following types of controls:

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

         These controls are subclasses of Component.

57) What is the difference between choice and list?

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

         and only one item may be selected from a choice.

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

         or more list items.

58) What is the difference between scrollbar and scrollpane?

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

         events and perform its own scrolling.

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

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

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

60) How are the elements of different layouts organized?

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

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

         and the center of a container.

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

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

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

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

         addition, the rows and columns may have different sizes.

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

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

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

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

63) What are wrapper classes?

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

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

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

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

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

         objects.

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

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

         locations.

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

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

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

65) What is the difference between set and list?

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

         elements in an ordered way but may contain duplicate elements.

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

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

         and they are:

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

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

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

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

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

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

68) What is an I/O filter?

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

         way as it is passed from one stream to another.

69) What is serialization and deserialization?

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

         Deserialization is the process of restoring these objects.

70) What is JDBC?

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

         to enable programs to write pure Java Database applications.

71) What are drivers available?

Ans:  a)    JDBC-ODBC Bridge driver

b)   Native API Partly-Java driver

a)      JDBC-Net Pure Java driver

b)      Native-Protocol Pure Java driver

72) What is the difference between JDBC and ODBC?

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

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

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

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

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

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

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

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

a)      Two tier model and b) Three tier model

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

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

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

b)      Receiving results from database to the client and

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

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

       database?

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

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

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

                database driver.

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

       DriverMnaager.getConnection( ) method is used.

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

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

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

            Statement stmt  = con.createStatement( );

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

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

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

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

                       while(rs.next( ) ) {

                           String event = rs.getString(“event”);

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

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

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

         ODBC driver to access a database engine).

76) What are the types of statements in JDBC?

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

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

                                               over

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

77) What is stored procedure?

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

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

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

         combination of input/output parameters.

78) How to create and call stored procedures?

Ans:   To create stored procedures:

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

          BEGIN

          Any multiple SQL statement;

          END;

         To call stored procedures:

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

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

          csmt.setInt(column no., column name)

          csmt.execute( ); 

79) What is servlet?

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

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

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

80) What are the classes and  interfaces for servlets?

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

          Javax.servlet contains:

               Interfaces                       Classes

               Servlet                            Generic Servlet

               ServletRequest               ServletInputStream

               ServletResponse             ServletOutputStream

               ServletConfig                 ServletException

               ServletContext                UnavailableException

               SingleThreadModel

          Javax.servlet.http contains:

               Interfaces                                 Classes

               HttpServletRequest                  Cookie

               HttpServletResponse                HttpServlet

               HttpSession                              HttpSessionBindingEvent

               HttpSessionCintext                   HttpUtils

               HttpSeesionBindingListener

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

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

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

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

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

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

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

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

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

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

83) What is the life cycle of a servlet?

Ans: Each Servlet has the same life cycle:

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

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

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

84) Who is loading the init() method of servlet?

Ans: Web server

85) What are the different servers available for developing
    and deploying Servlets?

Ans:  
a)    Java Web Server

b)
             JRun

g)
             Apache Server

h)
             Netscape Information Server

i)
               Web Logic    
                   
            
        

86) How many ways can we track client 
    and what are they?

Ans: The servlet API provides two ways to track client state
        and they are:

          a) Using Session
    tracking and b) Using Cookies.
    

87) What is session tracking and how do you track a
    user session in servlets?

Ans: Session tracking is a mechanism that servlets use to
        maintain state about a series requests
        
    

         from the same
    user across some period of time. The methods used for session tracking are:

a)
             User Authentication – occurs when a web server restricts
            access to some of its resources to only those clients that log in using a recognized
            username and password
            
        

b)
             Hidden form fields – fields are added to an HTML form
            that are not displayed in the client’s browser. When the form containing the fields
            is submitted, the fields are sent back to the server
            
        

c)
              URL rewriting – every URL that the user clicks on is
            dynamically modified or rewritten to include extra information. The extra information
            can be in the form of extra path information, added parameters or some custom, server-specific
            URL change.

d)
             Cookies – a bit of information that is sent by a web
            server to a browser and which can later be read back from that browser.

e)
             HttpSession- places a limit on the number of sessions
            that can exist in memory. This limit is set in the session.maxresidents property

88) What is Server-Side Includes(SSI)?

