A: A comment that is sent to the client in the viewable page source.The JSP engine handles an output comment as uninterpreted HTML text, returning the comment in the HTML output sent to the client. You can see the comment by viewing the page source from your Web browser.
JSP Syntax
Example 1
Displays in the page source:
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Q: What is a Hidden Comment?
A: A comments that documents the JSP page but is not sent to the client. The JSP engine ignores a hidden comment, and does not process any code within hidden comment tags. A hidden comment is not sent to the client, either in the displayed JSP page or the HTML page source. The hidden comment is useful when you want to hide or "comment out" part of your JSP page.
You can use any characters in the body of the comment except the closing --%> combination. If you need to use --%> in your comment, you can escape it by typing --%\>.
JSP Syntax
<%-- comment --%>
Examples
<%@ page language="java" %>
<%-- This comment will not be visible to the colent in the page source --%>
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Q: What is a Expression?
A: An expression tag contains a scripting language expression that is evaluated, converted to a String, and inserted where the expression appears in the JSP file. Because the value of an expression is converted to a String, you can use an expression within text in a JSP file. Like
<%= someexpression %>
<%= (new java.util.Date()).toLocaleString() %>
You cannot use a semicolon to end an expression
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Q: What is a Declaration?
A: A declaration declares one or more variables or methods for use later in the JSP source file.
A declaration must contain at least one complete declarative statement. You can declare any number of variables or methods within one declaration tag, as long as they are separated by semicolons. The declaration must be valid in the scripting language used in the JSP file.
<%! somedeclarations %>
<%! int i = 0; %>
<%! int a, b, c; %>
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Q: What is a Scriptlet?
A: A scriptlet can contain any number of language statements, variable or method declarations, or expressions that are valid in the page scripting language.Within scriptlet tags, you can
1.Declare variables or methods to use later in the file (see also Declaration).
2.Write expressions valid in the page scripting language (see also Expression).
3.Use any of the JSP implicit objects or any object declared with a
You must write plain text, HTML-encoded text, or other JSP tags outside the scriptlet.
Scriptlets are executed at request time, when the JSP engine processes the client request. If the scriptlet produces output, the output is stored in the out object, from which you can display it.
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Q: What are implicit objects? List them?
A: Certain 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 re listed below
request
response
pageContext
session
application
out
config
page
exception
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Q: Difference between forward and sendRedirect?
A: When you invoke a forward request, the request is sent to another resource on the server, without the client being informed that a different resource is going to process the request. This process occurs completly with in the web container. When a sendRedirtect method is invoked, it causes the web container to return to the browser indicating that a new URL should be requested. Because the browser issues a completly new request any object that are stored as request attributes before the redirect occurs will be lost. This extra round trip a redirect is slower than forward.
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Q: What are the different scope valiues for the
A: The different scope values for
1. page
2. request
3.session
4.application
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Q: Explain the life-cycle mehtods in JSP?
A: THe generated servlet class for a JSP page implements the HttpJspPage interface of the javax.servlet.jsp package. Hte HttpJspPage interface extends the JspPage interface which inturn extends the Servlet interface of the javax.servlet package. the generated servlet class thus implements all the methods of the these three interfaces. The JspPage interface declares only two mehtods - jspInit() and jspDestroy() that must be implemented by all JSP pages regardless of the client-server protocol. However the JSP specification has provided the HttpJspPage interfaec specifically for the JSp pages serving HTTP requests. This interface declares one method _jspService().
The jspInit()- The container calls the jspInit() to initialize te servlet instance.It is called before any other method, and is called only once for a servlet instance.
The _jspservice()- The container calls the _jspservice() for each request, passing it the request and the response objects.
The jspDestroy()- The container calls this when it decides take the instance out of service. It is the last method called n the servlet instance.
Q: How do I prevent the output of my JSP or Servlet pages from being cached by the browser?
A: You will need to set the appropriate HTTP header attributes to prevent the dynamic content output by the JSP page from being cached by the browser. Just execute the following scriptlet at the beginning of your JSP pages to prevent them from being cached at the browser. You need both the statements to take care of some of the older browser versions.
<%
response.setHeader("Cache-Control","no-store"); //HTTP 1.1
response.setHeader("Pragma\","no-cache"); //HTTP 1.0
response.setDateHeader ("Expires", 0); //prevents caching at the proxy server
%>
[ Received from Sumit Dhamija ] TOP
Q: How does JSP handle run-time exceptions?
