Best JSP/Servlets interview questions Answers

1. What is the difference between JSP and Servlets ?
JSP is used mainly for presentation only. A JSP can only be HttpServlet that means the only supported protocol in JSP is HTTP. But a servlet can support any protocol like HTTP, FTP, SMTP etc.
2. What is difference between custom JSP tags and beans?
Custom JSP tag is a tag you defined. You define how a tag, its attributes and its body are interpreted, and then group your tags into collections called tag libraries that can be used in any number of JSP files. To use custom JSP tags, you need to define three separate components: the tag handler class that defines the tag’s behavior ,the tag library descriptor file that maps the XML element names to the tag implementations and the JSP file that uses the tag library
JavaBeans are Java utility classes you defined. Beans have a standard format for Java classes. You use tags
Custom tags and beans accomplish the same goals — encapsulating complex behavior into simple and accessible forms. There are several differences:
Custom tags can manipulate JSP content; beans cannot. Complex operations can be reduced to a significantly simpler form with custom tags than with beans. Custom tags require quite a bit more work to set up than do beans. Custom tags usually define relatively self-contained behavior, whereas beans are often defined in one servlet and used in a different servlet or JSP page. Custom tags are available only in JSP 1.1 and later, but beans can be used in all JSP 1.x versions.
3. What are the different ways for session tracking?
Cookies, URL rewriting, HttpSession, Hidden form fields
4. What mechanisms are used by a Servlet Container to maintain session information?
Cookies, URL rewriting, and HTTPS protocol information are used to maintain session information
5. Difference between GET and POST
In GET your entire form submission can be encapsulated in one URL, like a hyperlink. query length is limited to 255 characters, not secure, faster, quick and easy. The data is submitted as part of URL.
In POST data is submitted inside body of the HTTP request. The data is not visible on the URL and it is more secure.
6. What is session?
The session is an object used by a servlet to track a user’s interaction with a Web application across multiple HTTP requests. The session is stored on the server.
7. What is servlet mapping?
The servlet mapping defines an association between a URL pattern and a servlet. The mapping is used to map requests to Servlets.
8. What is servlet context ?
The servlet context is an object that contains a information about the Web application and container. Using the context, a servlet can log events, obtain URL references to resources, and set and store attributes that other servlets in the context can use.
9. What is a servlet ?
servlet is a java program that runs inside a web container.
10. Can we use the constructor, instead of init(), to initialize servlet?
Yes. But you will not get the servlet specific things from constructor. The original reason for init() was that ancient versions of Java couldn’t dynamically invoke constructors with arguments, so there was no way to give the constructor a ServletConfig. That no longer applies, but servlet containers still will only call your no-arg constructor. So you won’t have access to a ServletConfig or ServletContext.
12. How many JSP scripting elements are there and what are they?
There are three scripting language elements: declarations, scriptlets, expressions.
13. How do I include static files within a JSP page?
Static resources should always be included using the JSP include directive. This way, the inclusion is performed just once during the translation phase.
14. How can I implement a thread-safe JSP page?
You can make your JSPs thread-safe adding the directive <%@ page isThreadSafe="false" % > within your JSP page.
15. What is the difference in using request.getRequestDispatcher() and context.getRequestDispatcher()?
In request.getRequestDispatcher(path) in order to create it we need to give the relative path of the resource. But in resourcecontext.getRequestDispatcher(path) in order to create it we need to give the absolute path of the resource.
16. What are the lifecycle of JSP?
When presented with JSP page the JSP engine does the following 7 phases.
Page translation: -page is parsed, and a java file which is a servlet is created.
Page compilation: page is compiled into a class file
Page loading : This class file is loaded.
Create an instance :- Instance of servlet is created
jspInit() method is called
_jspService is called to handle service calls
_jspDestroy is called to destroy it when the servlet is not required.
17. What are context initialization parameters?
Context initialization parameters are specified by the in the web.xml file, these are initialization parameter for the whole application.
18. What is a Expression?
Expressions are act as place holders for language expression, expression is evaluated each time the page is accessed. This will be included in the service method of the generated servlet.
19. What is a Declaration?
It 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 semicolons separate them. The declaration must be valid in the scripting language used in the JSP file. This will be included in the declaration section of the generated servlet.
20. What is a Scriptlet?
A scriptlet can contain any number of language statements, variable or expressions that are valid in the page scripting language. Within scriptlet tags, you can declare variables to use later in the file, write expressions valid in the page scripting language, use any of the JSP implicit objects or any object declared with a . Generally a scriptlet can contain any java code that are valid inside a normal java method. This will become the part of generated servlet’s service method.

