract.

b) In abstract class, key word abstract must be used for the methods whereas interface we need not use that

keyword for the methods.

c) Abstract class must have subclasses whereas interface can’t have subclasses.

33) Can you have an inner class inside a method and what variables can you access?

Ans: Yes, we can have an inner class inside a method and final variables can be accessed.

34) What is the difference between String and String Buffer?

Ans: a) String objects are constants and immutable whereas StringBuffer objects are not.

b) String class supports constant strings whereas StringBuffer class supports growable and modifiable

strings.

35) What is the difference between Array and vector?

Ans: Array is a set of related data type and static whereas vector is a growable array of objects and dynamic.

36) What is the difference between exception and error?

Ans: The exception class defines mild error conditions that your program encounters.

Ex: Arithmetic Exception, FilenotFound exception

Exceptions can occur when

-- try to open the file, which does not exist

-- the network connection is disrupted

-- operands being manipulated are out of prescribed ranges

-- the class file you are interested in loading is missing

The error class defines serious error conditions that you should not attempt to recover from. In most cases it

is advisable to let the program terminate when such an error is encountered.

Ex: Running out of memory error, Stack overflow error.

37) What is the difference between process and thread?

Ans: Process is a program in execution whereas thread is a separate path of execution in a program.

38) What is multithreading and what are the methods for inter-thread communication and what is the class

in which these methods are defined?

Ans: Multithreading is the mechanism in which more than one thread run independent of each other within the

process.

wait(), notify() and notifyAll() methods can be used for inter-thread communication and these methods are

in Object class.

wait( ) : When a thread executes a call to wait( ) method, it surrenders the object lock and enters into a

waiting state.

notify( ) or notifyAll( ) : To remove a thread from the waiting state, some other thread must make a call to

notify( ) or notifyAll( ) method on the same object.

39) What is the class and interface in java to create thread and which is the most advantageous method?

Ans: Thread class and Runnable interface can be used to create threads and using Runnable interface is the most

advantageous method to create threads because we need not extend thread class here.

40) What are the states associated in the thread?

Ans: Thread contains ready, running, waiting and dead states.

41) What is synchronization?

Ans: Synchronization is the mechanism that ensures that only one thread is accessed the resources at a time.

42) When you will synchronize a piece of your code?
Ans: When you expect your code will be accessed by different threads and these threads may change a particular 
         data causing data corruption.
  

43) What is deadlock?

Ans: When two threads are waiting each other and can’t proceed the program is said to be deadlock.

44) What is daemon thread and which method is used to create the daemon thread?

Ans: Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage

collection operation for the java runtime system.

setDaemon method is used to create a daemon thread.

45) Are there any global variables in Java, which can be accessed by other part of your program?

Ans: No, it is not the main method in which you define variables. Global variables is not possible because

concept of encapsulation is eliminated here.

46) What is an applet?

Ans: Applet is a dynamic and interactive program that runs inside a web page displayed by a java capable

browser.

47) What is the difference between applications and applets?

Ans: a) Application must be run on local machine whereas applet needs no explicit installation on local

machine.

b) Application must be run explicitly within a java-compatible virtual machine whereas applet loads and

runs itself automatically in a java-enabled browser.

d) Application starts execution with its main method whereas applet starts execution with its init method.

e) Application can run with or without graphical user interface whereas applet must run within a graphical user interface.

48) How does applet recognize the height and width?

Ans: Using getParameters() method.

49) When do you use codebase in applet?

Ans: When the applet class file is not in the same directory, codebase is used.

50) What is the lifecycle of an applet?

Ans: init( ) method - Can be called when an applet is first loaded

start( ) method - Can be called each time an applet is started

paint( ) method - Can be called when the applet is minimized or maximized

stop( ) method - Can be used when the browser moves off the applet’s page

destroy( ) method - Can be called when the browser is finished with the applet

51) How do you set security in applets?

Ans: using setSecurityManager() method

52) What is an event and what are the models available for event handling?

Ans: An event is an event object that describes a state of change in a source. In other words, event occurs when an

action is generated, like pressing button, clicking mouse, selecting a list,etc.

There are two types of models for handling events and they are:

a) event-inheritance model and b) event-delegation model

53) What are the advantages of the model over the event-inheritance model?

Ans: The event-delegation model has two advantages over the event-inheritance model. They are:

a) It enables event handling by objects other than the ones that generate the events. This allows a clean separation between a component’s design and its use.

b) It performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to be repeatedly process unhandled events as is the case of the event-inheritance.

54) What is source and listener ?

Ans: source : A source is an object that generates an event. This occurs when the internal state of that object

changes in some way.

listener : A listener is an object that is notified when an event occurs. It has two major requirements. First, it

must have been registered with one or more sources to receive notifications about specific types of events.

Second, it must implement methods to receive and process these notifications.

55) What is adapter class?

Ans: An adapter class provides an empty implementation of all methods in an event listener interface. Adapter

classes are useful when you want to receive and process only some of the events that are handled by a

particular event listener interface. You can define a new class to act listener by extending one of the adapter

classes and implementing only those events in which you are interested.

For example, the MouseMotionAdapter class has two methods, mouseDragged( )and mouseMoved( ). The

signatures of these empty are exactly as defined in the MouseMotionListener interface. If you are interested

in only mouse drag events, then you could simply extend MouseMotionAdapter and implement

mouseDragged( ) .

56) What is meant by controls and what are different types of controls in AWT?

Ans: Controls are components that allow a user to interact with your application and the AWT supports the

following types of controls:

Labels, Push Buttons, Check Boxes, Choice Lists, Lists, Scrollbars, Text Components.

These controls are subclasses of Component.

57) What is the difference between choice and list?

Ans: A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices

and only one item may be selected from a choice.

A List may be displayed in such a way that several list items are visible and it supports the selection of one

or more list items.

58) What is the difference between scrollbar and scrollpane?

Ans: A Scrollbar is a Component, but not a Container whereas Scrollpane is a Conatiner and handles its own

events and perform its own scrolling.

59) What is a layout manager and what are different types of layout managers available in java.awt?

Ans: A layout manager is an object that is used to organize components in a container. The different layouts are

available are FlowLayout, BorderLayout, CardLayout, GridLayout and GridBagLayout.

60) How are the elements of different layouts organized?

Ans: FlowLayout: The elements of a FlowLayout are organized in a top to bottom, left to right fashion.

BorderLayout: The elements of a BorderLayout are organized at the borders(North, South, East and West)

and the center of a container.

CardLayout: The elements of a CardLayout are stacked, on top of the other, like a deck of cards.

GridLayout: The elements of a GridLayout are of equal size and are laid out using the square of a grid.

GridBagLayout: The elements of a GridBagLayout are organized according to a grid.

However, the elements are of different size and may occupy more than one row or column of the grid. In

addition, the rows and columns may have different sizes.

61) Which containers use a Border layout as their default layout?

Ans: Window, Frame and Dialog classes use a BorderLayout as their layout.

62) Which containers use a Flow layout as their default layout?

Ans: Panel and Applet classes use the FlowLayout as their default layout.

63) What are wrapper classes?

Ans: Wrapper classes are classes that allow primitive types to be accessed as objects.

64) What are Vector, Hashtable, LinkedList and Enumeration?

Ans: Vector : The Vector class provides the capability to implement a growable array of objects.

Hashtable : The Hashtable class implements a Hashtable data structure. A Hashtable indexes and stores

objects in a dictionary using hash codes as the object’s keys. Hash codes are integer values that identify

objects.

