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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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