A: You can use the errorPage attribute of the page directive to have uncaught run-time exceptions automatically forwarded to an error processing page. For example:
<%@ page errorPage=\"error.jsp\" %> redirects the browser to the JSP page error.jsp if an uncaught exception is encountered during request processing. Within error.jsp, if you indicate that it is an error-processing page, via the directive: <%@ page isErrorPage=\"true\" %> Throwable object describing the exception may be accessed within the error page via the exception implicit object. Note: You must always use a relative URL as the value for the errorPage attribute.
[ Received from Sumit Dhamija ] TOP
Q: How can I implement a thread-safe JSP page? What are the advantages and Disadvantages of using it?
A: You can make your JSPs thread-safe by having them implement the SingleThreadModel interface. This is done by adding the directive <%@ page isThreadSafe="false" %> within your JSP page. With this, instead of a single instance of the servlet generated for your JSP page loaded in memory, you will have N instances of the servlet loaded and initialized, with the service method of each instance effectively synchronized. You can typically control the number of instances (N) that are instantiated for all servlets implementing SingleThreadModel through the admin screen for your JSP engine. More importantly, avoid using the tag for variables. If you do use this tag, then you should set isThreadSafe to true, as mentioned above. Otherwise, all requests to that page will access those variables, causing a nasty race condition. SingleThreadModel is not recommended for normal use. There are many pitfalls, including the example above of not being able to use <%! %>. You should try really hard to make them thread-safe the old fashioned way: by making them thread-safe .
[ Received from Sumit Dhamija ] TOP
Q: How do I use a scriptlet to initialize a newly instantiated bean?
A: A jsp:useBean action may optionally have a body. If the body is specified, its contents will be automatically invoked when the specified bean is instantiated. Typically, the body will contain scriptlets or jsp:setProperty tags to initialize the newly instantiated bean, although you are not restricted to using those alone.
The following example shows the “today” property of the Foo bean initialized to the current date when it is instantiated. Note that here, we make use of a JSP expression within the jsp:setProperty action.
<%-- scriptlets calling bean setter methods go here --%>
[ Received from Sumit Dhamija ] TOP
Q: How can I prevent the word "null" from appearing in my HTML input text fields when I populate them with a resultset that has null values?
A: You could make a simple wrapper function, like
<%!
String blanknull(String s) {
return (s == null) ? \"\" : s;
}
%>
then use it inside your JSP form, like
" >
[ Received from Sumit Dhamija ] TOP
Q: What's a better approach for enabling thread-safe servlets and JSPs? SingleThreadModel Interface or Synchronization?
A: Although the SingleThreadModel technique is easy to use, and works well for low volume sites, it does not scale well. If you anticipate your users to increase in the future, you may be better off implementing explicit synchronization for your shared data. The key however, is to effectively minimize the amount of code that is synchronzied so that you take maximum advantage of multithreading.
Also, note that SingleThreadModel is pretty resource intensive from the server\'s perspective. The most serious issue however is when the number of concurrent requests exhaust the servlet instance pool. In that case, all the unserviced requests are queued until something becomes free - which results in poor performance. Since the usage is non-deterministic, it may not help much even if you did add more memory and increased the size of the instance pool.
[ Received from Sumit Dhamija ] TOP
Q: How can I enable session tracking for JSP pages if the browser has disabled cookies?
A: We know that session tracking uses cookies by default to associate a session identifier with a unique user. If the browser does not support cookies, or if cookies are disabled, you can still enable session tracking using URL rewriting. URL rewriting essentially includes the session ID within the link itself as a name/value pair. However, for this to be effective, you need to append the session ID for each and every link that is part of your servlet response. Adding the session ID to a link is greatly simplified by means of of a couple of methods: response.encodeURL() associates a session ID with a given URL, and if you are using redirection, response.encodeRedirectURL() can be used by giving the redirected URL as input. Both encodeURL() and encodeRedirectedURL() first determine whether cookies are supported by the browser; if so, the input URL is returned unchanged since the session ID will be persisted as a cookie.