21. What are the implicit objects?
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 are: request, response, pageContext, session, application, out, config, page, exception
22. What’s the difference between forward and sendRedirect?
forward is server side redirect and sendRedirect is client side redirect. 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 completely with in the web container And then returns to the calling method. When a sendRedirect 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 completely 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. Client can disable sendRedirect.
23. What are the different scopes available ?
page, request, session, application
24. Is JSP extensible ?
Yes, it is. JSP technology is extensible through the development of custom actions, or tags, which are encapsulated in tag libraries.
25. What’s the Servlet Interface?
The central abstraction in the Servlet API is the Servlet interface. All servlets implement this interface, either directly or, more commonly, by extending a class that implements it such as HttpServlet.
26. What are two different types of Servlets ?
GenericServlet and HttpServlet. HttpServlet is used to implement HTTP protocol, where as Generic servlet can implement any protocol.
27. What is the life cycle of servlet?
Each servlet has the same life cycle: first, the server loads and initializes the servlet by calling the init method. This init() method will be executed only once during the life time of a servlet. Then when a client makes a request, it executes the service method. finally it executes the destroy() method when server removes the servlet.
28. Can we call destroy() method on servlets from service method ?
Yes.
29. What is the need of super.init (config) in servlets ?
Then only we will be able to access the ServletConfig from our servlet. If there is no ServletConfig our servlet will not have any servlet nature.
30. What is the difference between GenericServlet and HttpServlet?
GenericServlet supports any protocol. HttpServlet supports only HTTP protocol. By extending GenericServlet we can write a servlet that supports our own custom protocol or any other protocol.
32. Can we write a constructor for servlet ?
Yes. But the container will always call the default constructor only. If default constructor is not present , the container will throw an exception.
33. What is the difference between <%@ include ...> (directive include) and ?
@ include is static include. It is inline inclusion. The contents of the file will get included on Translation phase. It is something like inline inclusion. We cannot have a dynamic filename for directive include. is dynamic include. Here the included file will be processed as a separate file and the response will be included. We can have a dynamic filename for . We can also pass parameters to
34. Can I just abort processing a JSP?
Yes. You can put a return statement to abort JSP processing.
35. What are the parameters for service method ?
ServletRequest and ServletResponse
36. What are cookies ?
Cookies are small textual information that are stored on client computer. Cookies are used for session tracking.
37. How do I prevent the output of my JSP or Servlet pages from being cached by the browser?
By setting appropriate HTTP header attributes we can prevent caching by the browser

<% 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 %>

38. How to refer the “this” variable within a JSP page?
Under JSP 1.0, the page implicit object page is equivalent to “this”, and returns a reference to the servlet generated by the JSP page.
39. How many JSP scripting elements and what are they?
There are three scripting elements in JSP . They are declarations, scriptlets, expressions.
40. Can we implement an interface in JSP ?
No.

41. What is the meaning of response has already been committed error?
You will get this error only when you try to redirect a page after you already have flushed the output buffer. This happens because HTTP specification force the header to be set up before the lay out of the page can be shown. When you try to send a redirect status, your HTTP server cannot send it right now if it hasn’t finished to set up the header. Simply it is giving the error due to the specification of HTTP 1.0 and 1.1
42. How do I use a scriptlet to initialize a newly instantiated bean?
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 (Only at the time of instantiation.) 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.
43. What is JSP?
JSP is a server side scripting technology. JSP allows Java as well as a few special tags to be embedded into a web file (HTML/XML, etc). The suffix must ends with .jsp.
44. What are JSP Actions?
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:include, jsp:useBean, jsp:setProperty, jsp:getProperty, jsp:forward and Jsp: plugin
45. What is the difference between ServletContext and ServletConfig?
The ServletConfig gives the information about the servlet initialization parameters. The servlet engine implements the ServletConfig interface in order to pass configuration information to a servlet. The server passes an object that implements the ServletConfig interface to the servlet’s init() method. The ServletContext gives information about the container. The ServletContext interface provides information to servlets regarding the environment in which they are running. It also provides standard way for servlets to write events to a log file.
46. How can a servlet refresh automatically?
We can use a client-side Refresh or Server Push
47. What is Server side push?
Server Side push is useful when data needs to change regularly on the clients application or browser, without intervention from client. The mechanism used is, when client first connects to Server, then Server keeps the TCP/IP connection open.
48. What is client side refresh?
The standard HTTP protocols ways of refreshing the page, which is normally supported by all browsers.