LinkedList : Removing or inserting elements in the middle of an array can be done using LinkedList. A

LinkedList stores each object in a separate link whereas an array stores object references in consecutive

locations.

Enumeration: An object that implements the Enumeration interface generates a series of elements, one at a time. It

has two methods,namely hasMoreElements( ) and nextElement( ). HasMoreElemnts( ) tests if this enumeration has

more elements and nextElement method returns successive elements of the series.

65) What is the difference between set and list?

Ans: Set stores elements in an unordered way but does not contain duplicate elements, whereas list stores

elements in an ordered way but may contain duplicate elements.

66) What is a stream and what are the types of Streams and classes of the Streams?

Ans: A Stream is an abstraction that either produces or consumes information. There are two types of Streams

and they are:

Byte Streams: Provide a convenient means for handling input and output of bytes.

Character Streams: Provide a convenient means for handling input & output of characters.

Byte Streams classes: Are defined by using two abstract classes, namely InputStream and OutputStream.

Character Streams classes: Are defined by using two abstract classes, namely Reader and Writer.

67) What is the difference between Reader/Writer and InputStream/Output Stream?

Ans: The Reader/Writer class is character-oriented and the InputStream/OutoutStream class is byte-oriented.

68) What is an I/O filter?

Ans: An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some

way as it is passed from one stream to another.

69) What is serialization and deserialization?

Ans: Serialization is the process of writing the state of an object to a byte stream.

Deserialization is the process of restoring these objects.

70) What is JDBC?

Ans: JDBC is a set of Java API for executing SQL statements. This API consists of a set of classes and interfaces

to enable programs to write pure Java Database applications.

71) What are drivers available?

Ans: a) JDBC-ODBC Bridge driver

b) Native API Partly-Java driver

c) JDBC-Net Pure Java driver

d) Native-Protocol Pure Java driver

72) What is the difference between JDBC and ODBC?

Ans: a) OBDC is for Microsoft and JDBC is for Java applications.

b) ODBC can’t be directly used with Java because it uses a C interface.

c) ODBC makes use of pointers which have been removed totally from Java.

d) ODBC mixes simple and advanced features together and has complex options for simple queries. But JDBC is designed to keep things simple while allowing advanced capabilities when required.

e) ODBC requires manual installation of the ODBC driver manager and driver on all client machines. JDBC drivers are written in Java and JDBC code is automatically installable, secure, and portable on all platforms.

f) JDBC API is a natural Java interface and is built on ODBC. JDBC retains some of the basic features of ODBC.

73) What are the types of JDBC Driver Models and explain them?

Ans: There are two types of JDBC Driver Models and they are:

a) Two tier model and b) Three tier model

Two tier model: In this model, Java applications interact directly with the database. A JDBC driver is required to communicate with the particular database management system that is being accessed. SQL statements are sent to the database and the results are given to user. This model is referred to as client/server configuration where user is the client and the machine that has the database is called as the server.

Three tier model: A middle tier is introduced in this model. The functions of this model are:

a) Collection of SQL statements from the client and handing it over to the database,

b) Receiving results from database to the client and

c) Maintaining control over accessing and updation of the above.

74) What are the steps involved for making a connection with a database or how do you connect to a

database?

Ans: a) Loading the driver : To load the driver, Class.forName( ) method is used.

Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);

When the driver is loaded, it registers itself with the java.sql.DriverManager class as an available

database driver.

b) Making a connection with database : To open a connection to a given database,

DriverMnaager.getConnection( ) method is used.

Connection con = DriverManager.getConnection(“jdbc:odbc:somedb”, “user”, ”password”);

c) Executing SQL statements : To execute a SQL query, java.sql.statements class is used.

createStatement( ) method of Connection to obtain a new Statement object.

Statement stmt = con.createStatement( );

A query that returns data can be executed using the executeQuery( ) method of Statement. This method

executes the statement and returns a java.sql.ResultSet that encapsulates the retrieved data:

ResultSet rs = stmt.executeQuery(“SELECT * FROM some table”);

d) Process the results : ResultSet returns one row at a time. Next( ) method of ResultSet object can be called to move to the next row. The getString( ) and getObject( ) methods are used for retrieving column values:

while(rs.next( ) ) {

String event = rs.getString(“event”);

Object count = (Integer) rs.getObject(“count”);

75) What type of driver did you use in project?

Ans: JDBC-ODBC Bridge driver (is a driver that uses native(C language) libraries and makes calls to an existing

ODBC driver to access a database engine).

76) What are the types of statements in JDBC?

Ans: Statement -- To be used createStatement( ) method for executing single SQL statement

PreparedStatement -- To be used preparStatement( ) method for executing same SQL statement over and

over

CallableStatement -- To be used prepareCall( ) method for multiple SQL statements over and over

77) What is stored procedure?

Ans: Stored procedure is a group of SQL statements that forms a logical unit and performs a particular task.

Stored Procedures are used to encapsulate a set of operations or queries to execute on database. Stored

procedures can be compiled and executed with different parameters and results and may have any

combination of input/output parameters.

78) How to create and call stored procedures?

Ans: To create stored procedures:

Create procedure procedure name (specify in, out and in out parameters)

BEGIN

Any multiple SQL statement;

END;

To call stored procedures:

CallableStatement csmt = con.prepareCall(“{call procedure name(?,?)}”);

csmt.registerOutParameter(column no., data type);

csmt.setInt(column no., column name)

csmt.execute( );

79) What is servlet?

Ans: Servlets are modules that extend request/response-oriented servers, such as java-enabled web servers.

For example, a servlet might be responsible for taking data in an HTML order-entry form and applying the

business logic used to update a company’s order database.

80) What are the classes and interfaces for servlets?

Ans: There are two packages in servlets and they are javax.servlet and javax.servlet.http.

Javax.servlet contains:

Interfaces Classes

Servlet Generic Servlet

ServletRequest ServletInputStream

ServletResponse ServletOutputStream

ServletConfig ServletException

ServletContext UnavailableException

SingleThreadModel

Javax.servlet.http contains:

Interfaces Classes

HttpServletRequest Cookie

HttpServletResponse HttpServlet

HttpSession HttpSessionBindingEvent

HttpSessionCintext HttpUtils

HttpSeesionBindingListener

81) What is the difference between an applet and a servlet?

Ans: a) Servlets are to servers what applets are to browsers.

b) Applets must have graphical user interfaces whereas servlets have no graphical user interfaces.

82) What is the difference between doPost and doGet methods?

Ans: a) doGet( ) method is used to get information, while doPost( ) method is used for posting information.

b) doGet( ) requests can’t send large amount of information and is limited to 240-255 characters. However,

doPost( )requests passes all of its data, of unlimited length.

c) A doGet( ) request is appended to the request URL in a query string and this allows the exchange is

visible to the client, whereas a doPost( ) request passes directly over the socket connection as part of its

HTTP request body and the exchange is invisible to the client.

83) What is the life cycle of a servlet?

Ans: Each Servlet has the same life cycle:

a) A server loads and initializes the servlet by init( ) method.

b) The servlet handles zero or more client’s requests through service( ) method.

c) The server removes the servlet through destroy( ) method.

84) Who is loading the init() method of servlet?
Ans: Web server
 
85) What are the different servers available for developing and deploying Servlets?
Ans:  a)    Java Web Server
b)      JRun
g)      Apache Server
h)      Netscape Information Server
i)        Web Logic        
 
86) How many ways can we track client  and what are they?
Ans: The servlet API provides two ways to track client state and they are:
          a) Using Session tracking and b) Using Cookies. 
 