Consider the following example, in which two JSP files, say hello1.jsp and hello2.jsp, interact with each other. Basically, we create a new session within hello1.jsp and place an object within this session. The user can then traverse to hello2.jsp by clicking on the link present within the page. Within hello2.jsp, we simply extract the object that was earlier placed in the session and display its contents. Notice that we invoke the encodeURL() within hello1.jsp on the link used to invoke hello2.jsp; if cookies are disabled, the session ID is automatically appended to the URL, allowing hello2.jsp to still retrieve the session object. Try this example first with cookies enabled. Then disable cookie support, restart the brower, and try again. Each time you should see the maintenance of the session across pages. Do note that to get this example to work with cookies disabled at the browser, your JSP engine has to support URL rewriting.
hello1.jsp
<%@ page session=\"true\" %>
<%
Integer num = new Integer(100);
session.putValue("num",num);
String url =response.encodeURL("hello2.jsp");
%>
\'>hello2.jsp
hello2.jsp
<%@ page session="true" %>
<%
Integer i= (Integer )session.getValue("num");
out.println("Num value in session is " + i.intValue());
%>
Q: What is the difference b/w variable declared inside a declaration part and variable declared in scriplet part?
A: Variable declared inside declaration part is treated as a global variable.that means after convertion jsp file into servlet that variable will be in outside of service method or it will be declared as instance variable.And the scope is available to complete jsp and to complete in the converted servlet class.where as if u declare a variable inside a scriplet that variable will be declared inside a service method and the scope is with in the service method.
[ Received from Neelam Gangadhar] TOP
Q: Is there a way to execute a JSP from the comandline or from my own application?
A: There is a little tool called JSPExecutor that allows you to do just that. The developers (Hendrik Schreiber
Response has already been commited error. What does it mean? - This error show only when you try to redirect a page after you already have written something in your page. This happens because HTTP specification force the header to be set up before the lay out of the page can be shown (to make sure of how it should be displayed, content-type=”text/html” or “text/xml” or “plain-text” or “image/jpg”, etc.) When you try to send a redirect status (Number is line_status_402), your HTTP server cannot send it right now if it hasn’t finished to set up the header. If not starter to set up the header, there are no problems, but if it ’s already begin to set up the header, then your HTTP server expects these headers to be finished setting up and it cannot be the case if the stream of the page is not over… In this last case it’s like you have a file started with
1. Is there a way I can set the inactivity lease period on a per-session basis? - Typically, a default inactivity lease period for all sessions is set within your JSP engine admin screen or associated properties file. However, if your JSP engine supports the Servlet 2.1 API, you can manage the inactivity lease period on a per-session basis. This is done by invoking the HttpSession.setMaxInactiveInterval() method, right after the session has been created. For example:
2. <%
3. session.setMaxInactiveInterval(300);
4. %>
would reset the inactivity period for this session to 5 minutes. The inactivity interval is set in seconds.
5. How can I set a cookie and delete a cookie from within a JSP page? - A cookie, mycookie, can be deleted using the following scriptlet:
6. <%
7. //creating a cookie
8. Cookie mycookie = new Cookie("aName","aValue");
9. response.addCookie(mycookie);
10. //delete a cookie
11. Cookie killMyCookie = new Cookie("mycookie", null);
12. killMyCookie.setMaxAge(0);
13. killMyCookie.setPath("/");
14. response.addCookie(killMyCookie);
15. %>
16. How does a servlet communicate with a JSP page? - The following code snippet shows how a servlet instantiates a bean and initializes it with FORM data posted by a browser. The bean is then placed into the request, and the call is then forwarded to the JSP page, Bean1.jsp, by means of a request dispatcher for downstream processing.
17. public void doPost (HttpServletRequest request, HttpServletResponse response) {
18. try {
19. govi.FormBean f = new govi.FormBean();
20. String id = request.getParameter("id");
21. f.setName(request.getParameter("name"));
22. f.setAddr(request.getParameter("addr"));
23. f.setAge(request.getParameter("age"));
24. //use the id to compute
25. //additional bean properties like info
26. //maybe perform a db query, etc.
27. // . . .
28. f.setPersonalizationInfo(info);
29. request.setAttribute("fBean",f);
30. getServletConfig().getServletContext().getRequestDispatcher
31. ("/jsp/Bean1.jsp").forward(request, response);
32. } catch (Exception ex) {
33. . . .