Ans: Server-Side Includes allows to embed servlets within
        HTML pages using a special servlet tag. In many
        
    

         servlets that
    support servlets, a page can be processed by the server to include output from servlets
    at certain
    

         points inside
    the HTML page. This is accomplished using a  
    special internal SSINCLUDE, which processes
    

         the servlet tags.
    SSINCLUDE servlet will be invoked whenever a file with an .shtml extension is requested.
    

         So HTML files
    that include  server-side includes must
    be stored with an .shtml extension.

89) What are cookies and how will you use them?

Ans: Cookies are 
        a mechanism that a servlet uses to have a client hold a small amount of state-information
        
    

         associated with
    the user.

         a)
           Create a cookie with the Cookie constructor:

               
                 public Cookie(String name,
    String value)

b)
              A servlet can send a cookie to the client by passing
            a Cookie object to the addCookie( ) method of
            
        

      
          HttpServletResponse:

               
                 public void HttpServletResponse.addCookie(Cookie
    cookie)

c)
              A servlet retrieves cookies by calling the getCookies(
            ) method of HttpServletRequest:

               
       public Cookie[ ] HttpServletRequest.getCookie( ).

90) Is it possible to communicate from an applet to
    servlet and how many ways and how?

Ans: Yes, there are three ways to communicate from an applet
        to servlet and they are:

a)
             HTTP Communication(Text-based and object-based)

b)
             Socket Communication

c)
              RMI Communication

          (You can say
    , by using URL object open the connection to server and get the InputStream  from
    

            URLConnection
    object).

           Steps involved
        for applet-servlet communication:

1)
             Get the server URL.

URL url
    = new URL( );

2)
             Connect to the host

URLConnection
    Con = url.openConnection( );

3)
             Initialize the connection

Con.setUseCatches(false):

Con.setDoOutput(true);

Con.setDoInput(true);

4)
             Data will be written to a byte array buffer so that
            we can tell the server the length of the data .

ByteArrayOutputStream
    byteout  = new ByteArrayOutputStream(
    );

5)
             Create the OutputStream to be used to write the data
            to the buffer.

DataOutputStream
    out = new DataOutputStream(byteout); 
        
    

91) What is connection pooling?

Ans: With servlets, opening a database connection is a major
        bottleneck because we are creating and tearing
        
    

         down a new connection
    for every page request and the time taken to create connection will be more.
    

         Creating a connection
    pool is an ideal approach for a complicated servlet. With a connection pool, we
    can
    

         duplicate only
    the resources we need to duplicate rather than the entire servlet. A connection
    pool can also
    

         intelligently
    manage the size of  the pool and make
    sure each connection remains valid. A number of
    

         connection pool  packages are currently available. Some
    like DbConnectionBroker are freely available from
    

         Java Exchange
    Works by creating an object that dispenses connections and connection Ids on request.

         The ConnectionPool
    class maintains a Hastable, using Connection objects as keys and Boolean values
    as
    

         stored values.
    The Boolean value indicates whether a connection is in use or not. A program calls     
    

         getConnection(
    ) method of the ConnectionPool for getting  
    Connection object it can use; it calls
    

         returnConnection(
    ) to give the connection back to the pool.

92) Why should we go for interservlet communication?

Ans: Servlets running together in the same server communicate
        with each other in several ways.

         The three major
    reasons to use interservlet communication are:

a)
             Direct servlet manipulation – allows to gain access
            to the other currently loaded servlets and perform certain tasks (through the ServletContext
            object)

b)
             Servlet reuse – allows the servlet to reuse the public
            methods of another servlet.

c)
              Servlet collaboration – requires to communicate with
            each other by sharing specific information (through method invocation)

93) Is it possible to call servlet with parameters in
    the URL?
    