87) What is session tracking and how do you track a user session in servlets?
Ans: Session tracking is a mechanism that servlets use to maintain state about a series requests 
         from the same user across some period of time. The methods used for session tracking are:
a)      User Authentication – occurs when a web server restricts access to some of its resources to only those clients that log in using a recognized username and password 
b)      Hidden form fields – fields are added to an HTML form that are not displayed in the client’s browser. When the form containing the fields is submitted, the fields are sent back to the server 
c)       URL rewriting – every URL that the user clicks on is dynamically modified or rewritten to include extra information. The extra information can be in the form of extra path information, added parameters or some custom, server-specific URL change.
d)      Cookies – a bit of information that is sent by a web server to a browser and which can later be read back from that browser.
e)      HttpSession- places a limit on the number of sessions that can exist in memory. This limit is set in the session.maxresidents property
 
88) What is Server-Side Includes(SSI)?
Ans: Server-Side Includes allows to embed servlets within HTML pages using a special servlet tag. In many 
         servlets that support servlets, a page can be processed by the server to include output from servlets at certain 
         points inside the HTML page. This is accomplished using a  special internal SSINCLUDE, which processes 
         the servlet tags. SSINCLUDE servlet will be invoked whenever a file with an .shtml extension is requested. 
         So HTML files that include  server-side includes must be stored with an .shtml extension.
 
89) What are cookies and how will you use them?
Ans: Cookies are a mechanism that a servlet uses to have a client hold a small amount of state-information 
         associated with the user.
         a)    Create a cookie with the Cookie constructor:
                             public Cookie(String name, String value)
b)       A servlet can send a cookie to the client by passing a Cookie object to the addCookie( ) method of 
        HttpServletResponse:
                             public void HttpServletResponse.addCookie(Cookie cookie)
c)       A servlet retrieves cookies by calling the getCookies( ) method of HttpServletRequest:
                   public Cookie[ ] HttpServletRequest.getCookie( ).
 
 
 
 
90) Is it possible to communicate from an applet to servlet and how many ways and how?
Ans: Yes, there are three ways to communicate from an applet to servlet and they are:
a)      HTTP Communication(Text-based and object-based)
b)      Socket Communication
c)       RMI Communication
          (You can say , by using URL object open the connection to server and get the InputStream  from 
            URLConnection object).
           Steps involved for applet-servlet communication:
1)      Get the server URL.
URL url = new URL( );
2)      Connect to the host
URLConnection Con = url.openConnection( );
3)      Initialize the connection
Con.setUseCatches(false):
Con.setDoOutput(true);
Con.setDoInput(true);
4)      Data will be written to a byte array buffer so that we can tell the server the length of the data .
ByteArrayOutputStream byteout  = new ByteArrayOutputStream( );
5)      Create the OutputStream to be used to write the data to the buffer.
DataOutputStream out = new DataOutputStream(byteout); 
 
91) What is connection pooling?
Ans: With servlets, opening a database connection is a major bottleneck because we are creating and tearing 
         down a new connection for every page request and the time taken to create connection will be more. 
         Creating a connection pool is an ideal approach for a complicated servlet. With a connection pool, we can 
         duplicate only the resources we need to duplicate rather than the entire servlet. A connection pool can also 
         intelligently manage the size of  the pool and make sure each connection remains valid. A number of 
         connection pool  packages are currently available. Some like DbConnectionBroker are freely available from 
         Java Exchange Works by creating an object that dispenses connections and connection Ids on request.
         The ConnectionPool class maintains a Hastable, using Connection objects as keys and Boolean values as 
         stored values. The Boolean value indicates whether a connection is in use or not. A program calls     
         getConnection( ) method of the ConnectionPool for getting  Connection object it can use; it calls 
         returnConnection( ) to give the connection back to the pool.
 
92) Why should we go for interservlet communication?
Ans: Servlets running together in the same server communicate with each other in several ways.
         The three major reasons to use interservlet communication are:
a)      Direct servlet manipulation – allows to gain access to the other currently loaded servlets and perform certain tasks (through the ServletContext object)
b)      Servlet reuse – allows the servlet to reuse the public methods of another servlet.
c)       Servlet collaboration – requires to communicate with each other by sharing specific information (through method invocation)
 
93) Is it possible to call servlet with parameters in the URL? 
Ans: Yes. You can call a servlet with parameters in the syntax as(?Param1 = xxx|| m2 = yyy).
 
94) What is Servlet chaining?
Ans: Servlet chaining is a technique in which two or more servlets can cooperate in servicing a single request.
         In servlet chaining, one servlet’s output is piped to the next servlet’s input. This process continues until the 
         last servlet is reached. Its output is then sent back to the client.
 
95) How do servlets handle multiple simultaneous requests?
Ans: The server has multiple threads that are available to handle requests. When a request comes  in, it is 
         assigned to a thread, which calls a service method(for example: doGet( ), doPost( ) and service( ) ) of the 
         servlet. For this reason, a single servlet object can have its service  methods called by many threads at once.
 
96) What is the difference between TCP/IP and UDP ?
Ans: TCP/IP is a two-way communication between the client and the server and it is a reliable and there is a 
         confirmation regarding reaching the message to the destination. It is like a phone call. 
         UDP is a one-way communication only between the client and the server and it is not a reliable and there is 
         no confirmation regarding reaching the message to the destination. It is like a postal mail.
 
97) What is Inet address?
Ans: Every computer connected to a network has an IP address. An IP address is a number that  uniquely 
          identifies each computeron the Net. An IP address is a 32-bit number.
 
98) What is Domain Naming Service(DNS)?
Ans: It is very difficult to remember a set of numbers(IP address) to connect to the Internet. The Domain Naming 
         Service(DNS) is used to overcome this problem. It maps one particular IP address to a string of characters.
         For example, www.mascom.com implies com is the domain name reserved for US commercial sites, 
         moscom is the name of the company and www is the name of the specific computer, which is mascom’s 
         server.
 
99) What is URL?
Ans: URL stands for Uniform Resource Locator and it points to resource files on the Internet.
         URL has four components:
         http://www.Pentafour.com:80/index.html
         http – protocol name, Pentafour – IP address or host name, 80 – port number and  index.html – file path.
 
100) What is RMI and steps involved in developing an RMI object?
Ans: Remote Method Invocation(RMI) allows java object that executes on one machine and to invoke the method 
         of a Java object to execute on another machine.
         The steps involved in developing an RMI object are:
a)      Define the interfaces
b)      Implementing these interfaces
c)       Compile the interfaces and their implementations with the java compiler 
d)      Compile the server implementation with RMI compiler
e)      Run the RMI registry
f)        Run the application
 
101) What is RMI architecture ? 
Ans: - RMI architecture consists of four layers and each layer performs specific functions:
            a) Application layer             ----   contains the actual object definition
            b) Proxy layer                      ----   consists of stub and skeleton
            c) Remote Reference layer  ----   gets the stream of bytes from the transport layer and sends it to the proxy 
                                                                 layer 
            d) Transportation layer        ----   responsible for handling the actual machine to machine communication
 
102) What is UnicastRemoteObject?
Ans: All remote objects must extend UnicastRemoteObject, which provides functionality that is  needed to make 
         objects available from remote machines.
 