34. }
35. }
The JSP page Bean1.jsp can then process fBean, after first extracting it from the default request scope via the useBean action.
jsp:useBean id="fBean" class="govi.FormBean" scope="request"
/ jsp:getProperty name="fBean" property="name"
/ jsp:getProperty name="fBean" property="addr"
/ jsp:getProperty name="fBean" property="age"
/ jsp:getProperty name="fBean" property="personalizationInfo" /
36. How do I have the JSP-generated servlet subclass my own custom servlet class, instead of the default? - One should be very careful when having JSP pages extend custom servlet classes as opposed to the default one generated by the JSP engine. In doing so, you may lose out on any advanced optimization that may be provided by the JSP engine. In any case, your new superclass has to fulfill the contract with the JSP engine by:
Implementing the HttpJspPage interface, if the protocol used is HTTP, or implementing JspPage otherwise Ensuring that all the methods in the Servlet interface are declared final Additionally, your servlet superclass also needs to do the following:
o The service() method has to invoke the _jspService() method
o The init() method has to invoke the jspInit() method
o The destroy() method has to invoke jspDestroy()
If any of the above conditions are not satisfied, the JSP engine may throw a translation error.
Once the superclass has been developed, you can have your JSP extend it as follows:
<%@ page extends="packageName.ServletName" %>
37. How can I prevent the word "null" from appearing in my HTML input text fields when I populate them with a resultset that has null values? - You could make a simple wrapper function, like
38. <%!
39. String blanknull(String s) {
40. return (s == null) ? "" : s;
41. }
42. %>
43. then use it inside your JSP form, like
44. " >
45. How can I get to print the stacktrace for an exception occuring within my JSP page? - By printing out the exception’s stack trace, you can usually diagonse a problem better when debugging JSP pages. By looking at a stack trace, a programmer should be able to discern which method threw the exception and which method called that method. However, you cannot print the stacktrace using the JSP out implicit variable, which is of type JspWriter. You will have to use a PrintWriter object instead. The following snippet demonstrates how you can print a stacktrace from within a JSP error page:
46. <%@ page isErrorPage="true" %>
47. <%
48. out.println(" ");
49. PrintWriter pw = response.getWriter();
50. exception.printStackTrace(pw);
51. out.println(" ");
52. %>
53. How do you pass an InitParameter to a JSP? - The JspPage interface defines the jspInit() and jspDestroy() method which the page writer can use in their pages and are invoked in much the same manner as the init() and destory() methods of a servlet. The example page below enumerates through all the parameters and prints them to the console.
54. <%@ page import="java.util.*" %>
55. <%!
56. ServletConfig cfg =null;
57. public void jspInit(){
58. ServletConfig cfg=getServletConfig();
59. for (Enumeration e=cfg.getInitParameterNames(); e.hasMoreElements();) {
60. String name=(String)e.nextElement();
61. String value = cfg.getInitParameter(name);
62. System.out.println(name+"="+value);
63. }
64. }
65. %>
66. How can my JSP page communicate with an EJB Session Bean? - The following is a code snippet that demonstrates how a JSP page can interact with an EJB session bean:
67. <%@ page import="javax.naming.*, javax.rmi.PortableRemoteObject, foo.AccountHome, foo.Account" %>
68. <%!
69. //declare a "global" reference to an instance of the home interface of the session bean
70. AccountHome accHome=null;
71. public void jspInit() {
72. //obtain an instance of the home interface
73. InitialContext cntxt = new InitialContext( );
74. Object ref= cntxt.lookup("java:comp/env/ejb/AccountEJB");
75. accHome = (AccountHome)PortableRemoteObject.narrow(ref,AccountHome.class);
76. }
77. %>
78. <%
79. //instantiate the session bean
80. Account acct = accHome.create();
81. //invoke the remote methods
82. acct.doWhatever(...);
83. // etc etc...
84. %>
RequestDispatcher description.
RequestDispatcher defines an object that receives requests from the client and sends them to any resource (such as a servlet, HTML file, or JSP file) on the server. The servlet container creates the RequestDispatcher object, which is used as a wrapper around a server resource located at a particular path or given by a particular name.
An object implementing the RequestDispatcher interface may be obtained via the following methods:
ServletContext.getRequestDispatcher(String path)
ServletContext.getNamedDispatcher(String name)
ServletRequest.getRequestDispatcher(String path)
The ServletContext.getRequestDispatcher method takes a String argument describing a path within the scope of the ServletContext. This path must be relative to the root of the ServletContext and begin with a '/'. The method uses the path to look up a servlet, using the servlet path matching rules, wraps it with a RequestDispatcher object, and returns the resulting object. If no servlet can be resolved based on the given path, a RequestDispatcher is provided that returns the content for that path.
The ServletContext.getNamedDispatcher method takes a String argument indicating the NAME of a servlet known to the ServletContext. If a servlet is found, it is wrapped with a RequestDispatcher object and the object is returned. If no servlet is associated with the given name, the method must return null.