<META HTTP-EQUIV="Refresh" CONTENT="5; URL=/servlet/MyServlet/">

This will refresh the page in the browser automatically and loads the new data every 5 seconds.
49. What is the Max amount of information that can be saved in a Session Object ?
There is no such limit on the amount of information that can be saved in a Session Object. The only limit is the Session ID length , which should not exceed more than 4K.
50. Why should we go for inter servlet communication?
The three major reasons to use inter servlet 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)
51. What is a output comment?
A comment that is sent to the client in the viewable page source. The JSP engine handles an output comment as un interpreted 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.
52. What is a Hidden Comment
Hidden Comments are JSP comments. 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.
53. What are the differences between a session and a cookie?
Session is stored in server but cookie stored in client. Session should work regardless of the settings on the client browser. There is no limit on the amount of data that can be stored on session. But it is limited in cookie. Session can store objects and cookies can store only strings. Cookies are faster than session.
54. What is HttpTunneling?
HTTP tunneling is used to encapsulate other protocols within the HTTP or HTTPS protocols. Normally the intranet is blocked by a firewall and the network is exposed to the outer world only through a specific web server port, that listens for only HTTP requests. To use any other protocol, that by passes the firewall, the protocol is embedded in HTTP and send as HttpRequest.
55. How to pass information from JSP to included JSP?
By using tag.
56. What is the better way to enable thread-safe servlets and JSPs? SingleThreadModel Synchronization?
The better approach is to use synchronization. Because SingleThreadModel is not scalable. SingleThreadModel is pretty resource intensive from web server’s perspective. The most serious issue is when the number of concurrent requests exhaust the servlet instance pool. In that case, all the un serviced requests are queued until something becomes free – which results in poor performance.
57. What is the difference between ServletContext and PageContext?
ServletContext gives the information about the container and PageContext gives the information about the Request
58. Why in Servlet 2.4 specification SingleThreadModel has been deprecated?
SingleThreadModel is pretty resource intensive from web server’s perspective. When the number of concurrent requests exhaust the servlet instance pool, all the un serviced requests are queued until something becomes free – which results in poor performance.
59. How do you pass data (including JavaBeans) to a JSP from a servlet?
By forwarding the request to the servlet ( the data must be there in the request scope) we can pass the data from a JSP to servlet. Also we can use a session to pass the data.
60. How can I set a cookie?

 Cookie c = new Cookie("name","value");
    response.addCookie(c);

61. How will you delete a cookie?

1. Cookie c = new Cookie ("name", null);  
2. c.setMaxAge(0);  
3. response.addCookie(killCookie);  

62. What is the difference between Context init parameter and Servlet init parameter?
Servlet init parameters are for a single servlet only. No body out side that servlet can access that. It is declared inside the tag inside Deployment Descriptor, where as context init parameter is for the entire web application. Any servlet or JSP in that web application can access context init parameter. Context parameters are declared in a tag directly inside the tag. The methods for accessing context init parameter is getServletContext ().getInitParamter (“name”) where as method for accessing servlet init parameter is getServletConfig ().getInitParamter (“name”);
63. What are the different ways for getting a servlet context?
We will get ServletContext by calling getServletConfig ().getServletContext (). This is because a ServletConfig always hold a reference to ServletContext. By calling this.getServletContext () also we will get a ServletContext object.
64. What is the difference between an attribute and a parameter?
The return type of attribute is object, where the return type of parameter is String. The method to retrieve attribute is getAttribute () where as for parameter is getParameter (). We have a method setAttribute to set an attribute. But there is no setters available for setting a parameter.
65. How to make a context thread safe?
Synchronizing the ServletContext is the only solution to make a ServletContext thread safe.
Eg:

1. synchronized (getServletContext ()) {  
2.       // do whatever you want with thread safe context.  
3. }  