103) Explain the methods, rebind( ) and lookup() in Naming class?
Ans: rebind( ) of the Naming class(found in java.rmi) is used to update the RMI registry on the server machine.
         Naming.rebind(“AddSever”, AddServerImpl); 
          
         lookup( ) of the Naming class accepts one argument, the rmi URL and returns a  reference to an object of 
         type AddServerImpl.
 
104) What is a Java Bean?
Ans: A Java Bean is a software component that has been designed to be reusable in a variety of different 
         environments.   
 
105) What is a Jar file?
Ans: Jar file allows to efficiently deploy a set of classes and their associated resources. The elements in a jar file 
         are compressed, which makes downloading a Jar file much faster than separately downloading several 
         uncompressed files.
         The package java.util.zip contains classes that read and write jar files. 
 
106) What is BDK?
Ans: BDK,Bean Development Kit is a tool that enables to create, configure and connect a set of set of Beans and 
         it can be used to test Beans without writing a code. 
  

107) What is JSP?

Ans: JSP is a dynamic scripting capability for web pages that allows Java as well as a few special tags to be

embedded into a web file (HTML/XML, etc). The suffix traditionally ends with .jsp to indicate to the web

server that the file is a JSP file. JSP is a server side technology - you can't do any client side validation with it.

The advantages are:

a) The JSP assists in making the HTML more functional. Servlets on the other hand allow outputting of

HTML but it is a tedious process.

b) It is easy to make a change and then let the JSP capability of the web server you are using deal with

compiling it into a servlet and running it.

108) What are JSP scripting elements?

Ans: JSP scripting elements lets to insert Java code into the servlet that will be generated from the current JSP page. There

are three forms:

a) Expressions of the form <%= expression %> that are evaluated and inserted into the output,

b) Scriptlets of the form <% code %> that are inserted into the servlet's service method, and

c) Declarations of the form <%! Code %> that are inserted into the body of the servlet class, outside of any existing

methods.

109) What are JSP Directives?

Ans: A JSP directive affects the overall structure of the servlet class. It usually has the following form:

    <%@ directive attribute="value" %>

However, you can also combine multiple attribute settings for a single directive, as follows:

    <%@ directive attribute1="value1" 
              attribute2="value2"
              ...
              attributeN="valueN" %>

There are two main types of directive: page, which lets to do things like import classes, customize the servlet

superclass, and the like; and include, which lets to insert a file into the servlet class at the time the JSP file is

translated into a servlet

110) What are Predefined variables or implicit objects?

Ans: To simplify code in JSP expressions and scriptlets, we can use eight automatically defined variables, sometimes

called implicit objects. They are request, response, out, session, application, config,

pageContext, and page.

111) What are JSP ACTIONS?

Ans: JSP actions use constructs in XML syntax to control the behavior of the servlet engine. You can dynamically insert a

file, reuse JavaBeans components, forward the user to another page, or generate HTML for the Java plugin. Available

  • actions include:
  • jsp:useBean - Find or instantiate a JavaBean.
  • jsp:setProperty - Set the property of a JavaBean.
  • jsp:getProperty – Insert the property of a JavaBean into the output.
  • jsp:forward – Forward the requester to a newpage.
  • Jsp:plugin – Generate browser-specific code that makes an OBJECT or EMBED

112) How do you pass data (including JavaBeans) to a JSP from a servlet?

Ans: (1) Request Lifetime: Using this technique to pass beans, a request dispatcher (using either "include" or

forward") can be called. This bean will disappear after processing this request has been completed.

Servlet:

    request.setAttribute("theBean", myBean);
    RequestDispatcher rd =
        getServletContext().getRequestDispatcher('/thepage.jsp");
    rd.forward(request, response);
JSP PAGE:
    

(2) Session Lifetime: Using this technique to pass beans that are relevant to a particular session

(such as in individual user login) over a number of requests. This bean will disappear when the

session is invalidated or it times out, or when you remove it.

Servlet:
    HttpSession session = request.getSession (true);
    session.putValue("theBean", myBean);
    /* You can do a request dispatcher here,
        or just let the bean be visible on the
        next request */
JSP Page:
    

3) Application Lifetime: Using this technique to pass beans that are relevant to all servlets and JSP pages in a particular app, for all users. For example, I use this to make a JDBC connection pool object available to the various servlets and JSP pages in my apps. This bean will disappear when the servlet engine is shut down, or when you remove it.

Servlet:
    getServletContext().setAttribute("theBean", myBean);
JSP PAGE:
    
 
113) How can I set a cookie in JSP?
Ans: response.setHeader("Set-Cookie", "cookie string");

To give the response-object to a bean, write a method setResponse
(HttpServletResponse response)
- to the bean, and in jsp-file:

<%
bean.setResponse (response);
%>

114) How can I delete a cookie with JSP?

Ans: Say that I have a cookie called "foo," that I set a while ago & I want it to go away. I simply:

<%

Cookie killCookie = new Cookie("foo", null);

killCookie.setPath("/");

killCookie.setMaxAge(0);

response.addCookie(killCookie);

%>

115) How are Servlets and JSP Pages related?

Ans: JSP pages are focused around HTML (or XML) with Java codes and JSP tags inside them. When a web

server that has JSP support is asked for a JSP page, it checks to see if it has already compiled the page into a

servlet. Thus, JSP pages become servlets and are transformed into pure Java and then compiled, loaded into

the server and executed.

116) What will be the out put of this programme

public class try1

{

public static void main(String args[])

{

try1 t1 = new try1();

t1.test();

}

public String test()

{

System.out.println("Alpesh u r here1");

return "try1";

}

public void test()

{

System.out.println("Alpesh u r here2");

}

}

Ans: Compilation error - Methods can't be redefined with a different return type.

117) What will be the o/p of this programme if I execute the programme at command prompt command
java try1
public class try1

{

public static void main(String args[])

{

System.out.println("Hi "+args[0]);

}

}
Ans: Runtime error : Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0

118) What will be the o/p of this programme

public class try1

{

public static void main(String args[])

{

String s1 = new String("amit");

String s2 = s1.replace('i','o');

s1.concat("Poddar");

System.out.println(s1);

System.out.println((s1+s2).charAt(5));

}

}

Ans: amit

m

public class try1

{

public static void main(String args[])

{

String s1 = new String("amit");//s1 is amit

String s2 = s1.replace('i','o');// s2 is amot and s1 is amit

s1.concat("Poddar");//s1 is amit as you have not assigned the o/p to s1 and s2 is amot

System.out.println(s1);//amit

System.out.println((s1+s2).charAt(5));//amitamot so “m”

}

}

119.) What are the differences between the == operator and the equals() method?

Ans.) Well, first off, == is a fundamental operator in the language. The result type of the expression is a boolean. For comparing boolean types, it compares the operands for the same truth value. For comparing reference types, it compares the operands for the same reference value (i.e., refer to the same object or are both null). For numeric types, it compares the operands for the same integer value or equivalent floating point values. See the Java Language Specification, section 15.20 for more information.

In contrast, equals() is an instance method which is fundamentally defined by the java.lang.Object class. This method, by convention, indicates whether the receiver object is "equal to" the passed in object. The base implementation of this method in the Object class checks for reference equality. Other classes, including those you write, may override this method to perform more specialized equivalence testing. See the Java Language Specification, section 20.1.3 for more information.