To allow RequestDispatcher objects to be obtained using relative paths that are relative to the path of the current request (not relative to the root of the ServletContext), the ServletRequest.getRequestDispatcher method is provided in the ServletRequest interface. The behavior of this method is similar to the method of the same name in the ServletContext. The servlet container uses information in the request object to transform the given relative path against the current servlet to a complete path. For example, in a context rooted at '/' and a request to /garden/tools.html, a request dispatcher obtained via ServletRequest.getRequestDispatcher("header.html") will behave exactly like a call to ServletContext.getRequestDispatcher("/garden/header.html").
RequestDispatcher creation and using.
public class Dispatcher extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse res) {
RequestDispatcher dispatcher =
request.getRequestDispatcher("/template.jsp");
if (dispatcher != null) dispatcher.forward(request, response);
}
}
forward should be called before the response has been committed to the client (before response body output has been flushed). If the response already has been committed, this method throws an IllegalStateException. Uncommitted output in the response buffer is automatically cleared before the forward.
public class Dispatcher extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse res) {
RequestDispatcher dispatcher =
getServletContext().getRequestDispatcher("/banner");
if (dispatcher != null) dispatcher.include(request, response);
}
}
Includes the content of a resource (servlet, JSP page, HTML file) in the response. In essence, this method enables programmatic server-side includes. The ServletResponse object has its path elements and parameters remain unchanged from the caller's. The included servlet cannot change the response status code or set headers; any attempt to make a change is ignored.
package javax.servlet;
public interface RequestDispatcher {
public void forward(ServletRequest request, ServletResponse response)
throws ServletException, java.io.IOException;
public void include(ServletRequest request, ServletResponse response)
throws ServletException, java.io.IOException;
}
The include method of the RequestDispatcher interface may be called at ANY time. The target servlet of the include method has access to all aspects of the request object, but its use of the response object is more limited. It can only write information to the ServletOutputStream or Writer of the response object and commit a response by writing content past the end of the response buffer, or by explicitly calling the flushBuffer method of the ServletResponse interface. It CANNOT set headers or call any method that affects the headers of the response. Any attempt to do so must be ignored.
The forward method of the RequestDispatcher interface may be called by the calling servlet ONLY when NO output has been committed to the client. If output data exists in the response buffer that has not been committed, the content must be cleared before the target servlet's service method is called. If the response has been committed, an IllegalStateException must be thrown.
The path elements of the request object exposed to the target servlet must reflect the path used to obtain the RequestDispatcher. The only exception to this is if the RequestDispatcher was obtained via the getNamedDispatcher method. In this case, the path elements of the request object must reflect those of the original request. Before the forward method of the RequestDispatcher interface returns, the response content MUST be sent and committed, and closed by the servlet container.
The ServletContext and ServletRequest methods that create RequestDispatcher objects using path information allow the optional attachment of query string information to the path. For example, a Developer may obtain a RequestDispatcher by using the following code:
String path = "/raisins.jsp?orderno=5";
RequestDispatcher rd = context.getRequestDispatcher(path);
rd.include(request, response);
Parameters specified in the query string used to create the RequestDispatcher take precedence over other parameters of the same name passed to the included servlet. The parameters associated with a RequestDispatcher are scoped to apply only for the duration of the include or forward call.
Additional request-scoped attributes.
Except for servlets obtained by using the getNamedDispatcher method, a servlet that has been invoked by another servlet using the include method of RequestDispatcher has access to the path by which it was invoked.
The following request attributes must be set:
javax.servlet.include.request_uri
javax.servlet.include.context_path
javax.servlet.include.servlet_path
javax.servlet.include.path_info
javax.servlet.include.query_string
These attributes are accessible from the included servlet via the getAttribute method on the request object and their values must be equal to the request URI, context path, servlet path, path info, and query string of the INCLUDED servlet, respectively. If the request is subsequently included, these attributes are replaced for that include.
If the included servlet was obtained by using the getNamedDispatcher method, these attributes MUST NOT be set.
Except for servlets obtained by using the getNamedDispatcher method, a servlet that has been invoked by another servlet using the forward method of RequestDispatcher has access to the path of the ORIGINAL request.