66. What is the difference between setting the session time out in deployment descriptor and setting the time out programmatically?
In DD time out is specified in terms of minutes only. But in programmatically it is specified in seconds. A session time out value of zero or less in DD means that the session will never expire. To specify session will never expire programmatically it must be negative value.
67. What JSP lifecycle methods we can override?
You cannot override the _jspService() method within a JSP page. You can however, override the jspInit() and jspDestroy() methods within a JSP page.
68. How will you include a static file in a JSP page?
You can include a static resource to a JSP using or <%@ inlcude >.
69. How you can perform browser redirection?
We can use the method sendRedirect of HttpServletResponse or forward method of RequestDispatcher.
70. Can we use ServletOutputStream object from a JSP page?
No. You are supposed to use JSPWriter object (given to you in the form of the implicit object out) only for replying to clients.
71. How can you stop JSP execution in the middle of processing a request?
We can use the return statement to stop the processing of JSP. Because JSP is compiled to servlet and all the statements will go inside service method, any time you can stop the processing using return statement.
72. How can I invoke a JSP error page from a servlet?
You can invoke the JSP error page and pass the exception object to it from within a servlet. For that you need to create a request dispatcher for the JSP error page, and pass the exception object as a javax.servlet.jsp.jspException request attribute.
73. How will you pass information from JSP to included JSP?
By using <%jsp:param> tag.
74. How does JSP handle runtime exceptions?
Using errorPage attribute of page directive JSP handles runtime exceptions. We need to specify isErrorPage=true if the current page is intended to use as a JSP error page.
75. How can I enable session tracking for JSP pages if the browser has disabled cookies?
By default session tracking uses cookies to associate a session identifier with a user. If the browser does not support cookies, or if cookies are disabled, you can still use session tracking using URL rewriting. For URL rewriting to be effective, you need to append the session ID for each and every link that is part of your servlet response. By using the methods response.encodeURL() and response.encodeRedirectURL() we can achieve this.
76. How can you declare methods in your JSP page?
You can declare methods as declarations in your JSP page. The methods can then be invoked within any other methods you declare, or within JSP scriptlets and expressions.
77. How can we set the inactivity period on a per-session basis?
We can set the session time out programmatically by using the method setMaxInactiveInterval() of HttpSession.
78. How do you pass an init parameter to a JSP?
You need to configure the DD for passing init parameter to a JSP. You can configure the DD as follows.

1.     <servlet>  
2.           <servlet-name>test.jsp</servlet-name>  
3.           <jsp-file>test.jsp  
4.                <init-param>  
5. <param-name>Abc</param-name>  
6. <param-value>Xyz</param-value>  
7.            </init-param>  
8.     </jsp-file></servlet>  
9.       

79. How can my application get to know when a HttpSession is removed?
You can define a class which implements HttpSessionBindingListener and override the valueUnbound() method.
80. How many cookies can one set in the response object of the servlet? Also, are there any restrictions on the size of cookies?
If the client is using Netscape, the browser can receive and store 300 total cookies and 4 kilobytes per cookie. And the no of cookie is restricted to 20 cookies per server or domain

