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Lecture 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 appletviewer to 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 Applet or 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 packages import java.awt.; import javax.swing.;
// extending class from JApplet so that our class also becomes an //applet public class HelloApplet extends JApplet {
// overriding paint method public void paint(Graphics g) {
// write code here u want to display & draw by using // Graphics object g.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.
Simple Applet
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.
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
init()
start()
stop()
destroy()
paint
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
The DemoTest.html file is using this applet. The code snippet of it given below:
Applet Life Cycle Methods
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
— 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
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 values Color 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 font int fontChoice = (int) (Math.random() * 4) + 1;
// declaring font reference Font f = null;
// using switch based logic for selecting font 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;
} // 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 & color for (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
// 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);
}
} // end class
