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Replies to This Discussion

Q:

Explain the life cycle methods of a Servlet.

A:

The javax.servlet.Servlet interface defines the three methods known as life-cycle method.
public void init(ServletConfig config) throws ServletException
public void service( ServletRequest req, ServletResponse res) throws ServletException, IOException
public void destroy()
First the servlet is constructed, then initialized wih the init() method.
Any request from client are handled initially by the service() method before delegating to the doXxx() methods in the case of HttpServlet.

The servlet is removed from service, destroyed with the destroy() method, then garbage collected and finalized.

 

 

 

 

Q:

What is the difference between the getRequestDispatcher(String path) method of javax.servlet.ServletRequest interface and javax.servlet.ServletContext interface?

A:

The getRequestDispatcher(String path) method of javax.servlet.ServletRequest interface accepts parameter the path to the resource to be included or forwarded to, which can be relative to the request of the calling servlet. If the path begins with a "/" it is interpreted as relative to the current context root.

The getRequestDispatcher(String path) method of javax.servlet.ServletContext interface cannot accepts relative paths. All path must sart with a "/" and are interpreted as relative to curent context root.

 

 

 

 

Q: Explain the directory structure of a web application.

A: The directory structure of a web application consists of two parts.
A private directory called WEB-INF
A public resource directory which contains public resource folder.

WEB-INF folder consists of
1. web.xml
2. classes directory
3. lib directory

                                                          

 

Q:

What are the common mechanisms used for session tracking?

A:

Cookies
SSL sessions
URL- rewriting

 

 

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Q:

Explain ServletContext.

A:

ServletContext interface is a window for a servlet to view it's environment. A servlet can use this interface to get information such as initialization parameters for the web application or servlet container's version. Every web application has one and only one ServletContext and is accessible to all active resource of that application.

 

 

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Q:

What is preinitialization of a servlet?

A:

A container doesnot initialize the servlets ass soon as 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 <load-on-startup> 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 preinitializing a servlet.

Q:

What is the difference between Difference between doGet() and doPost()?

A:

A doGet() method is limited with 2k of data to be sent, and doPost() method doesn't have this limitation. A request string for doGet() looks like the following:
http://www.allapplabs.com/svt1?p1=v1&p2=v2&...&pN=vN
doPost() method call doesn't need a long text tail after a servlet name in a request. All parameters are stored in a request itself, not in a request string, and it's impossible to guess the data transmitted to a servlet only looking at a request string.

Q:

What is the difference between HttpServlet and GenericServlet?

A:

A GenericServlet has a service() method aimed to handle requests. HttpServlet extends GenericServlet and adds support for doGet(), doPost(), doHead() methods (HTTP 1.0) plus doPut(), doOptions(), doDelete(), doTrace() methods (HTTP 1.1).
Both these classes are abstract.

Q:

What is the difference between ServletContext and ServletConfig?

A:

ServletContext: Defines a set of methods that a servlet uses to communicate with its servlet container, for example, to get the MIME type of a file, dispatch requests, or write to a log file.The ServletContext object is contained within the ServletConfig object, which the Web server provides the servlet when the servlet is initialized

ServletConfig: The object created after a servlet is instantiated and its default constructor is read. It is created to pass initialization information to the servlet.

 

 

 

Q:

What is a output comment?

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
<!-- comment [ <%= expression %> ] -->

Example 1
<!-- This is a commnet sent to client on
<%= (new java.util.Date()).toLocaleString() %>
-->

Displays in the page source:
<!-- This is a commnet sent to client on January 24, 2004 -->

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" %>
<html>
<head><title>A Hidden Comment </title></head>
<body>
<%-- This comment will not be visible to the colent in the page source --%>
</body>
</html>

 

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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 <jsp:useBean> tag.
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.

 

 

 

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.

 

 

 

Q:

What are the different scope valiues for the <jsp:useBean>?

A:

The different scope values for <jsp:useBean> are

1. page
2. request
3.session
4.application

 

 

 

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
%>

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\" %> Throw able 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.

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 .

 

 

 

 

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.

<jsp:useBean id="foo" class="com.Bar.Foo" >

<jsp:setProperty name="foo" property="today"
value="<%=java.text.DateFormat.getDateInstance().format(new java.util.Date()) %>" / >

<%-- scriptlets calling bean setter methods go here --%>

</jsp:useBean >

 

 

 

 

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

<input type="text" name="lastName" value="<%=blanknull(lastName)% >" >

 

[ Received from Sumit Dhamija ]

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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");
%>
<a href=\'<%=url%>\'>hello2.jsp</a>

hello2.jsp
<%@ page session="true" %>
<%
Integer i= (Integer )session.getValue("num");
out.println("Num value in session is " + i.intValue());
%>

 

 

TOP

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.

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 <hs@webapp.de> & Peter Rossbach <pr@webapp.de>) aim was not to write a full blown servlet engine, but to provide means to use JSP for generating source code or reports. Therefore most HTTP-specific features (headers, sessions, etc) are not implemented, i.e. no reponseline or header is generated. Nevertheless you can use it to precompile JSP for your website.