81. When a session object gets added or removed to the session, which event will get notified ?
HttpSessionBindingListener will get notified When an object is added and/or removed from the session object, or when the session is invalidated, in which case the objects are first removed from the session, whether the session is invalidated manually or automatically (timeout).
82. What is URL Encoding and URL Decoding ?
URL encoding is the method of replacing all the spaces and other extra characters into their corresponding Hex Characters and Decoding is the process of converting all Hex Characters back to their normal form.
83. What is the difference between an applet and a servlet?
Servlets run inside a web server and applets run inside web browsers. Applets must have graphical user interfaces whereas servlets have no graphical user interfaces.
84. What are the different web servers available name few of them?
Tomcat, Java Web Server, JRun,
85. What is a Java Bean?
A Java Bean is a software component that has been designed to be reusable in a variety of different environments.
86. What are the different types of ServletEngines?
The different types of ServletEngines available are:- Standalone ServletEngine: This is a server that includes built-in support for servlets. Add-on ServletEngine: It is a plug-in to an existing server. It adds servlet support to a server that was not originally designed with servlets in mind.
87. What is a Session Id?
It is a unique id assigned by the server to the user when a user starts a session.
88. What is use of parseQueryString ?
It parses a query string and builds a Hashtable of key-value pairs, where the values are arrays of strings. The query string should have the form of a string packaged by the GET or POST method.
89. When init() and Destroy() will be called.
init() is called whenever the servlet is loaded for the first time into the web server. Destroy will be called whenever the servlet is removed from the web server.
90. What is the use of setComment and getComment methods in Cookies ?
setComment: If a web browser presents this cookie to a user, the cookie’s purpose will be described using this comment. This is not supported by version zero cookies.
getComment: Returns the comment describing the purpose of this cookie, or null if no such comment has been defined.
91. Why we are used setMaxAge() and getMaxAge() in Cookies ?
setMaxAge : Sets the maximum age of the cookie. The cookie will expire after that many seconds have passed. Negative values indicate the default behavior, and will be deleted when the browser closes.
getMaxAge : Returns the maximum specified age of the cookie. If none was specified, a negative value is returned, indicating the default behavior described with setMaxAge.
92. What is the use of setSecure() and getSecure() in Cookies ?
setSecure method indicates to the web browser that the cookie should only be sent using a secure protocol (https). getSecure method returns the value of the ‘secure’ flag.
93. How do you communicate between the servlets?
We can communicate between servlets by using RequestDespatcher interface and servlet chaining.
94. What is Servlet chaining?
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 the input of next servlet. This process continues until the last servlet is reached. Its output is then sent back to the client. We are achieving Servlet Chaining with the help of RequestDispatcher.
95. How will you communicate from an applet to servlet?
There are three ways to communicate from an applet to servlet and they are: HTTP Communication (Text-based and object-based) , Socket Communication and RMI Communication
96. Can we call a servlet with parameters in the URL?
Yes. You can call a servlet with parameters in the URL like ?param=value
97. How do servlets handle multiple simultaneous requests?
When a request comes in, the web server will start a new thread and the request is assigned to a thread, which calls a service method of the servlet.
98. Explain the directory structure of a web application?
The directory structure of a web application consists of two parts. A private directory called WEB-INF and a public resource directory which contains files server to public. WEB-INF folder consists of web.xml (the deployment descriptor), classes directory (where we keeps all our classes and servlets) and lib directory (where we keeps all our jar files). The public folder contains the static resources of the web application.
99. What is pre initialization of a servlet?
A container doesn’t initialize the servlets when it starts up. It initializes a servlet when it receives a request for that servlet first time. This is called lazy loading. The servlet specification defines the element, which can be specified in the deployment descriptor to make the servlet container load and initialize the servlet as soon as it starts up. The process of loading a servlet before any request comes in is called preloading or pre initializing a servlet.
100. What are the uses of ServletRequest?
The ServletRequest gives information such as the names of the parameters passed by the client, the protocol (scheme) being used by the client, and the names of the remote host that made the request and the server that received it. The input stream, ServletInputStream.


101. What are the uses of ServletResponse interface?
ServletResponse allows the servlet to set the content length and MIME type of that response. It provides an output stream, ServletOutputStream and a Writer through which the servlet can send data.
102. How HTTP Servlet handles client requests?
An HTTP Servlet handles client requests through its service method. The service method supports standard HTTP client requests by dispatching each request to a method designed to handle that request.
103. Is HTML page a web component?
No. Static HTML pages and applets are not considered as web components by J2EE specification. Even the server-side utility classes are not considered web components.
104. What is the container?
A container is a runtime support of a system-level entity. Containers provide components with services such as lifecycle management, security, deployment, and threading.
105. What is the web container?
Servlet and JSP containers are collectively referred to as Web containers.
106. What is deployment descriptor?
A deployment descriptor is an XML based file which describes a web application’s deployment settings. The name of deployment descriptor of a web application is web.xml.
107. How can I print the stack trace of an exception from a JSP page?
By creating an object of PrintWriter we can print the stack trace in JSP page. Eg:
PrintWriter pw = response.getWriter();
exception.printStackTrace(pw);
108. Do objects stored in a HTTP Session need to be serializable? Or can it store any object?
No, the objects need not to be serializable. We can store any type of objects in session.
109. What is the difference between JspWriter and PrintWriter?
JspWriter is buffered.
110. How can you implement singleton pattern in servlets ?
All servlets are singleton only , if the servlet is not implementing SingleThreadModel.
111. What is the difference between an application server and a web server?
Everything in web server must be achieved programmatically. But application server provides a lot of services like security, transaction , scalability. Application server will be having EJB support where web server don’t have an EJB support.
112. What is servlet exception?
It indicates that there is an exception occurred in the servlet.
113. Can we implement Runnable interface from within our servlet?
Our servlet can implement the Runnable interface.
114. What is a WAR file?
WAR stands for Web Archive. It is a compressed version of your web application. You can use this WAR file to deploy your web application.
115. What is a servlet filter?
Servlet filters are pluggable Web components that allow us to implement pre-processing and post-processing logic in our Web applications.