The following request attributes must be set:
javax.servlet.forward.request_uri
javax.servlet.forward.context_path
javax.servlet.forward.servlet_path
javax.servlet.forward.path_info
javax.servlet.forward.query_string
The values of these attributes must be equal to the return values of the HttpServletRequest methods getRequestURI, getContextPath, getServletPath, getPathInfo, getQueryString respectively, invoked on the request object passed to the first servlet object in the call chain that received the request from the client.
These attributes are accessible from the forwarded servlet via the getAttribute method on the request object. Note that these attributes must always reflect the information in the original request even under the situation that multiple forwards and subsequent includes are called.
If the forwarded servlet was obtained by using the getNamedDispatcher method, these attributes must not be set.
What if the main method is declared as private?
The program compiles properly but at runtime it will give "Main method not public." message.
What is meant by pass by reference and pass by value in Java?
Pass by reference means, passing the address itself rather than passing the value. Pass by value means passing a copy of the value.
If you’re overriding the method equals() of an object, which other method you might also consider?
hashCode()
What is Byte Code?
Or
What gives java it’s “write once and run anywhere” nature?
All Java programs are compiled into class files that contain bytecodes. These byte codes can be run in any platform and hence java is said to be platform independent.
Expain the reason for each keyword of public static void main(String args[])?
public- main(..) is the first method called by java environment when a program is executed so it has to accessible from java environment. Hence the access specifier has to be public.
static: Java environment should be able to call this method without creating an instance of the class , so this method must be declared as static.
void: main does not return anything so the return type must be void
The argument String indicates the argument type which is given at the command line and arg is an array for string given during command line.
What are the differences between == and .equals() ?
Or
what is difference between == and equals
Or
Difference between == and equals method
Or
What would you use to compare two String variables - the operator == or the method equals()?
Or
How is it possible for two String objects with identical values not to be equal under the == operator?
The == operator compares two objects to determine if they are the same object in memory i.e. present in the same memory location. It is possible for two String objects to have the same value, but located in different areas of memory.
== compares references while .equals compares contents. The method public boolean equals(Object obj) is provided by the Object class and can be overridden. The default implementation returns true only if the object is compared with itself, which is equivalent to the equality operator == being used to compare aliases to the object. String, BitSet, Date, and File override the equals() method. For two String objects, value equality means that they contain the same character sequence. For the Wrapper classes, value equality means that the primitive values are equal.
public class EqualsTest{
public static void main(String[] args){
String s1 = "abc";
String s2 = s1;
String s5 = "abc";
String s3 = new String("abc");
String s4 = new String("abc");
// if we remove the brackets around "s1 == s5' it gives a different result.
System.out.println("== comparison : " +(s1 == s5));
System.out.println("== comparison : " +(s1 == s2));
System.out.println("Using equals method : " +s1.equals(s2));
System.out.println("== comparison : " +s3 == s4);
System.out.println("Using equals method : " +s3.equals(s4));
}
}
Output
== comparison : true
== comparison : true
Using equals method : true
false
Using equals method : true
What if the static modifier is removed from the signature of the main method?
Or
What if I do not provide the String array as the argument to the method?
Program compiles. But at runtime throws an error "NoSuchMethodError".
Why oracle Type 4 driver is named as oracle thin driver?
Oracle provides a Type 4 JDBC driver, referred to as the Oracle “thin” driver. This driver includes its own implementation of a TCP/IP version of Oracle’s Net8 written entirely in Java, so it is platform independent, can be downloaded to a browser at runtime, and does not require any Oracle software on the client side. This driver requires a TCP/IP listener on the server side, and the client connection string uses the TCP/IP port address, not the TNSNAMES entry for the database name.
What is the difference between final, finally and finalize? What do you understand by the java final keyword?
Or
What is final, finalize() and finally?
Or
What is finalize() method?
Or
What is the difference between final, finally and finalize?
Or
What does it mean that a class or member is final?
o final - declare constant
o finally - handles exception
o finalize - helps in garbage collection
Variables defined in an interface are implicitly final. A final class can't be extended i.e., final class may not be subclassed. This is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant). finalize() method is used just before an object is destroyed and garbage collected. finally, a key word used in exception handling and will be executed whether or not an exception is thrown. For example, closing of open connections is done in the finally method.
What is the Java API?
The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets.
What is the GregorianCalendar class?
The GregorianCalendar provides support for traditional Western calendars.
What is the ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.
Why there are no global variables in Java?
Global variables are globally accessible. Java does not support globally accessible variables due to following reasons:
* The global variables breaks the referential transparency
* Global variables creates collisions in namespace.