Q:

What is the difference between an Interface and an Abstract class?

A:

An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.
.

 

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Q:

What is the purpose of garbage collection in Java, and when is it used?

A:

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.

 

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Q:

Describe synchronization in respect to multithreading.

A:

With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors. 

 

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Q:

Explain different way of using thread?

A:

The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.

 

TOP

 

Q:

What are pass by reference and passby value?

A:

Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed. 

 

TOP

 

Q:

What is HashMap and Map?

A:

Map is Interface and Hashmap is class that implements that.

 

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Q:

Difference between HashMap and HashTable?

A:

The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.

 

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Q:

Difference between Vector and ArrayList?

A:

Vector is synchronized whereas arraylist is not.

 

TOP

 

Q:

Difference between Swing and Awt?

A:

AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.

 

TOP

 

Q:

What is the difference between a constructor and a method?

A:

A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.
A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

 

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Q:

What is an Iterator?

A:

Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.

 

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Q:

State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.

A:

public : Public class is visible in other packages, field is visible everywhere (class must be public too)
private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.
protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.
default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.

 

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Q:

What is an abstract class?

A:

Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.
A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

 

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Q:

What is static in java?

A:

Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.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 nonstatic method. In other words, you can't change a static method into an instance method in a subclass.

 

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Q:

What is final?

A:

A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).

:

Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?

A:

Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol
symbol : class ABCD
location: package io
import java.io.ABCD;

Q:

Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?

A:

No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.

Q:

What is the difference between declaring a variable and defining a variable?

A:

In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.
e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.

 

[ Received from Sandesh Sadhale]

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Q:

What is the default value of an object reference declared as an instance variable?

A:

null unless we define it explicitly.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

Can a top level class be private or protected?

A:

No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected.

 

[ Received from Sandesh Sadhale]

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Q:

What type of parameter passing does Java support?

A:

In Java the arguments are always passed by value .

 

[ Update from Eki and Jyothish Venu]

TOP

 

Q:

Primitive data types are passed by reference or pass by value?

A:

Primitive data types are passed by value.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

Objects are passed by value or by reference?

A:

Java only supports pass 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 .

 

[ Update from Eki and Jyothish Venu]

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Q:

What is serialization?

A:

Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.

 

[ Received from Sandesh Sadhale]

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Q:

How do I serialize an object to a file?

A:

The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.

 

[ Received from Sandesh Sadhale]

TOP

Q:

Which methods of Serializable interface should I implement?

A:

The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

How can I customize the seralization process? i.e. how can one have a control over the serialization process?

A:

Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

What is the common usage of serialization?

A:

Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

What is Externalizable interface?

A:

Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.

 

[ Received from Sandesh Sadhale]

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Q:

When you serialize an object, what happens to the object references included in the object?

A:

The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect.

 

[ Received from Sandesh Sadhale]

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Q:

What one should take care of while serializing the object?

A:

One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

What happens to the static fields of a class during serialization?

A:

There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of ay particular state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields.

 

[ Received from Sandesh Sadhale Modified after P.John David comments.]

TOP

:

Does Java provide any construct to find out the size of an object?

A:

No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java.

 

[ Received from Sandesh Sadhale]

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Q:

Give a simplest way to find out the time a method takes for execution without using any profiling tool?

A:

Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution.

To put it in code...

long start = System.currentTimeMillis ();
method ();
long end = System.currentTimeMillis ();

System.out.println ("Time taken for execution is " + (end - start));

Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amout of processing.

 

[ Received from Sandesh Sadhale]

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Q:

What are wrapper classes?

A:

Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc.

 

[ Received from Sandesh Sadhale]

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Q:

Why do we need wrapper classes?

A:

It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

What are checked exceptions?

A:

Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

What are runtime exceptions?

A:

Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

What is the difference between error and an exception?

A:

An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.).

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

How to create custom exceptions?

A:

Your class should extend class Exception, or some more specific type thereof.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

If I want an object of my class to be thrown as an exception object, what should I do?

A:

The class should extend from Exception class. Or you can extend your class from some more precise exception type also.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?

A:

One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

How does an exception permeate through the code?

A:

An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.

Q:

What are the different ways to handle exceptions?

A:

There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and
2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions.

Q:

What is the basic difference between the 2 approaches to exception handling.
1> try catch block and
2> specifying the candidate exceptions in the throws clause?
When should you use which approach?

A:

In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.

Q:

Is it necessary that each try block must be followed by a catch block?

A:

It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.

Q:

If I write return at the end of the try block, will the finally block still execute?

A:

Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.

 

 

T

 

Q:

If I write System.exit (0); at the end of the try block, will the finally block still execute?

A:

No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.

Q:

Is Empty .java file a valid source file?

A:

Yes, an empty .java file is a perfectly valid source file.

 

[Received from Sandesh Sadhale]

TOP

 

Q:

Can a .java file contain more than one java classes?

A:

Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

Is String a primitive data type in Java?

A:

No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

Is main a keyword in Java?

A:

No, main is not a keyword in Java.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

Is next a keyword in Java?