Long Term Solution for java.lang.OutOfMemoryError:

This Error is thrown when the Java Virtual Machine cannot allocate an object because it is out of memory, and no more memory could be made available by the garbage collector. Note: Its an Error (extends java.lang.Error) not Exception. Two important types of OutOfMemoryError are often encountered

1. java.lang.OutOfMemoryError: Java heap space

The quick solution is to add these flags to JVM command line when Java runtime is started:


1. -Xms1024m -Xmx1024m   

2. java.lang.OutOfMemoryError: PermGen space

The solution is to add these flags to JVM command line when Java runtime is started:


1. -XX:+CMSClassUnloadingEnabled-XX:+CMSPermGenSweepingEnabled  

Long Term Solution: Increasing the Start/Max Heap size or changing Garbage Collection options may not always be a long term solution for your Out Of Memory Error problem. Best approach is to understand the memory needs of your program and ensure it uses memory wisely and does not have leaks. You can use a Java memory profiler to determine what methods in your program are allocating large number of objects and then determine if there is a way to make sure they are no longer referenced, or to not allocate them in the first place.

Master list of Java interview questions and Answers

1. What is the difference between procedural and object-oriented programs?- a) In procedural program, programming logic follows certain procedures and the instructions are executed one after another. In OOP 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.
2. What are Encapsulation, Inheritance and Polymorphism?- 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.
3. What is the difference between Assignment and Initialization?- Assignment can be done as many times as desired whereas initialization can be done only once.


4. What is OOPs?- 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.
5. What are Class, Constructor and Primitive data types?- 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?- 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?- Constructor will be automatically invoked when an object is created whereas method has to be called explicitly.
8. What are methods and how are they defined?- 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?- Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.
10. What is casting?- 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?- 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?- 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?- 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?- 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?- 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?- 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?- 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?- 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?- 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?- 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) Overloading must have different method signatures whereas overriding must have same signature.
21. What is meant by Inheritance and what are its advantages?- 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 super class by subclasses.
22. What is the difference between this() and super()?- 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 superclass and subclass?- 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?- public, abstract and final can be used for top-level class.
25. What are inner class and anonymous class?- 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?- 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?- java. lang. reflect package has the ability to analyze itself in runtime.
28. What is interface and its use?- 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?- 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?- 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?- It is not having any method because it is a TAGGED or MARKER interface.
32. What is the difference between abstract class and interface?- 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?- 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?- 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?- 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?- The exception class defines mild error conditions that your program encounters. Exceptions can occur when trying 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.
37. What is the difference between process and thread?- 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?- 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?- 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?- Thread contains ready, running, waiting and dead states.
41. What is synchronization?- 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?- 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?- When two threads are waiting each other and can’t precede the program is said to be deadlock.
44. What is daemon thread and which method is used to create the daemon thread?- 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?- 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?- 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?- 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?- Using getParameters() method.
49. When do you use codebase in applet?- When the applet class file is not in the same directory, codebase is used.
50. What is the lifecycle of an applet?- 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?- using setSecurityManager() method
52. What is an event and what are the models available for event handling?- 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?- 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?- 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?- 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?- 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?- 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?- 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?- 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?- 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?- Window, Frame and Dialog classes use a BorderLayout as their layout.
62. Which containers use a Flow layout as their default layout?- Panel and Applet classes use the FlowLayout as their default layout.
63. What are wrapper classes?- Wrapper classes are classes that allow primitive types to be accessed as objects.
64. What are Vector, Hashtable, LinkedList and Enumeration?- 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?- 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?- 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?- The Reader/Writer class is character-oriented and the InputStream/OutputStream class is byte-oriented.
68. What is an I/O filter?- 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?- 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?- 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?- 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?- 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?- 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 updating of the above.
74. What are the steps involved for making a connection with a database or how do you connect to a database?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, DriverManager. 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?- 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?- Statement: to be used createStatement() method for executing single SQL statement PreparedStatement — To be used preparedStatement() 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?- 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?- To create stored procedures: Create procedure procedurename (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?- 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?- There are two packages in servlets and they are javax. servlet and
81. What is the difference between an applet and a servlet?- 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?- 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 are invisible to the client.
83. What is the life cycle of a servlet?- 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?- Web server
85. What are the different servers available for developing and deploying Servlets?- 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?- 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?- 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)?- Server-Side Includes allows embedding 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?- 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?- 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
91. What is connection pooling?- 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?- 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?- Yes. You can call a servlet with parameters in the syntax as (?Param1 = xxx || m2 = yyy).
94. What is Servlet chaining?- 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?- 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?- 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?- Every computer connected to a network has an IP address. An IP address is a number that uniquely identifies each computer on the Net. An IP address is a 32-bit number.
98. What is Domain Naming Service(DNS)?- 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?- URL stands for Uniform Resource Locator and it points to resource files on the Internet. URL has four components: http://www. address. com:80/index.html, where http - protocol name, address - 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?- 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?- 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?- 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?- 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?- 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?- Jar file allows to efficiently deploying 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?- 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?- 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 files. 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?- 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?- 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″ attribute 2=”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?- 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?- 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:include - Include a file at the time the page is requested. 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?- (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:<jsp: useBean id=”theBean” scope=”request” class=”. . . . . ” />(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:<jsp:useBean id=”theBean” scope=”session” class=”. . . ” /> 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:<jsp:useBean id=”theBean” scope=”application” class=”. . . ” />
113. How can I set a cookie in JSP?- 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?- 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?- 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.