Ans: Yes. You can call a servlet with parameters in the syntax
        as(?Param1 = xxx|| m2 = yyy).

94) What is Servlet chaining?

Ans: Servlet chaining is a technique in which two or more
        servlets can cooperate in servicing a single request.

     
            In servlet chaining, one servlet’s output is piped to the
    next servlet’s input. This process continues until the
    

         last servlet
    is reached. Its output is then sent back to the client.

95) How do servlets handle multiple simultaneous requests?

Ans: The server has multiple threads that are available to
        handle requests. When a request comes  
        in, it is
        
    

         assigned to a
    thread, which calls a service method(for example: doGet( ), doPost( ) and service(
    ) ) of the
    

         servlet. For
    this reason, a single servlet object can have its service 
    methods called by many threads at once.

96) What is the difference between TCP/IP and UDP ?

Ans: TCP/IP is a two-way communication between the client and the
            server and it is a reliable and there is a
            
        

     
           confirmation regarding reaching the message to the destination.
    It is like a phone call.
    

         UDP is
    a one-way communication only between the client and the server and it is not a reliable
    and there is
    

no confirmation
    regarding reaching the message to the destination. It is like a postal mail.

97) What is Inet address?

Ans: Every computer connected to a network has an IP address.
        An IP address is a number that  uniquely
        
    

          identifies each
    computeron the Net. An IP address is a 32-bit number.

98) What is Domain Naming Service(DNS)?

Ans: It is very difficult to remember a set of numbers(IP
        address) to connect to the Internet. The Domain Naming
        
    

         Service(DNS)
    is used to overcome this problem. It maps one particular IP address to a string
    of characters.

         For example,
    www.mascom.com implies com is the domain name
    reserved for US commercial sites,
    

         moscom is the
    name of the company and www is the name of the specific computer, which is mascom’s
    

         server.

99) What is URL?

Ans: URL stands for Uniform Resource Locator and it points
        to resource files on the Internet.

         URL has four
    components:

         http://www.Pentafour.com:80/index.html

         http – protocol
    name, Pentafour – IP address or host name, 80 – port number and 
    index.html – file path.

100) What is RMI and steps involved in developing an
    RMI object?

Ans: Remote Method Invocation(RMI) allows java object that
        executes on one machine and to invoke the method
        
    

         of a Java object
    to execute on another machine.

         The steps involved
    in developing an RMI object are:

a)
             Define the interfaces

b)
             Implementing these interfaces

c)
              Compile the interfaces and their implementations with
            the java compiler
            
        

d)
             Compile the server implementation with RMI compiler

e)
             Run the RMI registry

f)
               Run the application

101) What is RMI architecture ?
    

Ans: - RMI architecture consists of four layers and each layer
        performs specific functions:

       
            a) Application layer 
                       ----   contains the actual object definition

            b) Proxy
    layer              
               ----   
    consists of stub and skeleton

            c) Remote
    Reference layer  ----
          gets the stream of bytes from the transport layer and sends it
    to the proxy
    

               
                           
                           
     layer
    

            d) Transportation
    layer        ----   responsible for handling the actual
    machine to machine communication

102) What is UnicastRemoteObject?

Ans: All remote objects must extend UnicastRemoteObject,
        which provides functionality that is  
        needed to make
        
    

         objects available
    from remote machines.

103) Explain the methods, rebind( ) and lookup() in
    Naming class?

Ans: rebind( ) of the Naming class(found in java.rmi) is used to update
            the RMI registry on the server machine.

         Naming.rebind(“AddSever”,
    AddServerImpl);
        
    

         lookup( )
    of the Naming class accepts one argument, the rmi URL and returns a 
    reference to an object of
    

         type AddServerImpl.