The typical "gotcha" for most people is in using == to compare two strings when they really should be using the String class's equals() method. From above, you know that the operator will only return "true" when both of the references refer to the same actual object. But, with strings, most uses want to know whether or not the value of the two strings are the same -- since two different String objects may both have the same (or different) values.

120.) When should I use an interface instead of an abstract class?
Ans.)

There are two primary axes of "inheritance" in object-oriented languages like Java. "Implementation" inheritance is where the sub-class inherits the actual code implementation from the parent. "Interface" inheritance is where the "sub-class" adheres to the public interface of the "parent".

Alas, Java actually mixes the two notions together a bit... Java interfaces are nice and clean -- when you "implement" an interface, you are stipulating that your class adheres to the "contract" of the interface that you specified. Java class inheritance isn't so clean -- when you sub-class in Java you are getting both the code inheritance but you are also stipulating that your sub-class adheres to the "contract" of the interface of the parent class.

Abstract classes in Java are just like regular Java classes but with the added constraint that you cannot instantiate them directly. In terms of that added constraint, they are basically classes which don't actually implement all of the code specified by their "contract".

So, it's generally considered good OO practice to specify the "contract" which you want to adhere to via Java interfaces. Then use normal Java class inheritance primarily for code reuse purposes. Use abstract Java classes when you want to provide some standard base code but want/need to force the user's of your class to complete the implementation (i.e., you create a skeleton implementation and the sub-classes must flesh it out).

121.) What are the differences between instance and class variables?
Ans.

Instance variables have separate values for each instance of a class. Class variables maintain a single shared value for all instances of the class, even if no instance object of that class exists.

You would use the static keyword to change an instance variable into a class variable.

Both instance and class variables are declared at the class level, not within methods:

public class Foo {

static private int count; // class variable

String name; // instance variable

private void Bar() {

int halfCount; // local variable

}

}

Note also that classes are loaded by classloaders therefore class variables are unique on a per-classloader basis.


Enterprise Java Beans notes

1.) What is an enterprise bean?

Ans. An enterprise bean is a server-side component -- defined in the Java technology -- which adheres to the Enterprise JavaBeans server-side component model. A server-side component is business object that can be accessed remotely. Many server-side component models exist: CORBA specifies CORBA objects; Microsoft Transaction Server (MTS) defines COM/DCOM; and EJB specifies enterprise beans.

What is an Enterprise java Bean

Ans. The Enterprise JavaBeans architecture is a component architecture for the development and deployment of component-based distributed business applications. Applications written using the Enterprise JavaBeans architecture are scalable, transactional, and multi-user secure. These applications may be written once, and then deployed on any server platform that supports the Enterprise JavaBeans specification.

Enterprise beans can be developed to represent business concepts like Employee, Order, TravelAgent, etc. Enterprise beans can be assembled into applications that solve enterprise business problems.

EJB has two basic types of enterprise beans: Session and Entity. Depending on the type of enterprise bean used, features like persistence, transactions, security, and multiple concurrent access can be managed automatically.

(2) Type of EJBS?

Ans.) Entity and session beans

Session Bean

A session bean is a type of enterprise bean; a type of EJB server-side component. Session bean components implement the javax.ejb.SessionBean interface and can be stateless or stateful. Stateless session beans are components that perform transient services; stateful session beans are components that are dedicated to one client and act as a server-side extension of that client.

Session beans can act as agents modeling workflow or provide access to special transient business services. As an agent, a stateful session bean might represent a customer's session at an online shopping site. As a transitive service, a stateless session bean might provide access to validate and process credit card orders.

Session beans do not normally represent persistent business concepts like Employee or Order. This is the domain of a different component type called an entity bean.

Entity Bean

An entity bean is a type of enterprise bean; a type of EJB server-side component. Entity bean components implement the javax.ejb.EntityBean interface and can be container-managed (CMP) or bean-managed (BMP). Entity beans are designed to represent data in the database; they wrapper data with business object semantics and read and update data automatically.

Entity beans have identity, where each bean represents a unique set of data. An entity bean of type Customer, for example, would have thousands of identities -- one for each customer in the database. Entity beans can be safely shared by many clients, so that several applications can access the same bean identity at the concurrently.

A bean developer might choose to create a Customer bean to represent customer data in the database. The bean developer decides what data from the database represents a customer and how customers are uniquely identified. The bean developer also decides what business methods the Customer bean expose and what data can be changed or read. Below is the server-side component definition of a Customer bean.

 
public class CustomerBean implements javax.ejb.EntityBean {
      
         // persistent fields
         public long customerNumber;
         public Date lastActive;
         public String lastName;
         public String firstName;
 
         // initialization method for creating a new customer
         public CustomerKey ejbCreate(long id, PersonName name){
                   customerNumber = id;
                   setName(name);
                   lastActive = new Date();
                   return new CustomerKey(id);
        }
         
         // business methods
         public PersonName getName(  ){
                return new PersonName(firstName, lastName);
         }
         public void setName(PersonName name){
               lastName = name.getLastName();
               fistName = name.getFirstName();
         }
         public Date lastActive( ){
                    return lastActive;
         }
         ... 
}

Note: Only a subset of the bean code normally implemented is shown here for brevity. Ordinarily the bean would also implement special EJB specific callback methods that are not important to this FAQ entry.

The ejbCreate () method is used only once in the life type of a bean identity when its first created. In this case we initialize the bean with the customer's id number and name. The lastActive value is set within the method. The ejbCreate( ) creates and returns the customer bean's identity, which is called a primary key in EJB. A primary key is a object that represents a unique identifier for a bean. In the case of the Customer bean the primary key is the CustomerKey which is an object that wrappers the customer's id. Primary keys can be as simple as a String value or more be more complicated like a key that wraps several values that together represent a unique index to the bean identity. Below is the definition of the CustomerKey object.

 
public class CustomerKey implements java.io.Serializable {
          public long id;
          
          public CustomerKey(long l)
         { 
                   id = l;
         }
         public int hashCode( ) { 
                   return (int)id;
          }
          public boolean equals(Object otherKey){
                    if(otherKey instanceof CustomerKey)
                           return ((CustomerKey)otherKey).id == this.id;
                   else
                           return false;
          }
}

You will notice that the business methods do not exactly match the persistent fields. This illustrates that the persistent state of the bean need not map exactly to the business methods, which provides more flexibility in how components are designed. For example, instead of having an accessor for lastName and firstName, the Customer bean uses a serializable PersonName to pass this information to and from the bean's client. In addition, you will have noticed that the lastActive can only be read by a bean client. The bean itself is responsible for updating this value.

In addition to the business methods and the ejbCreate( ) methods there are a number of notification methods that the bean container (EJB server) uses to alert that bean that some significant event in its lifecycle is about to, or just has, occurred. Of particular importance are the two notification methods (ejbLoad( ) and ejbStore( )) used to alert the bean when its being synchronized with the database. The behavior of these methods changes depending on whether the bean is designed for BMP or CMP.

Persistence in entity beans is of two types:

Container-managed persistence: Here, the EJB container is responsible for saving the state of the entity bean. Because it is container-managed, the implementation is independent of the data source. All container-managed fields need to be specified in the deployment descriptor, however, for the persistence to be automatically handled by the container.

Bean-managed persistence: Here, the entity bean is directly responsible for saving its own state and the container does not need to generate any database calls. Consequently, this implementation is less adaptable than the previous one as the persistence needs to be hard-coded into the bean.