Useful J2EE Design Patterns

Q) What is a Desin Pattern? Why use patterns?

A) A pattern describes a proven solution to a recurring design problem. Patterns provide a ready-made solution that can be adapted to different problems as necessary.

Q) J2EE Design Patterns?

Dispatcher View: Combines a Dispatcher component with the Front Controller and View Helper patterns, deferring many activities to View processing.

Service to Worker: Combines a Dispatcher component with the Front Controller and View Helper patterns.

Transfer Object Assembler: It is used to build the required model or submodel. The Transfer Object Assembler uses Transfer Objects to retrieve data from various business objects and other objects that define the model or part of the model.

Composite Entity :It model, represent, and manage a set of interrelated persistent objects rather than representing them as individual fine-grained entity beans. A Composite Entity bean represents a graph of objects.

Service Activator: Service Activator enables asynchronous access to enterprise beans and other business services. It receives asynchronous client requests and messages. On receiving a message, the Service Activator locates and invokes the necessary business methods on the business service components to fulfill the request asynchronously. In EJB2.0, Message Driven beans can be used to implement Service Activator for message based enterprise applications. The Service Activator is a JMS Listener and delegation service that creates a message façade for the EJBs.

Q) What is architectural design pattern?

A) Describe MVC2 & Front Controller.

Front Controller

 It will dispatch the request to the correct resource, Centralized controller for managing and holding of a request.

Service Locator

To access different resources/services, J2EE compatible server binds these resources/services to the JNDI server so that the clients can lookup those resources/services through JNDI lookup process from anywhere in the network. The resources/services can be

                1. EJBHome objects    2. DataSource objects               3. JMS ConnectionFactory   4. JMS Topic/Queue etc.

All these services need to bind to the JNDI services and the clients need to lookup JNDI to get those services. Clients have to go through JNDI lookup process every time to work with these services. JNDI lookup process is expensive because clients need to get network connection to the JNDI server if the JNDI server is located on a different machine and need to go through lookup process every time, this is redundant and expensive.

The solution for the redundant and expensive JNDI lookup process problem is to cache those service objects when the client performs JNDI lookup first time and reuse that service object from the cache second time onwards for other clients. This technique maintains a cache of service objects and looks up the JNDI only first time for a service object.

Session Façade

EJB clients (swing, servlets, jsps etc) can access entity beans directly. If EJB clients access entity beans directly over the network, it takes more network calls and imposes network overhead.          

Here the servlet calls multiple entity beans directly to accomplish a business process, thereby increasing the number of network calls.

The solution for avoiding number of network calls due to directly accessing multiple entity beans is to wrap entity beans with session bean (Facade). The EJB client accesses session bean (Facade) instead of entity beans through coarse grained method call to accomplish a business process.

Message Facade

Session bean and entity bean methods execute synchronously that means the method caller has to wait till a value is returned. In some situations like sending hundred's of mails or firing a batch process or updating processes, the client does not have to bother about return value. If you use synchronous session and entity beans in such situations, they take a long time to process methods and clients have to wait till the method returns a value.

The client has to wait till all the eight synchronous steps complete. This synchronous execution takes a long time and has an impact on performance when the method process is huge.

To avoid blocking of a client, use asynchronous message driven beans, so that client does not have to wait for a return value. If a client uses asynchronous messaging then the client need not wait for a return value but can continue its flow of execution after sending the message.

