APPLETS03.doc

7/30/2019 APPLETS03.doc

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APPLETS

When a html page wants to communicate with the user on

Internet, it can use a special java program called as

applet to communicate and respond to the user.

The user can interact by typing some details or by clicking

the components available in the applet program.

The program then processes and then displays results.

APPLET = JAVA BYTE CODE + HTML PAGE

That is, we can understand an applet as a java byte code

embedded in a html page.

Therefore, applet can be defined as java byte code embedded

in a web page.

The java.applet package provides an API ( application

programming interface) that gives applets some capabilities

that applications dont have. Ex: applets can play sounds

which other programs cant do yet.

When you use a Java technology-enabled browser to view a

page that contains an applet, the applet's code is

transferred to your system and executed by the browser's

Java Virtual Machine (JVM).

Applets are classified as: LOCAL APPLET & REMOTE APPLET

Local applet is developed by user and it is present inour local machine only.

Remote applet is developed by others and is present ininternet. It can be downloaded to our local machine and

it can run in our machine also.

Syntax for writing html code:

. . . . .


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. . . . . .

. . . . . .

Creating an APPLET:

To create an applet we need to write a java program and

compile it to get byte code.

Then we should include it into a HTML page on a

particular location wherever we want it to be

displayed.

This page is the stored in web server.

A client machine communicates with web server, the

server then sends the HTML page that contains the

applet.

The page is then transmitted to the client where

the applet is executed on the clients web

browser.

Thus, applets are executed at client side by the

web browser.

Thus, applets travel thousands of kilometers of

distance on internet and reach the client machines

before their execution on the client.

To create an applet we have Applet class of

java.applet package and JApplet class of

javax.swing package. The classes use the following

methods which are automatically run by any appletprogram. So these methods should be overridden in

applet program.

1.public void init():

This method is the first method to be called bythe browser and it is executed only once.


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So the programmer can use this method toinitialize any variables, creating components

and creating threads etc.,

When execution of this method is completedbrowser looks for the next method start().

2.Public void start():

This method is called after init() method and each timethe applet is revisited by the user.

For example, the user has minimized the web page thatcontains the applet and moved to another page then this

methods execution is stopped.

When the user comes back to view the webpage againstart() method execution will resume.

Any calculations and processing of data should be donein this method and the results are also displayed.

3. Public void stop():

This method is called by the browser when the applet isto be stopped.

If the user minimizes the web page, then this method iscalled and when the user again comes back to this page,

then start() method is called.

In this way, start() and stop() methods can be calledrepeatedly.

For example, an applet with some animation might want touse the start() method to resume animation, and the

stop() method to suspend animation.

4. public void destroy():

This method is called when applet is being terminatedfrom memory.

The stop() method will always be called beforedestroy().

The code related to releasing the memory allocated tothe applet and stopping any running threads should be

written in this method.


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EXECUTING init(),start(),stop() AND destroy() METHODS IN

THE ABOVE SEQUENCE IS CALLED AS LIFE CYCLE OF APPLET.

Note: writing all these methods is not compulsory while

writing an applet.

Applet dosent have main() method. That is,public static

void main(String args[]) is not available in applet. So

applet code can be compiled but cannot be executed by JVM.

Now the question is where does this applets run?

Once the applet is created we compile and obtain its byte

code.

This byte code is embedded in HTML page and the page is

sent to client computer.

The client machine contains a browser like HTML page and

the page in sent to client computer.

The client machine contains a browser like internet

explorer, netscape navigator or Mozilla firefox where HTML

page is viewed by user.

The same b rowser will execute the applet of the HTML page.

The browser contains a small virtual machine called applet

engine which understands and run the applet.


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APPLET LIFE CYCLE:

Initialization state:

Stop()

Start()

Paint()

Display

Born

Dead

Running

Idle

Begin

(load Applet)initialization

Start()

stopped

Destroy()

Destroyed

Exit of Browser

Applet state transition diagram


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Applet enters the initialization state when it is

first loaded. This is achieved by calling the init() method of Applet

class.