To summarize, every entity bean has the following characteristics:

  • Entity beans can share access from multiple users.
  • Entity beans can participate in transactions.
  • Entity beans represent data in a domain model.
  • Entity beans are persistent. They live as long as the data lives in the domain model.
  • Entity beans can survive EJB server crashes. Any EJB server crash is always transparent to the client.
  • Entity beans have a persistent object reference. The object reference encapsulates the persistent key for this bean.

Q) Diff betn Session and Entity Bean

Session Bean

Entity Bean

The data members of the session bean contain conversational state.

The data members of the entity bean represent actual data in the domain model.

A session bean may handle database access for a single client.

Entity beans share database access for multiple clients.

Because session beans are about a conversation with a single client, session beans are allowed to store per-client state information..

Because entity beans are shared between multiple clients, they do not allow storage of per-client state information.

The relationship between a session bean and its client is one-to-one.

The relationship between an entity bean and a row in the domain model is one-to-one.

The life of the session bean is limited to the life of its client.

An entity bean persists as long as the data exists in the database.

Session beans can be transaction aware.

Entity beans are transactional.

Session beans do not survive server crashes.

Entity beans survive server crashes.

(3) Types of Entity and Session Bean?

Ans. Stateless and stateful

Stateful Session Bean

A stateful session bean is an enterprise bean (EJB component) that acts as a server-side extension of the client that uses it. The stateful session bean is created by a client and will work for only that client until the client connection is dropped or the bean is explicitly removed.

The stateful session bean is EJB component that implements the javax.ejb.SessionBean interface and is deployed with the declarative attribute "stateful". Stateful session beans are called "stateful" because they maintain a conversational state with the client. In other words, they have state or instance fields that can be initialized and changed by the client with each method invocation. The bean can use the conversational state as it process business methods invoked by the client.

Stateful session beans are usually developed to act as agents for the client, managing the interaction of other beans and performing work on behalf of the client application. An example is a shopping cart stateful session bean that tracks a client's product choices and can execute a sale when requested. Below is partial example of a stateful session bean that is used as a shopping cart.

 
public class ShoppingCartBean implements javax.ejb.SessionBean {
          
          // instance fields; The conversational state
          Vector products = new Vector ();
          Customer customer;
          
          // Initializes bean with reference to customer bean
          public void ejbCreate(Customer cust){
                    customer = cust;
          }
 
           //add a product to shopping cart
          public void addItem(Product item) {
                       products.addElement(item);
          }
 
          //charge customer for products and create a shipping order
          public void executeSale(){
 
                         // get the customer's credit card object
                        CreditCard card = customer.getCreditCard( );
 
                        // calculate a total price from products chosen
                        double total;
                        for(int i = 0; i  products.size(); i++){
                               Product product = (Product)products.elementAt(i);
                               total += product.getPrice( );
                        }
 
                        // get reference to the CreditService bean and process the customers charge
                        CreditService cardSwipper = ... get credit card service bean
                        Charge charge = cardSwipper.charge(CreditCard, total);
 
                        // get reference to the OrderHome and create a new shipping order bean (record)
                        OrderHome orderHome = ... get the Order beans Home object
                        Order order = orderHome.create(customer, products, charge);
          }
...
}


In the example above the
ShoppingCartBean keeps track of the Customer and the items chosen by the client application. This is the bean's conversational state. Once the client application is ready to execute a sale, the ShoppingCartBean manages the interactions of the Customer, Product, CreditServer, and Order beans in a workflow that results in a charge against the customers credit card and the creation of a shipping order. The ShoppingCartBean behaves as an agent for the client managing the interaction of these other beans, tracking chosen items, and executing the sale. The below table identifies the beans types used in the ShoppingCartBean example above.

Bean Name

Bean Type

ShoppingCartBean

Stateful Session

CreditService

Stateless Session

Customer

Entity

Product

Entity

Order

Entity

From the clients perspective only the Customer and ShoppingCart beans are visible, because the client application works with the bean's remote interface while the bean itself resides on the server. This means that the client need only be concerned with the business methods made public by the ShoppingCartBean. Below is an example of the client applications view of the ShoppingCartBean. (Note: The client interacts with the beans remote interface not the bean itself. The remote interface is called the ShoppingCart.)

// somewhere in the client application (applet, servlet, etc.)
 
// obtain the home objects (factories) for the Customer and ShoppingCart beans
CustomerHome custHm = ... get CustomerHome
ShoppingCartHome shpCrtHm = ... get ShoppingCartHome 
 
// locate the bean representing the customer named, Bob Johnsten
Customer customer = custHome.findByName("Bob","Johnsten");
 
// create a new ShoppingCart initialized with the Bob Johnsten Customer bean
ShoppingCart shoppingCart = shpCrtHm.create(customer);
 ...
Product product = ... get reference to bean representing the product chosen
// add product to customer's shopping cart
shoppingCart.addItem(product);
...
//charge customer for products and create a shipping order
shoppingCart.executeSale( );

Session beans like the shoppingCartBean only live as long as the client maintains a connection. In other words, they represent some aspect of the client's current session with the system and die when the client ends that session. Session beans are generally not fault tolerant. A system failure or shut down will result in the death of session bean and a loss of any conversation state it maintained prior to the failure. (This is a conceptual lifecycle of a session bean. Some vendors can make them more fault tolerant and longer lasting).

Stateless Session Bean

A stateless session bean is an enterprise bean (EJB component) that provides a stateless service to the client. Conceptually, the business methods on a stateless session bean are similar to procedural applications or static methods; there is no instance state, so all the data needed to execute the method is provided by the method arguments.

The stateless session bean is an EJB component that implements the javax.ejb.SessionBean interface and is deployed with the declarative attribute "stateless". Stateless session beans are called "stateless" because they do not maintain conversational state specific to a client session. In other words, the instance fields in a stateless session bean do not maintain data relative to a client session. This makes stateless session beans very lightweight and fast, but also limits their behavior.

Stateless session beans are usually developed as a set of related and simple services. Think of a session bean as a functional API where each business method represents an isolated independent service. An example is a CreditService bean that provides methods for making charges against different types of credit cards (MC, Visa, Discovery, etc).

 
 
public class CreditService implements javax.ejb.SessionBean {
 
    public Charge charge (String accountToCredit, CreditCard card, double amount)
    throws ValidationException, RemoteException{
               
               // attempt to obtain Merchant bean representing the retailer making the charge.
               try{
                   MerchantHome mrchntHome = ... get VendorHome reference
                   Merchant merchant = mrchntHome.findByPrimaryKey(vendorToCredit);      
               }catch(ObjectNotFoundException onfe){
                         throw new ValidationException("Invalid merchant account number");
               }         
 
               // attempt to create a Charge bean based on the vendor, amount, and credit card info
               try{
                   ChargeHome chgHome = ... get ChargeHome reference
                   Charge charge =  chgHome.create(merchant, card, amount);
                    return charge;
              } catch(CreateException ce){
                        throw new ValidationException(ce.getMessage());
              }
   }
...
}

In the above example the CreditService bean method charge () is completely independent of the bean state or any other methods. The charge () method is a stateless service and the CreditService bean is a stateless bean. The charge () method uses two other bean types to process the charge. Its normal for stateless beans to use other beans but they can also access the database directly or even other resources like proprietary connections. Below is a list the beans used in this example.

Bean Name

Bean Type

CreditService

Stateless Session

Merchant

Entity

Charge

Entity

An example of how the CreditService bean could be used by a client application or bean is shown in the FAQ entry

(4) Difference betn java bean and EJB

JavaBeans

Enterprise JavaBeans

JavaBeans may be visible or nonvisible at runtime. For example, the visual GUI component may be a button, list box, graphic, or a chart.