Value Object (DTO-DataTransfer Object)

When a client calls a remote method there will be process of marshalling, network calls and unmarshalling involved for the remote method invocation. If you choose fine-grained approach when calling methods remotely, there will be a significant network overhead involved. For example if you call fine grained method like this,

        remoteObject.getName();

        remoteObject.getCity();

        remoteObject.getState();

there are three network calls from client to the remote object because every method call is remote method call.

The solution for avoiding many network calls due to fine-grained method calls is to use coarse-grained approach. For example:

        // Create a Value Object and fill that object locally

        PersonInfo person = new PersonInfo();

        person.setName("Ravi");

        person.setCity("Austin");

        // send Value Object through network

        remoteObject.getPersonInfo(person);

Here, there is only one network call instead of three network calls and PersonInfo object is a Value Object. The following figure illustrates the coarse grained approach that is passing a Value Object through network.

Value Object is an object that is passed over the network rather than passing each attributes separately thus increasing performance by reducing network calls.

ValueObjectFactory

For a single request, a client might need to access multiple server side components such as different session beans and entity beans. In such situations the client accesses multiple components over the network, this increases the network traffic and has an impact on the performance.

To reduce the network traffic due to accessing multiple components by a client for a single request, let ValueObjectFactory hold different ValueObjects as placeholders and respond with a single ValueObject for a client request.

Value List Handler (DAO)

J2EE applications generally have the search facility and have to search huge data and retrieve results. If an application returns huge queried data to the client, the client takes long time to retrieve that large data and If that application uses entity bean to search data, it has an impact on.

1. Use Data Access Objects (DAO) rather than Entity beans

2. Return small quantity of data multiple times iteratively rather than returning large amount of data at once to the client.

DAO encapsulates JDBC access logic. ValueListHandler caches list of Value objects that are retrieved through DAO. When client wants to search data, It calls ValueListHandler that is in turn responsible for caching data and returning data to the client iteratively. 

Singleton

à You can achieve this by having the private constructor in the class, so that other classes can't create a new instance. Its intent is to ensure that a class has only one instance, and to provide a global point of access to it. There are many situations in which a singleton object is necessary: a GUI application must have a single mouse, an active modem needs one and only one telephone line, an operating system can only have one window manager, and a PC is connected to a single keyboard

1.Create a Private constructor, so that outside class can not access this constructor. And declare a private static reference of same class.

2.Write a public Factory method which creates an object. Assign this object to private static Reference and return the object

public class Singleton

{

private static Singleton ref;

    private Singleton (){

    }

    public static Singleton getSingleton()

    {

      if (ref == null)

          ref = new Singleton ();

      return ref;

    }

}

Business Delegate

The B.D acts as a client-side business abstraction and hides the implementation of the business services. such as lookup & access details of the EJB architecture.

The delegate may cache results and references to remote business services. Caching can significantly improve performance, because it limits unnecessary and potentially costly round trips over the network.

B.D uses a component called the Lookup Service. The Lookup Service is responsible for hiding the underlying implementation details of the business service lookup code.

The client requests the BusinessDelegate to provide access to the underlying business service. The BusinessDelegate uses a LookupService to locate the required BusinessService component.

Q). Where do you use singleton pattern and why?

A) If I require a single instance of an object in a particular JVM, ex while designing database connection pool. I would require a single connection object for all the users coming in, not a separate one for each user.

Q). Why Factory Pattern is used and an example?

A) Factory pattern is used in place where the implementation varies over time. Factory pattern suggest creating many different instances from interfaces. Interfaces are the one that faces client and implementation of those methods come from factory depending on a specific condition.

     ex: If the OS is Windows, look and feel of the application changes to Window style, for Linux it is Metal and for machintosh it will be different.

    

Q). Where do you use visitor pattern and why?

A) If I want to defrag business logic in different sets of modules and the final processing requires all these module to be included in a particular fashion. Visitor pattern generally calls a visit method of these modules /objects and

all the different logic stored in different modules and call one by one. It is something like visiting many modules one at a time.

    

Q). What problem an observer pattern solves?

A) If a particular event has to be notified to many objects, and list grows over time and it is hardly possible to call /notify each and every listeners at design time. We use observer pattern , just to register many objects listening to a particular event and getting notified automatically, as and when the event occurs.

    

Q) What is the difference between J2EE design patterns and the Gang of Four patterns?

A) The GOF design patterns apply generically to any object-oriented programming language. J2EE design patterns address common problems encountered in designing J2EE architecture. This course presents the key J2EE design patterns required when implementing a J2EE system.