Java Applet

Java Applet is a small application.
It is transported from server to client machine. Then it runs on client machine. It uses limited resources while running at client machine.
Applet is used to make the web page dynamic and for processing of inputs received from user.
Applet provides a graphic user interface for user for interaction.
Applet does not require main() method for running at client machine.
Applet program extends 'Applet' class.

Simple Applet Example

import java.awt.*;
import java.applet.*;
public class HelloApplet extends Applet {
	public void paint(Graphics g) {
	g.drawString("Hello Applet", 30, 30);
	}
}

Explanation of the program

To write an Applet program, first import packages java.awt.* and java.applet.* , then creates a class, in this case, HelloApplet which extends Applet class.
Remember, Applet class is pre-defined class provided by Java and HelloApplet is class which is defined by programmer.
Within HelloApplet class, paint() method is overridden.
Method paint() has one argument of type Graphics to create Graphical Context for the applet program.
Method drawString() of Graphics class is called through the object g which is of Graphics type.
Method drawString() is used to display text message on the Applet window.

There are three methods to run an applet.

Method 1: Running Applet using Java compatible browser

To execute an applet in a web browser, you need to write a HTML file that contains Applet tag.
Applet tag of HTML loads the applet.
Following tag should be included in the HTML file.

<applet code="HelloApplet.class" width=200 height=60 >

Method 2: Running Applet using standard tool appletviewer

HTML file made in method 1 can be run using applet tool appletviewer .
Assume HTML file name is AppletRun.html, then this file can be run using appletviewer .
For Example:

C:\>appletviewer AppletRun.html

Method 3: Including Applet tag in Java source file

In this method, Applet tag is included in Java source file in the comment.
To including Applet tag in the above example, it will look like:

import java.awt.*;
import java.applet.*;
/*
<applet code="HelloApplet.class" width=200 height=60>
*/
public class HelloApplet extends Applet {
	public void paint(Graphics g) {
	g.drawString("Hello Applet", 30, 30);
	}
}

By doing this, compiled code is documented with a prototype of the necessary HTML statements.
Now, you can directly run the applet using appletviewer .

Important methods of Applet programming

init() method
- This is first method which is called when an applet is running.
- It is used to initialize the variables.
- It is called only once during the run time of the applet.
start( ) method
- This method is called after init() method.
- It is used to restart the applet.
- When a user leaves a web page and comes back, then the start() method resumes the execution of the applet.
- start() method is called each time whenever an applet needs to be displayed on the screen.
paint() method
- This is the method where you will do most of the applet programming.
- This method is called each time when applet's output is to be redrawn.

Example:Showing init(), start( ) & paint() method

import java.awt.*;
import java.applet.*;
/*
<applet code="DemoApplet.class" width=300 height=50>
*/
public class DemoApplet extends Applet {
String m1;
// set the foreground and background colors.
public void init() {
setBackground(Color.green);
setForeground(Color.red);
m1 = " Within init method (),";
}
// Initialize the string to be displayed.
public void start() {
m1 += " Within Start() Method, ";
}
// Display msg in applet window.
public void paint(Graphics g) {
m1 += " Within paint() Method.";
g.drawString(m1, 20, 40);
}
}

Program is showing sequenece of Applet methods call.

Example: Calculator Program using Applet

import java.applet.*;
import java.awt.*;
import java.awt.event.*;

public class calculatorDemo extends Applet implements ActionListener
{
 String s,s1,s2,s3,s4;
 Button b1,b2,b3,b4,b5,b6,b7,b8,b9,b0;
 Button add,sub,eq,cl,mul,div;
 TextField t1;
 int a,b,c;
 
 public void init()
 {
	 // Creating TextField and Buttons 
  t1=new TextField(10);
  b1=new Button("1");
  b2=new Button("2");
  b3=new Button("3");
  b4=new Button("4");
  b5=new Button("5");
  b6=new Button("6");
  b7=new Button("7");
  b8=new Button("8");
  b9=new Button("9");
  b0=new Button("0");
  add=new Button("+");
  sub=new Button("-");
  mul=new Button("*");
  div=new Button("/");
  eq=new Button("=");
  cl=new Button("Clear");
   
  GridLayout gb=new GridLayout(4,5);   // setting the Grid Layout
  setLayout(gb);
  
  // adding AWT controls Textfield and Buttons to the Grid Layout
  add(t1);
  add(b1);
  add(b2);
  add(b3);
  add(b4);
  add(b5);
  add(b6);
  add(b7);
  add(b8);
  add(b9);
  add(b0);
  add(add);
  add(sub);
  add(mul);
  add(div);
  add(eq);
  add(cl);
  
  // Registration of Event Listner 
  b1.addActionListener(this);
  b2.addActionListener(this);
  b3.addActionListener(this);
  b4.addActionListener(this);
  b5.addActionListener(this);
  b6.addActionListener(this);
  b7.addActionListener(this);
  b8.addActionListener(this);
  b9.addActionListener(this);
  b0.addActionListener(this);
  add.addActionListener(this);
  sub.addActionListener(this);
  mul.addActionListener(this);
  div.addActionListener(this);
  eq.addActionListener(this);
  cl.addActionListener(this);
 
  paint();
 }
 public void paint()
 {
  setBackground(Color.blue);
 }
 
 // Following method is called when event (like click event) occurs
 public void actionPerformed(ActionEvent e)
 {
  s=e.getActionCommand();
  if(s.equals("0")||s.equals("1")||s.equals("2")||
s.equals("3")||s.equals("4")||s.equals("5")||s.equals("6")||s.equals("7")||s.equals("8")||
s.equals("9")||s.equals("0"))
  {
   s1=t1.getText()+s;
   t1.setText(s1);
  }
  if(s.equals("+"))
  {
     s2=t1.getText();
     t1.setText("");
     s3="+";
  }
  if(s.equals("-"))
  {
     s2=t1.getText();
     t1.setText("");
     s3="-";
  }
  if(s.equals("*"))
  {
     s2=t1.getText();
     t1.setText("");
     s3="*";
  }
  if(s.equals("/"))
  {
     s2=t1.getText();
     t1.setText("");
     s3="/";
  }
  if(s.equals("="))  // On pressing '=' button, arithmetic operation is performed. 
  {
   s4=t1.getText();
   a=Integer.parseInt(s2);
   b=Integer.parseInt(s4);
   if(s3.equals("+"))
    c=a+b;
   if(s3.equals("-"))
    c=a-b;
   if(s3.equals("*"))
    c=a*b;
   if(s3.equals("/"))
    c=a/b;
   t1.setText(String.valueOf(c));
  }
  if(s.equals("Clear"))
  {
   t1.setText("");
  }
 }
}

HTML file of Calculator Applet program

calculatorDemo.html

<html>
<body>
<applet code="calculatorDemo.class" width="300" height="300">
</applet>
</body>
</html>

Running the calculator Program

Use appletviewer command of java

c:\> javac calculatorDemo.java
c:\> appletviewer calculatorDemo.html

Remember calculatorDemo class file and calculatorDemo.html file should be in the same director.
Html file for running the applet may have any name. It may have name different from the java class name.

Output

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