An EJB is a nonvisual, remote object.

JavaBeans are intended to be local to a single process and are primarily intended to run on the client side. Although one can develop server-side JavaBeans, it is far easier to develop them using the EJB specification instead.

EJBs are remotely executable components or business objects that can be deployed only on the server.

JavaBeans is a component technology to create generic Java components that can be composed together into applets and applications.

Even though EJB is a component technology, it neither builds upon nor extends the original JavaBean specification.

JavaBeans have an external interface called the Properties interface, which allows a builder tool to interpret the functionality of the bean.

EJBs have a deployment descriptor that describes its functionality to an external builder tool or IDE.

JavaBeans may have BeanInfo classes, property editors, or customizers.

EJBs have no concept of BeanInfo classes, property editors ,or customizers and provide no additional information other than that described in the deployment descriptor.

JavaBeans are not typed.

EJBs are of two types—session beans and entity beans.

No explicit support exists for transactions in JavaBeans.

EJBs may be transactional and the EJB Servers provide transactional support.

Component bridges are available for JavaBeans. For example, a JavaBean can also be deployed as an ActiveX control.

An EJB cannot be deployed as an ActiveX control because ActiveX controls are intended to run at the desktop and EJBs are server side components. However, CORBA-IIOP compatibility via the EJB-to-CORBA mapping is defined by the OMG.

(5) Life cycle of Stateful and Stateless Session Bean?

Stateless Session Bean
A stateless session bean has only two states: Does Not Exists and Method Ready Pool.
A bean has not yet instantiated (so it is not an instance in memory) when it is in the Does Not Exists state.
When the EJB container needs one or more beans, it creates and set them in the Method Ready Pool state. This happens through the creation of a new instance (Class.newInstance()), then it is set its context (setSessionContext()) and finally calls the ejbCreate() method.
The ejbRemove() method is called to move a bean from the Method Ready Pool back to Does Not Exists state.

Stateful Session Bean
Unlike Stateless, a Stateful Session Bean has three states. Does not exists, Method Ready and Passivated states.
Like Stateless beans, when the Stateful Session Bean hasn't been instantiated yet (so it is not an instance in memory) is it in the Does not exists state.
Once a container creates one or more instances of a Stateful Session Bean it sets them in a Method Ready state. In this state it can serve requests from its clients. Like Stateless Session Beans, a new instance is created (Class.newInstance()), the context is passed (setSessionContext()) and finally the bean is created with ejbCreate().
During the life of a Stateful Session Bean, there are periods of inactivity. In these periods, the container can set the bean to the Passivate state. This happens through the ejbPassivate() method. From the Passivate state the bean can be moved back to the Method Ready state, via ejbActivate() method, or can go directly to the Does Not Exists state with ejbRemove().

(6) Life cycle of Entity Bean?

1. The does not exist state represents entity bean instances that have not

been instantiated yet.

2. To create a new instance, the container calls the newInstance() method

on the entity bean class. This calls your entity bean’s default constructor,

bringing a new instance into memory. Next, the container associates your

entity bean with an entity context object—this is done via a callback that

you implement, called setEntityContext(EntityContext ctx). Note that this

step occurs only when the container wants to increase the available pool of

entity bean instances—this is not necessarily done when a client connects.

3. After step 2, your entity bean is in a pool of other entity beans. At this

point your entity bean does not have any entity bean database data loaded

into it, and it does not hold any bean-specific resources, such as socket

connections. Your bean instance can be used in this mode to find entity

data in the database, by servicing a finder method on behalf of a client. If

the container wants to reduce its pool size, it can destroy your bean. The

container signals your bean instance that it is about to be destroyed by

calling the unsetEntityContext() method on your bean. Once this is done,

the container releases any references to your bean, and eventually, the

Java garbage collector will clean up the memory your instance had been

using. Therefore, your unsetEntityContext() method should prepare your

bean to be cleaned up, perhaps by releasing any bean-independent resources

your bean had claimed.

4. When the client wants to create some new database data (say, a new order

for goods placed over the Internet), it calls a create() method on your entity

bean’s home object. The container then grabs an entity bean instance from

the pool, and the instance’s ejbCreate() method is called. ejbCreate() initializes

the entity bean to a specific data set. For example, if a client calls

a create() method to create a bank account, it might pass the bank account

holder’s name and the initial balance as parameters. Your entity bean’s

ejbCreate() method would populate its member variables with these parameters.

It would also create the corresponding database representation (if

you’re using bean-managed persistence). Now your bean is in the “ready”

state.

(7) How will we do Insertion, Deletion by using Entity Bean?

Ans : Thru ejbCreate() and ejbRemove()

(8) Wt is the significance of ejbActivate(),ejbPassivate(),ejbStore(),ejbLoad(),ejbCreate() methods of Entity Bean?

FOR ENTITY BEAN

METHOD

EXPLANATION

TYPICAL

setEntityContext()

If the container wants to increase

its pool size of bean

instances, it will instantiate a

new entity bean instance. Following

this, the container calls

the instance’s setEntityContext().

This method associates a bean

with context information

information about the bean’s

environment. Once this method

is called, the bean can access

information about its

environment.

Stick the entity context somewhere, such as in a member

variable. You can then access the

context later to acquire environment information, such as security

information, from the container.

You should also request any

resources your instance will need

regardless of what data the bean

represents.

The bean is now in a pool, does

not have any specific database

data inside of it, and is not bound

to any particular EJB object.

ejbFind<…>(<…>)

(also called finder

methods)

While your bean instance is

still in the pool, the container

can use your bean to service a

finder method. Finder methods

locate one or more existing

entity bean data instances in

the underlying persistent store.

You must define at least one

finder method—

ejbFindByPrimaryKey().

You do not write finder methods

for container-managed beans.

Search through a data store using

a storage API such as JDBC or

SQL/J. For example, you might

perform a relational query such as

“SELECT id FROM accounts

WHERE balance > 0.”

When you’ve found some data, you

should return the primary keys for

that data back to the container by

creating one or more primary key

Java object instances. The container

will then create EJB objects

for the client to invoke on and

possibly associate some entity

bean instances with those EJB

objects. Those entity bean

instances are then no longer in

the pool—they now have specific

database data inside of them,

and they are bound to particular

EJB objects.(BMP)

You should not implement these

methods for container-managed

persistent entity beans.

ejbCreate(<…>)

Note: You do not need

to write any

ejbCreate()

methods if you

don’t want EJB

clients to be able to

create new data-

base data. Instead,

you could mandate

that all data is created

through other means,

such as via direct database

inserts or through

batch files.

When a client calls create() on

a home object, the container

then calls ejbCreate() on a

pooled bean instance.

ejbCreate() methods are

responsible for creating new

database data and for

initializing your bean.

Make sure the client’s initialization

parameters are valid. Explicitly

create the database representation of the data via a storage API

such as JDBC or SQL/J.

Your entity bean instance is then

no longer in the pool—it now has

specific database data inside of it.

The container will bind your

instance to a particular EJB object.

For CMP

Do not create database data in

this method. Rather, simply

check that the client parameters

are correct, and set your

container-managed fields to the

parameters passed in. After the

EJB container calls your

ejbCreate() method, it will

extract the container-managed

fields from your bean and create

the database representation of

the data for you.

ejbPostCreate(<…>)

ejbPostCreate()

Your bean class must define

one ejbPostCreate() for each

ejbCreate(). Each pair must

accept the same parameters.