104) What is a Java Bean?

Ans: A Java Bean is a software component that has been designed
        to be reusable in a variety of different
        
    

         environments.   

105) What is a Jar file?

Ans: Jar file allows to efficiently deploy a set of classes
        and their associated resources. The elements in a jar file
        
    

         are compressed,
    which makes downloading a Jar file much faster than separately downloading several
    

         uncompressed
    files.

         The package java.util.zip
    contains classes that read and write jar files.
    

106) What is BDK?

Ans: BDK,Bean Development Kit is a tool that enables to create,
        configure and connect a set of set of Beans and
        
    

         it can be used
    to test Beans without writing a code.
    

107) What is JSP?

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

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

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

         The advantages are:

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

HTML but it is a tedious process.

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

       compiling it into a servlet and running it.

108) What are JSP scripting elements?

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

          are three forms:

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

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

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

       methods.

109) What are JSP Directives?

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

    
    <%@
        directive attribute="value" %>

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

    
    <%@
        directive attribute1="value1"
        
    

              
    attribute2="value2"

             
     ...

              
    attributeN="valueN" %>

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

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

         translated into a servlet

110) What are Predefined variables or implicit objects?

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

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

          pageContext, and page.

111) What are JSP ACTIONS?

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

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

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

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

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

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

              Servlet:

    request.setAttribute("theBean", myBean);

    RequestDispatcher rd =

        getServletContext().getRequestDispatcher('/thepage.jsp");

    rd.forward(request, response);

JSP PAGE:

    <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?

Ans: response.setHeader("Set-Cookie", "cookie string");

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

<%

bean.setResponse
    (response);

%>

114) How can I delete a cookie with JSP?

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

         <%

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

           killCookie.setPath("/");

           killCookie.setMaxAge(0);

           response.addCookie(killCookie);

         %>

115) How are Servlets and JSP Pages related?

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

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

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

         the server and executed.

116) What will be the out put of this programme

public class try1

{

            public static void main(String args[])

            {

                        try1 t1 = new try1();

                        t1.test();

            }

            public String test()

            {

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

                        return "try1";

            }

            public void test()

            {

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

            }

}

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

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

{

  public static void main(String args[])

  {

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

  }

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

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

            public class try1

{

            public static void main(String args[])

            {

                        String s1 = new String("amit");

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

                        s1.concat("Poddar");

                        System.out.println(s1);

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

            }

}

Ans:     amit

m

public class try1

{

            public static void main(String args[])

            {

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

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

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

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

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

            }

}         

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

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

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

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

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

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

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

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

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

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

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

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

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

public class Foo {

  static private int count; // class variable

  String name; // instance variable

  private void Bar() {

    int halfCount; // local variable

  }

}

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

Enterprise Java Beans notes

1.) What is an enterprise bean?

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

What is an Enterprise java Bean

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

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

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

(2) Type of EJBS?

Ans.) Entity and session beans

Session Bean

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

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

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

Entity Bean

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

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

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

public class CustomerBean implements javax.ejb.EntityBean
    {

   
         // persistent fields

   
         public long customerNumber;

   
         public Date lastActive;

   
         public String lastName;

   
         public String firstName;

   
         // initialization method for creating a new customer

   
         public CustomerKey ejbCreate(long id, PersonName name){

   
                   customerNumber = id;

   
                   setName(name);

   
                   lastActive = new Date();

   
                   return new CustomerKey(id);

   
        }

   
         // business methods

   
         public PersonName getName( 
    ){

   
                return new PersonName(firstName,
    lastName);

   
         }

   
         public void setName(PersonName name){

   
               lastName = name.getLastName();

   
               fistName = name.getFirstName();

   
         }

   
         public Date lastActive( ){

   
                    return lastActive;

   
         }

   
         ...
    