How to convert String to Number in java program?
The valueOf() function of Integer class is is used to convert string to Number. Here is the code example:
String numString = "1000";
int id=Integer.valueOf(numString);
What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.
What is the difference between a while statement and a do statement?
A while statement (pre test) checks at the beginning of a loop to see whether the next loop iteration should occur. A do while statement (post test) checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the loop body at least once.
What is the Locale class?
The Locale class is used to tailor a program output to the conventions of a particular geographic, political, or cultural region.
Describe the principles of OOPS.
There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation.
Explain the Inheritance principle.
Inheritance is the process by which one object acquires the properties of another object. Inheritance allows well-tested procedures to be reused and enables changes to make once and have effect in all relevant places
What is implicit casting?
Implicit casting is the process of simply assigning one entity to another without any transformation guidance to the compiler. This type of casting is not permitted in all kinds of transformations and may not work for all scenarios.
Example
int i = 1000;
long j = i; //Implicit casting
Is sizeof a keyword in java?
The sizeof operator is not a keyword.
What is a native method?
A native method is a method that is implemented in a language other than Java.
In System.out.println(), what is System, out and println?
System is a predefined final class, out is a PrintStream object and println is a built-in overloaded method in the out object.
What are Encapsulation, Inheritance and Polymorphism
Or
Explain the Polymorphism principle. Explain the different forms of Polymorphism.
Polymorphism in simple terms means one name many forms. Polymorphism enables one entity to be used as a general category for different types of actions. The specific action is determined by the exact nature of the situation.
Polymorphism exists in three distinct forms in Java:
• Method overloading
• Method overriding through inheritance
• Method overriding through the Java interface
What is explicit casting?
Explicit casting in the process in which the complier are specifically informed to about transforming the object.
Example
long i = 700.20;
int j = (int) i; //Explicit casting
What is the Java Virtual Machine (JVM)?
The Java Virtual Machine is software that can be ported onto various hardware-based platforms
What do you understand by downcasting?
The process of Downcasting refers to the casting from a general to a more specific type, i.e. casting down the hierarchy
What are Java Access Specifiers?
Or
What is the difference between public, private, protected and default Access Specifiers?
Or
What are different types of access modifiers?
Access specifiers are keywords that determine the type of access to the member of a class. These keywords are for allowing privileges to parts of a program such as functions and variables. These are:
• Public: accessible to all classes
• Protected: accessible to the classes within the same package and any subclasses.
• Private: accessible only to the class to which they belong
• Default: accessible to the class to which they belong and to subclasses within the same package
Which class is the superclass of every class?
Object.
Name primitive Java types.
The 8 primitive types are byte, char, short, int, long, float, double, and boolean.
What is the difference between static and non-static variables?
Or
What are class variables?
Or
What is static in java?
Or
What is a static method?
A static variable is associated with the class as a whole rather than with specific instances of a class. Each object will share a common copy of the static variables i.e. there is only one copy per class, no matter how many objects are created from it. Class variables or static variables are declared with the static keyword in a class. These are declared outside a class and stored in static memory. Class variables are mostly used for constants. Static variables are always called by the class name. This variable is created when the program starts and gets destroyed when the programs stops. The scope of the class variable is same an instance variable. Its initial value is same as instance variable and gets a default value when its not initialized corresponding to the data type. Similarly, a static method is a method that belongs to the class rather than any object of the class and doesn't apply to an object or even require that any objects of the class have been instantiated.
Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a non-static method. In other words, you can't change a static method into an instance method in a subclass.
Non-static variables take on unique values with each object instance.
What is the difference between the boolean & operator and the && operator?
If an expression involving the boolean & operator is evaluated, both operands are evaluated, whereas the && operator is a short cut operator. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. If the first operand evaluates to false, the evaluation of the second operand is skipped.
How does Java handle integer overflows and underflows?
It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
What if I write static public void instead of public static void?
Program compiles and runs properly.
What is the difference between declaring a variable and defining a variable?
In declaration we only mention the type of the variable and its name without initializing it. Defining means declaration + initialization. E.g. String s; is just a declaration while String s = new String ("bob"); Or String s = "bob"; are both definitions.
What type of parameter passing does Java support?
In Java the arguments (primitives and objects) are always passed by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object.
Explain the Encapsulation principle.
Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. Objects allow procedures to be encapsulated with their data to reduce potential interference. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.
What do you understand by a variable?
Variable is a named memory location that can be easily referred in the program. The variable is used to hold the data and it can be changed during the course of the execution of the program.