The container calls

ejbPostCreate() right after

ejbCreate().

The container calls

after it has associated your bean

instance with an EJB object. You

can now complete your initial-

ization, by doing anything you

need to that requires that EJB

object, such as passing your

bean’s EJB object reference to

other beans.

You might also use this method to

reset certain transaction-related

parameters—for example, you

could keep a data status flag in

the bean to indicate whether a

field has been changed. Because

the bean instance may have been

used before, these fields might

have dirty data.

ejbActivate()

When a client calls a business

method on an EJB object but there

is no entity bean instance

bound to the EJB object, the

container needs to take a bean

from the pool and transition it

into a ready state. This is called

activation. Upon activation, the

ejbActivate() method is called

by the EJB container.

Note: ejbActivate() is never

called during a transaction.

Acquire any bean-specific

resources, such as socket connections, that your bean needs to

service a particular client when it’s

moved into the ready state. Note

that you should not read the entity

bean data from the database in

this method. That is handled by a

separate method, ejbLoad(),

which is called right after

ejbActivate().

ejbLoad()

The EJB container calls this to

load database data into your

bean instance, based on the

current transactional state.

First, your bean instance must

figure out what data it should

load. Call the getPrimaryKey()

method on the entity context; that

will tell your bean what data it

should be loading. Next, read database

data into your bean via a

storage API such as JDBC or

SQL/J.

ejbStore()

The EJB container calls this to

update the database to the

new values of your in-memory fields,

thus synchronizing the

database. The current trans-

actional state dictates when

this method is called. Also, this

method is called during passivation,

directly before

ejbPassivate().

Explicitly update the database

representation of the data via a

storage API such as JDBC. Typfields,

ically, you’ll write a number of

your member variable’s fields out

to disk.

FOR CMP

Do not update the database in

this method. Rather, the EJB

container will update the

database for you automatically

right after calling your ejbStore()

method. It does this by extracting your container-managed

fields and writing them to the

database.

In this method, you should

prepare your container-managed

fields to be written to the database.

For example, you can

compress the text of your fields

if necessary.

ejbPassivate()

The EJB container will call this

method when it wants to return

your entity bean to the pool.

This is called passivation and

is the opposite of activation. On

passivation, the ejbPassivate()

method is called by the EJB

container.

Note: ejbPassivate() is never

called during a transaction.

Release any resources, such as

socket connections, that you

allocated in ejbActivate() and that

your bean was holding during the

ready state for a particular client.

You should not save the entity

bean data into the database in

this method. That is handled by a

separate method, ejbStore(),

which is called right before

ejbPassivate().

ejbRemove()

Destroys database data. It is

not used to destroy the Java

Object; the object can be

pooled and reused for differ-

ent data.

First, figure out what data you

should be destroying via

getPrimaryKey() on the

EntityContext. Then explicitly

delete the database representation

of the data via a storage API

such as JDBC.

FOR CMP

Do not destroy database data in

this method. Rather, simply

perform any operations that

must be done before the data in

the database is destroyed. The

EJB container will destroy the

data for you right after

ejbRemove() is called.

unsetEntityContext()

This method disassociates a

bean from its environment.

The container calls this right

before your entity bean

instance is destroyed (when it

wants to reduce the pool size).

Release any resources you

allocated during setEntityContext(),

and get ready to be garbage

collected.

FOR SESSION BEAN

METHOD

DESCRIPTION

TYPICAL IMPLEMENTATION

setSessionContext

(SessionContext ctx)

Associates your bean with a

session context. Your bean can

query the context about its

current transactional state, its

current security state, and more.

.

Store the context away in a

member variable so the context

can be queried later.

.

.

ejbCreate(...)

Initializes your session bean.

You can define several

ejbCreate(...) methods, and

each can take different argu-

ments. You must provide at

least one ejbCreate() method

in your session bean.

Perform any initialization your

bean needs, such as setting

member variables to the

argument values passed in.

ejbPassivate()

Called immediately before your

bean is passivated (swapped

out to disk because there are

too many beans instantiated).

Does not apply to stateless

session beans.

Release any resources your bean

may be holding

ejbActivate()

Called immediately before your

bean is activated (swapped in

from disk because a client

needs your bean). Does not

apply to stateless session

beans.

Acquire any resources your bean

needs, such as those released

during ejbPassivate().

ejbRemove()

Called by the container immed-

iately before your bean is

removed from memory

Prepare your bean for destruction.

Free all resources you may have

allocated

(9) Wt is EntityContext and SessionContext?

Ans : Same as Above.

(10)Wt is Remote and Home Interface, How the client(jsp/servlets) access Entity Bean Class? Wt is EjbObject, homeObject?

(11) Same above question for local Interfaces?

(12) wt are ejb design patterns?

(13) wt is handle in EJB?

(14) wt are tranasaction attributes(Requied....ect) in EJB? Which interface u use if u want to work on transactions in EJB?

(15) then about deployment descripter?

(16) wt is jndi? which two things(InitialContext and jndi URL) u need to pass while look up?

(17) What is an EJB primary key? How is it implemented when the database doesn't have a primary key?

A primary key is an object that uniquely identifies the entity bean. According to the specification, the primary key must be unique for each entity bean within a container. Hence the bean's primary key usually maps to the PK in the database (provided its persisted to a database).

You may need to create a primary key in the database for the sake of referential integrity. This does not, however, mean you NEED a primary key in the database. As long as the bean's primary key (which maps to a column or set of columns) can uniquely identify the bean it should work.

(18) Difference between Stateful and StateLess Session Bean

Stateful or Stateless?

When deciding to use stateful session beans, you must first ask yourself whether the

business process you’re modeling inherently requires a stateful model. If it does, a stateful

session bean may be the ideal component to use. When using stateful session beans,

however, your inherent statefulness may limit your fault tolerance. For example, what

happens if an unexpected system-level error occurs, such as a bean crashing, the network

dying, or a machine rebooting? In a stateless model, the request could be transparently

rerouted to a different component because any component can service the

client’s needs. In stateful models, there is little that can be done to reroute the client’s

request because the client’s state is lost when the failure occurs (the state was kept

within the lost bean). Note, however, that some high-end EJB container implementations

are adding on stateful recovery services as an optional value feature. These services

allow for even stateful components to be transparently recovered, by continually

persisting the bean’s active state and recovering from permanent storage in case of

failure.

If you have a drawn-out business process, there is another alternative to using stateful

session beans. You can go with a stateless model and pass the entire client state as

parameters to the stateless bean during method invocations. Passing of state in such a

way could lead to severe performance degradation. This is especially true if the client is

remotely located from the bean and if the state passed over the network is large. Thus,

you may achieve lightweight fault tolerance, but the overall scalability of your system

may be compromised by the added network latency expense.

19) what are the different ways in which you maintain session. Explain how will it be done for servers in clustered mode.

20)How is load balancing done in clustered mode.

21) What is a Java Bean

22) What is the diffrence between an aplication server and a web server

Ans.)
A Web Server understands and supports only HTTP protocol whereas an Application Server supports HTTP,TCP/IP and many more protocols. Also many more features such as Caches,Clusters,Load Balancing are there in Application Servers which are not available in Web Servers. We can also Configure Application Servers to work as Web Server. In short, Applicaion Server is a super set of which Web Server is a sub set.

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