}

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

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

public class CustomerKey implements java.io.Serializable
    {

   
          public long id;

   
          public CustomerKey(long l)

   
         {
    

   
                   id = l;

   
         }

   
         public int hashCode( ) {
    

   
                   return (int)id;

   
          }

   
          public boolean equals(Object otherKey){

   
                    if(otherKey instanceof
    CustomerKey)

   
                          
return ((CustomerKey)otherKey).id == this.id;

   
                   else

   
                          
return false;

   
          }

}

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

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

Persistence in entity beans is of two types:

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

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

To summarize, every entity bean has the following characteristics:

• Entity beans can share access from multiple users.

• Entity beans can participate in transactions.

• Entity beans represent data in a domain model.

• Entity beans are persistent. They live as long as the data lives in the domain model.

• Entity beans can survive EJB server crashes. Any EJB server crash is always transparent to the client.

public class ShoppingCartBean implements javax.ejb.SessionBean
    {

   
          // instance fields; The conversational state
        
    

   
          Vector products = new Vector ();

   
          Customer customer;

   
          // Initializes bean with reference to customer bean
        
    

   
          public void ejbCreate(Customer cust){

   
                    customer = cust;

   
          }

   
           //add a product to shopping cart
        
    

   
          public void addItem(Product item) {

   
                       products.addElement(item);

  
       }

   
          //charge customer for products and create a shipping order
        
    

   
          public void executeSale(){

   
                         
// get the customer's credit card object
    

   
                        CreditCard
    card = customer.getCreditCard( );

  
             
           // calculate a total price from products chosen

   
                        double
    total;

   
                        for(int
    i = 0; i  products.size(); i++){

   
                           
       Product product = (Product)products.elementAt(i);

  
             
                  total += product.getPrice(
    );

   
                        }

   
                        // get reference to the CreditService bean and process the customers charge
        
    

   
                        CreditService
    cardSwipper = ... get credit card service bean

   
                        Charge
    charge = cardSwipper.charge(CreditCard, total);

   
                        // get reference to the OrderHome and create a new shipping order bean (record)
        
    

   
                        OrderHome
    orderHome = ... get the Order beans Home object

   
                        Order
    order = orderHome.create(customer, products, charge);

   
          }

...

}

// somewhere in the client application (applet, servlet, etc.)
    

// obtain the home objects (factories) for the Customer and ShoppingCart beans
    

CustomerHome custHm = ... get CustomerHome

ShoppingCartHome shpCrtHm = ... get ShoppingCartHome
    

// locate the bean representing the customer named, Bob Johnsten
    

Customer customer = custHome.findByName("Bob","Johnsten");

// create a new ShoppingCart initialized with the Bob Johnsten Customer bean
    

ShoppingCart shoppingCart = shpCrtHm.create(customer);

...

Product product = ... get reference to bean
    representing the product chosen

// add product to customer's shopping cart
    

shoppingCart.addItem(product);

...

//charge customer for products and create a shipping order
    

shoppingCart.executeSale( );

public class CreditService implements javax.ejb.SessionBean
    {

   
public Charge charge (String accountToCredit, CreditCard card, double amount)

   throws
    ValidationException, RemoteException{

   
               // attempt to obtain Merchant bean representing the retailer making the charge.

   
               try{

   
                   MerchantHome mrchntHome
    = ... get VendorHome reference

  
             
      Merchant merchant = mrchntHome.findByPrimaryKey(vendorToCredit);      
    

   
               }catch(ObjectNotFoundException onfe){

   
                         
    throw new ValidationException("Invalid merchant account number");

   
               }  
              
    

   
               // attempt to create a Charge bean based on the vendor, amount, and credit card info
        
    

   
               try{

   
                   ChargeHome chgHome
    = ... get ChargeHome reference

   
                   Charge charge =  chgHome.create(merchant, card, amount);

   
                    return charge;

   
              } catch(CreateException ce){

   
                        throw
    new ValidationException(ce.getMessage());

   
              }

  
}

...

}