What do you understand by numeric promotion?
The Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integral and floating-point operations may take place. In the numerical promotion process the byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.
What do you understand by casting in java language? What are the types of casting?
The process of converting one data type to another is called Casting. There are two types of casting in Java; these are implicit casting and explicit casting.
What is the first argument of the String array in main method?
The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name. If we do not provide any arguments on the command line, then the String array of main method will be empty but not null.
How can one prove that the array is not null but empty?
Print array.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print array.length.
Can an application have multiple classes having main method?
Yes. While starting the application we mention the class name to be run. The JVM will look for the main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.
When is static variable loaded? Is it at compile time or runtime? When exactly a static block is loaded in Java?
Static variable are loaded when classloader brings the class to the JVM. It is not necessary that an object has to be created. Static variables will be allocated memory space when they have been loaded. The code in a static block is loaded/executed only once i.e. when the class is first initialized. A class can have any number of static blocks. Static block is not member of a class, they do not have a return statement and they cannot be called directly. Cannot contain this or super. They are primarily used to initialize static fields.
Can I have multiple main methods in the same class?
No the program fails to compile. The compiler says that the main method is already defined in the class.
Explain working of Java Virtual Machine (JVM)?
JVM is an abstract computing machine like any other real computing machine which first converts .java file into .class file by using Compiler (.class is nothing but byte code file.) and Interpreter reads byte codes.
How can I swap two variables without using a third variable?
Add two variables and assign the value into First variable. Subtract the Second value with the result Value. and assign to Second variable. Subtract the Result of First Variable With Result of Second Variable and Assign to First Variable. Example:
int a=5,b=10;a=a+b; b=a-b; a=a-b;
What is data encapsulation?
Encapsulation may be used by creating 'get' and 'set' methods in a class (JAVABEAN) which are used to access the fields of the object. Typically the fields are made private while the get and set methods are public. Encapsulation can be used to validate the data that is to be stored, to do calculations on data that is stored in a field or fields, or for use in introspection (often the case when using javabeans in Struts, for instance). Wrapping of data and function into a single unit is called as data encapsulation. Encapsulation is nothing but wrapping up the data and associated methods into a single unit in such a way that data can be accessed with the help of associated methods. Encapsulation provides data security. It is nothing but data hiding.
What is reflection API? How are they implemented?
Reflection is the process of introspecting the features and state of a class at runtime and dynamically manipulate at run time. This is supported using Reflection API with built-in classes like Class, Method, Fields, Constructors etc. Example: Using Java Reflection API we can get the class name, by using the getName method.
Does JVM maintain a cache by itself? Does the JVM allocate objects in heap? Is this the OS heap or the heap maintained by the JVM? Why
Yes, the JVM maintains a cache by itself. It creates the Objects on the HEAP, but references to those objects are on the STACK.
What is phantom memory?
Phantom memory is false memory. Memory that does not exist in reality.
Can a method be static and synchronized?
A static method can be synchronized. If you do so, the JVM will obtain a lock on the java.lang. Class instance associated with the object. It is similar to saying:
synchronized(XYZ.class) {
}
What is difference between String and StringTokenizer?
A StringTokenizer is utility class used to break up string.
Example:
StringTokenizer st = new StringTokenizer("Hello World");
while (st.hasMoreTokens()) {
System.out.println(st.nextToken());
}
Output:
Hello
World
Explain Garbage collection mechanism in Java?
Garbage collection is one of the most important features of Java. The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use.
In Java on calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected. Garbage collection is an automatic process and can't be forced. There is no guarantee that Garbage collection will start immediately upon request of System.gc().
What kind of thread is the Garbage collector thread?
It is a daemon thread.
Can an object’s finalize() method be invoked while it is reachable?
An object’s finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object’s finalize() method may be invoked by other objects.
Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.
What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup, before the object gets garbage collected. For example, closing an opened database Connection.
If an object is garbage collected, can it become reachable again?
Once an object is garbage collected, It can no longer become reachable again.
1 comment:
Hi There,
Thank you! Thank you! Thank you! Your blog was a total game changer!
I have the following doubts that I would like to understand
- To read the color of a particular pixel of the screen (in its totality)
- To read the color of a particular pixel of an applet or window of application in Java
- To handle one second plate of VGA video.
Thanks a lot. This was a perfect step-by-step guide. Don’t think it could have been done better.
Shukran,
Stella
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