JAVA Applets

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Web Design & Development CS506

Lesson 20

Applets

A small program written in Java and included in a HTML page.

It is independent of the operating system on which it runs

An applet is a Panel that allows interaction with a Java program

A applet is typically embedded in a Web page and can be run from a browser

You need special HTML in the Web page to tell the browser about the applet

For security reasons, applets run in a sandbox: they have no access to the client's file system

Applets Support

Most modern browsers support Java 1.4 if they have the appropriate plugin

Sun provides an application appletviewerto view applets without using browser.

In general you should try to write applets that can be run with any browser

What an Applet is?

You write an applet by extending the class Appletor JApplet

Applet is just a class like any other; you can even use it in applications if you want

When you write an applet, you are only writing part of a program

The browser supplies the main method

The genealogy of Applet

The following figure shows the inheritance hierarchy of the JApplet class. This hierarchy determines much

of what an applet can do and how, as you'll see on the next few pages.

java.lang.Object

+----java.awt.Component

+----java.awt.Container|+----java.awt.Panel

+----java.applet.Applet|+----javax.swing.JApplet

Example Code 20.1: Writing a Simple Applet

Below is the source code for an applet called HelloApplet. This displays a "Hello World" string. Note that

no main method has been provided.

// File HelloApplet.java

//step 1: importing required packagesimport java.awt.*;import

javax.swing.*;

// extending class from JApplet so that our class also becomes an//appletpublic class HelloApplet extends

JApplet {

// overriding paint method

public void paint(Graphics g) {

// write code here u want to display & draw by using// Graphics objectg.drawString("Hello

World", 30 , 30);

} } // end class

After defining the HelloApplet.java, the next step is to write .html file. Below is the source code of

Test.html file. The Test.html contains the ordinary html code except one.

<html>

<head>

<title> Simple Applet

</title>

</head>

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<body>

<!-- providing the class name of applet with

width &height--!>

<applet code="HelloApplet.class"

width=150 height=100>

</applet>

</body>

</html>

Compile & Execute

By simply double clicking on Test.html file, you can view the applet in your browser. However, you can

also use the appletviewer java program for executing or running applets.

The applet viewer is invoked from the command line by the command

appletviewer htmlfile

where htmlfile is the name of the file that contains the html document. For our example, the command

looks like this:

appletviewer Test.html

As a result, you will see the following output

Applet Life Cycle Methods

When an applet is loaded, an instance of the applet's controlling class (an Applet subclass) is created. After

that an applet passes through some stages or methods, each of them are build for specific purpose

An applet can react to major events in the following ways:

It can initialize itself.

It can start running.

It can stop running.

It can perform a final cleanup, in preparation for being unloaded

The applet's life cycle methods are called in the specific order shown below. Not every applet needs to

override every one of these methods.

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Let's take a look on each method in detail and find out what they do

init( )

Is called only once.

The purpose of init( ) is to initialize the applet each time it's loaded (or reloaded).

You can think of it as a constructor

start( )

To start the applet's execution

For example, when the applet's loaded or when the user revisits a page that contains the applet

start( ) is also called whenever the browser is maximized

paint( )

paint( ) is called for the first time when the applet becomes visible

Whenever applet needs to be repainted, paint( ) is called again

Do all your painting in paint( ), or in a method that is called from paint( )

stop( )

To stop the applet's execution, such as when the user leaves the applet's page or quits the browser.

stop( ) is also called whenever the browser is minimized

destroy( )

Is called only once.

To perform a final cleanup in preparation for unloading

Example Code 20.2: Understanding Applet Life Cycle Methods

The following code example helps you in understanding the calling sequence of applet's life cycle

methods. These methods are only displaying debugging statements on the console.

// File AppletDemo.java

//step 1: importing required packagesimport java.awt.*;import

javax.swing.*;

// extending class from JApplet so that our class also becomes an//appletpublic

class AppletDemo extends JApplet {

// overriding init method

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public void init ( ) {

System.out.println("init() called");

}

// overriding start method

public void start ( ){

System.out.println("start() called");

}

// overriding paint method

public void paint(Graphics g){

System.out.println("paint() called");

}

// overriding stop method

public void stop(){

System.out.println("stop() called");

}

// overriding destroy method

public void destroy(){

System.out.println("destroy() called");

}

} // end class

The DemoTest.html file is using this applet. The code snippet of it given below:

<html> <head> <title> Applet Life Cycle Methods

</title></head>

<body>

<!-- providing the class name of applet with width &height--!>

<applet code="AppletDemo.class"

width=150 height=100>

</applet></body></html>

Compile & Execute

To understand the calling sequence of applet life cycle methods, you have to execute it by using

appletviewer command. Do experiments like maximizing, minimizing the applet, bringing another window

in front of applet and keep an eye on console output.

Example Code 20.3: Animated Java Word Sample Output

The browser output of the program is given below:

Design Process

The Program in a single call of paint method

Draws string "java" on 40 random locations

For every drawing, it selects random font out of 4 different fonts

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For every drawing, it selects random color out of 256 * 256 * 256 RGB colors

Repaint is called after every 1000 ms.

After 10 calls to repaint, screen is cleared

Generating Random Numbers

Use static method random of Math class

Math.random() ;

Returns positive double value greater than or equal to 0.0 or less than 1.0.

Multiply the number with appropriate scaling factor to increase the range and type cast it, if

needed.

int i = (int)( Math.random() * 5 );// will generate random numbers between 0 & 4.

