Applet

1.What is applet ?

Ans. An applet is an intelligent program designed to be transmitted over the Internet and
executed by a Java-compatible Web browser. An applet is actually a tiny java program,
dynamically downloaded across the network, just like an image, sound file or video clip.
An applet is not just an animation or media file. In other words, an applet is a program that can react to user input and dynamically change-not just run the same animation or sound over and
over.

 

2. How Applets differ from Applications.

 

        Although both the applets and stand-alone applications are java programs, there are significant                 difference between them.

        Applets are not full-featured application programs. They are usually written to accomplish a small         task or a component of a task.

        

        Since they are usually designed for use on the Internet, they impose certain limitations and

        restrictions in their design.

         Applets do not use main( ) method for initiating the execution of the code. Applets, when
        loaded, automatically call certain methods of Applet class to start and execute the applet
        code.
         Unlike stand-alone applications, applets cannot be run independently. They are run from
        inside a web page using a special feature known as HTML tag.
         Applets cannot read from or write to the files in the local computer.
         Applets cannot communicate with other servers on the network.
         Applets cannot run any program from the local computer.
         Applets are restricted from using libraries from other languages such as C or C++.
        (Remember, Java languages supports this feature through native methods)
        All these restrictions and limitations are placed in the interest of security of system. These
        restrictions ensure that an applet cannot do any damage to the local system.

 

3. Explain Lifecycle of an applet.

 

when an applet is loaded, it undergoes a series of changes in its state as shown in fig.
 The applet states include :
o Born or Initialization state
o Running state
o Idle state or stopped state
o Dead or destroyed state

 

 Initialization Applet enters the initialization state when it is first loaded. This is achieved by calling the init()
method of Applet class. The applet is born. At this stage, we may do the following, if required.
 Create objects needed by the applet
 Set up initial values
 Load images or fonts
 Set up colors
public void init()
{
….
}
Running state:
an applet become idel when the system calls the start() method of Applet class. This
occurs automatically after the applet is initialized. Starting can occur if the applet is already in‘stopped’ state. For example, we may leave the Web page containing the applet temporarily toanother page and return back to the page. This again starts the applet running. Note that, unlikeinit() method, the start() method may be called more than once. We may override the start()method to create a thread to control the applet.


public void start()
{
……
}
Display using paint( ) in Running State :
Applet moves to the display state whenever it has to perform some output operations on the
screen. This happens immediately after the applet enters into the running state. The paint()
method is called to accomplish this task. Almost every applet will have a paint() method.
It is to be noted that the display state is not considered as a part of applet’s life cycle. In
fact, the paint() method is defined in the Applet class. It is inherited from the Component class,a super class of Applet.
public void paint ( Graphics g )
{
….
}
Idle or stop state:An apple become idle when it is stopped from running. Stopping occurs automatically when we
leave the page containing the currently running applet. We can also do so by calling the stop()method explicitly. If we use a thread to run the applet, then we must use stop() method toterminate the thread. We can achieve this by overriding the stop() method.
public void stop()
{
….
}
Death state: An applet is said to be dead when it is removed from memory. This occurs automatically by
invoking the destroy() method when we quit the browser. Like initialization, destroying stage occurs only once in the applet’s life cycle. If the applet has created any resources, like threads, we may override the destroy() method to clean up these resources.


public void destroy()
{
….
}

 

4. Explain the class hierarchy of Applet class.

Ans. Applet class is available in java.applet package. The java.applet package also have
AppletContext, AppletStub and AudioClip interfaces. Applet class have init( ), start( ), stop( ), destroy( ) methods. Applet class is subclass of Panel class.

Panel class is belong to java.awt package. Panel class is subclass of Container class.
( Window class is also subclass of Container class. Frame and Dialog are the subclasses of this Window class.)

Container class is subclass of Component which is also in same package java.awt. Button,
Label, Canvas etc. are also subclasses of Component class.

Component class is subclass of Object. This Component class is belong to java.awt package. paint( Graphics g ) method which we use in Applet is actually method of Component class.

Graphics, Font, Color, Events classes which we are

used in Applet are subclass of Object
class. Object class is belong to java.lang package.

5.. How to pass parameters to Applet ?

Ans. We can pass user-defined parameters to applet using <param…> tags.
Each <param…> tag has a name attribute and value attribute.
Inside the applet code, the applet can refer to that parameter by name to find its value. This is done using the getParameter( ) method, which takes one string argument representing the name of the parameter and returns a string containing the value of that parameter.
Example :

import java.awt.*;
import java.applet.*;
/*
<applet code="MyApplet" width=800 height=600>
<param name="E_NAME" value=jd>
<param name="E_AGE" value=20>
</applet>
*/
public class MyApplet extends Applet
{
String name;
int age;
public void start()
{
name = getParameter("E_NAME");
age = Integer.parseInt(getParameter("E_AGE"));
}
public void paint(Graphics g)
{
g.drawString(name + " " + age,20,20);
}
}