A:

No, next is not a keyword.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

Is delete a keyword in Java?

A:

No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

Is exit a keyword in Java?

A:

No. To exit a program explicitly you use exit method in System object.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

What happens if you dont initialize an instance variable of any of the primitive types in Java?

A:

Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

What will be the initial value of an object reference which is defined as an instance variable?

A:

The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized references.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

What are the different scopes for Java variables?

A:

The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time.
1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.
2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.
3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

What is the default value of the local variables?

A:

The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized..

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

How many objects are created in the following piece of code?
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();

A:

Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

Can a public class MyClass be defined in a source file named YourClass.java?

A:

No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

Can main method be declared final?

A:

Yes, the main method can be declared final, in addition to being public static.

 

[ Received fromSandesh Sadhale]

TOP

 

Q:

What will be the output of the following statement?
System.out.println ("1" + 3);

A:

It will print 13.

 

[ Received from Sandesh Sadhale]

TOP

 

Q:

What will be the default values of all the elements of an array defined as an instance variable?

A:

If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null.

Can an unreachable object become reachable again?

 

A:

An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.

 

[Received from P Rajesh]

TOP

Q:

What method must be implemented by all threads?

A:

All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

 

[ Received from P Rajesh]

TOP

 

Q:

What are synchronized methods and synchronized statements?

A:

Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

 

[ Received from P Rajesh]

TOP

 

Q:

What is Externalizable?

A:

Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)

 

[ Received from Venkateswara Manam]

TOP

 

Q:

What modifiers are allowed for methods in an Interface?

A:

Only public and abstract modifiers are allowed for methods in interfaces.

 

[ Received from P Rajesh]

TOP

 

Q:

What are some alternatives to inheritance?

A:

Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).

 

[ Received from P Rajesh]

TOP

 

Q:

What does it mean that a method or field is "static"?

A:

Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.

Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.

 

[ Received from P Rajesh]

TOP

 

Q:

What is the difference between preemptive scheduling and time slicing?

A:

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

 

[ Received from P Rajesh]

TOP

 

Q:

What is the catch or declare rule for method declarations?

A:

If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

 

[ Received from P Rajesh]

TOP

 

How are Observer and Observable used?

A:

Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.

 

[Received from Venkateswara Manam]

TOP

 

Q:

What is synchronization and why is it important?

A:

With respect to multithreading, synchronization is the capability to control
the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.

 

[ Received from Venkateswara Manam]

TOP

 

Q:

How does Java handle integer overflows and underflows?

A:

It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.

 

[ Received from Venkateswara Manam]

TOP

 

Q:

Does garbage collection guarantee that a program will not run out of memory?

A:

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
.

 

[ Received from Venkateswara Manam]

TOP

 

Q:

What is the difference between preemptive scheduling and time slicing?

A:

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

 

[ Received from Venkateswara Manam]

TOP

 

Q:

When a thread is created and started, what is its initial state?

A:

A thread is in the ready state after it has been created and started.

 

[ Received from Venkateswara Manam]

TOP

 

Q:

What is the purpose of finalization?

A:

The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

 

[ Received from Venkateswara Manam]

TOP

 

Q:

What is the Locale class?

A:

The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.

 

[ Received from Venkateswara Manam]

TOP

 

Q:

What is the difference between a while statement and a do statement?

A:

A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement 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 body of a loop at least once.

 

[ Received from Venkateswara Manam]

TOP

 

Q:

What is the difference between static and non-static variables?

A:

A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

 

[ Received from Venkateswara Manam]

TOP

 

Q:

How are this() and super() used with constructors?

A:

This() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

 

[ Received from Venkateswara Manam]

TOP

 

Q:

What are synchronized methods and synchronized statements?

A:

Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

 

[ Received from Venkateswara Manam]

TOP

 

Q:

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

A:

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.

 

[ Received from Shipra Kamra]

TOP

 

Q:

Can applets communicate with each other?

A:

At this point in time applets may communicate with other applets running in the same virtual machine. If the applets are of the same class, they can communicate via shared static variables. If the applets are of different classes, then each will need a reference to the same class with static variables. In any case the basic idea is to pass the information back and forth through a static variable.

An applet can also get references to all other applets on the same page using the getApplets() method of java.applet.AppletContext. Once you get the reference to an applet, you can communicate with it by using its public members.

It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial.

 

[ Received from Krishna Kumar ]

TOP

 

Q:

What are the steps in the JDBC connection?

A:

  While making a JDBC connection we go through the following steps :

Step 1 : Register the database driver by using :

Class.forName(\" driver classs for that specific database\" );

Step 2 : Now create a database connection using :

Connection con = DriverManager.getConnection(url,username,password);

Step 3: Now Create a query using :

Statement stmt = Connection.Statement(\"select * from TABLE NAME\");

Step 4 : Exceute the query :

stmt.exceuteUpdate();

 

[ Received from Shri Prakash Kunwar]

TOP

 

Q:

How does a try statement determine which catch clause should be used to handle an exception?

A:

When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored.

 

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