Program's Modules

The program is build using many custom methods. Let's discuss each of them one by one that will help in

understanding the overall logic of the program.

. drawJava( )

As name indicates, this method will be used to write String "java" on random locations. The code is

given below:

// method drawJava

public void drawJava(Graphics2D g2) {

// generate first number randomly. The panel width is 1000int x = (int) (Math.random() * 1000); //

generate second number randomly. The panel height is 700

int y = (int) (Math.random() * 700); // draw String on these randomly

selected numebrsg2.drawString("java", x, y); }

. chooseColor( )

This method will choose color randomly out of 256 * 256 * 256 possible colors. The code snippet is given

below:

// method chooseColor

public Color chooseColor() {

// choosing red color value randomly

int r = (int) (Math.random() * 255);

// choosing green color value randomly

int g = (int) (Math.random() * 255);

// choosing blue color value randomly

int b = (int) (Math.random() * 255);

// constructing a color by providing R-G-B valuesColor c = new Color(r, g, b);

// returning color

return c;

}

. chooseFont( )

This method will choose a Font for text (java) to be displayed out of 4 available fonts. The code snippet is

given below:

// method chooseFont

public Font chooseFont() {

// generating a random value that helps in choosing a fontint fontChoice = (int)

(Math.random() * 4) + 1;

// declaring font reference

Font f = null;

// using switch based logic for selecting font

switch (fontChoice) {

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case 1: f = new Font("Serif", Font.BOLD + Font.ITALIC, 20);break;

case 2: f = new Font("SansSerif", Font.PLAIN, 17);break;

case 3: f = new Font("Monospaced", Font.ITALIC, 23);break;

case 4: f = new Font("Dialog", Font.ITALIC, 30);break;

} // end switch

// returns Font object

return f;

} //end chooseFont

. paint( )

The last method to be discussed here is paint( ). By overriding this method, we will print string "java"

on 40 random locations. For every drawing, it selects random font out of 4 different fonts & random

color out of 256 * 256 * 256 RGB colors.

Let's see, how it happens:

// overriding method paint

public void paint(Graphics g) {

// incrementing clear counter variable.

clearCounter++;

// printing 40 "java" strings on different locations by// selcting random font & colorfor (int i = 1; i

<= 40; i++) {

// choosing random color by calling chooseColor() method

Color c = chooseColor();

// setting color

g2.setColor(c);

// choosing random Font by calling chooseColor() method

Font f = chooseFont();

g2.setFont(f);

// drawing string "java" by calling drawJava() method

drawJava(g2);

} // end for loop Graphics2D g2 = (Graphics2D) g;

// checking if paint is called 10 times then clears the// screen and set counter again to zero if

(clearCounter == 10) {

g2.clearRect(0, 0, 1000, 700);clearCounter = 0; } } //

end paint method

Merging Pieces

By inserting all method inside JavaAnim.java class, the program will look like one given below. Notice that

it contains methods discussed above with some extra code with which you are already familiar.

// File JavaAnim.java

//step 1: importing required packages

import java.awt.*;

import java.awt.event.*;

import javax.swing.*;

public class JavaAnim extends JApplet implements ActionListener {

// used to count how many times paint is called

int clearCounter;

// declaring Timer reference

Timer t;

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// overriding init method, used to initialize variables

public void init() {

setBackground(Color.black);

clearCounter = 0;

Timer t = new Timer(1000, this);

t.start();

}

// overriding paint method discussed above

public void paint(Graphics g) {

clearCounter++;

Graphics2D g2 = (Graphics2D) g;

if (clearCounter == 10) {

g2.clearRect(0, 0, 1000, 700);

clearCounter = 0;

}

for (int i = 1; i <= 40; i++) {

Color c = chooseColor();

g2.setColor(c);

Font f = chooseFont();

g2.setFont(f);

drawJava(g2); }

}

// overriding actionPerformed()of ActionListener interface// called by Timer object

public void actionPerformed(ActionEvent ae) {

repaint();

}

// chooseColor method discussed above

public Color chooseColor() {

int r = (int) (Math.random() * 255);

int g = (int) (Math.random() * 255);

int b = (int) (Math.random() * 255);

Color c = new Color(r, g, b);

return c;

}

// chooseFont method discussed above

public Font chooseFont() {

int fontChoice = (int) (Math.random() * 4) + 1;

Font f = null;

switch (fontChoice) {

case 1: f = new Font("Serif", Font.BOLD + Font.ITALIC, 20);break;

case 2: f = new Font("SansSerif", Font.PLAIN, 17);break;

case 3: f = new Font("Monospaced", Font.ITALIC, 23);break;

case 4: f = new Font("Dialog", Font.ITALIC, 30);break;

}

return f;

}

// drawJava() method discussed above

public void drawJava(Graphics2D g2) {

int x = (int) (Math.random() * 1000);

int y = (int) (Math.random() * 700);

g2.drawString("java", x, y);

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}

} // end class

The AnimTest.html file is using this applet. The code snippet of it given below:

<html>

<head>

<title> Animated Java Word </title>

</head>

<body>

<applet code="JavaAnim.class" width=1000 height=700> </applet>

</body> </html>

Compile & Execute

You can execute it directly using browser or by using appletviewer application. For having fun, you can use

"your name" instead of "java" and watch it in different colors ☺

References:

. Java, A Lab Course by Umair Javed

. Writing Applets

. http://java.sun.com/docs/books/tutorial/applet/

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