Create objects needed by the applet.

Set up initial values. Load images or fonts.

Set up colors

Running state:

Applet enters the running state when the system calls

the start() method of Applet class. This occurs automatically after

the applet is initialized. Starting can also occur if the applet is

already in stopped (idle) state.

Idle or Stopped State:

An applet becomes idle when it is stopped from

running. Stopping occurs automatically when we leave the page

containing the currently running applet. We can calling the stop ()

method explicitly.

Dead 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.

Display State:

Applet moves to the display state whenever it has to

perform some output operation on the screen. This happens immediately


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after the applet enters into the running state. To display a message

we can take the help of paint() method of comoponent class of

java.awt package.

An applet life cycle is born with init() method and starts

functioning with start() method. To stop the applet the stop()

method is called and to terminate the applet completely from

memory the destroy() method is called. Once the applet is

terminated, we should reload the HTML page again to get the

applet start once again from init() method. This cyclic way of

executing the methods is called as applet life cyle.

CREATING A SIMPLE APPLET

Java program to create an applet with pink color background and

a message

import java.awt.*;

import java.applet.*;

public class myapplet extends Applet{

public void init()

{

setBackground(Color.pink);

}

public void paint(Graphics g)

{

g.drawString("this is my first applet",50,100);

}

}

Save the above program as : myapplet.java

Compile the above program as: myapplet.java

Now myapplet.class file is created.

This byte code should be embedded inti a html page using

tag as shown below:


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Save the above code asmyapplet.html

This html page contains the applet which can be opened in the

browser or an applet viewer supplied by sun micro systems can be

used to test the applet.

For this purpose, open browser and write the path of .html file

Along with directory path. The applet will be opened in the

browser or give the following command in command prompt :

appletviewer myapplet.html

output:


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Java program to create applet with some background color and

font color with a message. This message string is stored in msg

and is displayed in paint() method.

import java.awt.*;


public class myapplet1 extends Applet

{

String msg=" ";

public void init()

{

setBackground(Color.yellow);

setForeground(Color.red);

Font f = new Font("Arial",Font.BOLD,20);

setFont(f);

msg +=" this is init";}

public void start()

{

msg += " this is start";

}

public void stop()

{

msg += " this is stop";

}

public void destroy()

{

msg +=" this is destroy";

}


{

g.drawString(msg,50,100);

}

}

Output:


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Program to draw lines

import java.awt.*;



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public class lines extends Applet

{


{

setBackground(Color.cyan);

g.drawLine(10,20,170,40);

g.setColor(Color.pink);

g.drawLine(170,40,80,140);

g.setColor(Color.green);

g.drawLine(80,140,10,20);

}

}

Program to create rectangle

import java.awt.*;


public class rectangle extends Applet

{


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{


g.drawRect(30,30,60,60);

}

}

Program to fill rectangle with a color

import java.awt.*;


public class rectangle extends Applet

{


{



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g.drawRect(30,30,60,60);

g.fillRect(30,30,60,60);

setForeground(Color.yellow);

}

}

Program to draw polygons

import java.awt.*;


public class polygon extends Applet

{

int x1[]={100,10,100,200,100};

int y1[]={10,100,200,100,10};

int n1=5;

int x2[]={200,300,400,300,200};

int y2[]={100,200,100,10,100};

int n2=5;



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{

g.drawPolygon(x1,y1,n1);

g.fillPolygon(x2,y2,n2);

}

}

Program to display a banner which is moving

import java.awt.*;


/*

*/

public class Banner extends Applet implements Runnable

{Thread t;

String s;

public void init()

{

t = new Thread(this);


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s = "This is a moving banner";

t.start();

}

public void run()

{

char ch;

while(true) {

try{

repaint();

Thread.sleep(250);

ch = s.charAt(0);

s = s.substring(1, s.length());

s += ch;

} catch(InterruptedException e) {

System.out.println("Exception occured: " + e);

}

}

}


{

g.drawString(s, 20, 20);}

}

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APPLETS03.doc