Introduction to Java Applets

1
Introduction to Java Applets

• Chapter 20

2
Applets

• Applets are Java programs embedded in an HTML
  document.
• When a browser loads a web page containing an
  applet, the applet downloads into the browser and
  executes.
• The browser executing the applet is its
  container.
• For testing, another container called the applet
  viewer can be used.

3
Sample Applets

• The JDK comes with some sample applets in the
  demo directory.
• One such applet is the game of TicTacToe.

4
JApplet

• To create our own applet, we extend JApplet.
• JApplet has methods for init, start, stop, paint,
  and destroy.
• These can be overridden for our own applet.
• When a container loads the applet, it calls, in
  sequence, init, start, and paint.

5
JApplet

• init is used for initialization of the applet.
• start is called when the applet is loaded and
  each time the page is revisited by the browser.
• stop is called when the browser leaves the web
  page of the applet.
• paint is called whenever the applet needs to be
  painted.
• destroy is called when the container removes the
  applet from memory.

6
Applets

• Applets are embedded in web pages.
• You must create a web page for your applet to
  execute it.
• The applet viewer can be invoked like this
• appletviewer WelcomeApplet.html
• You can also open the web page from your browser.

7
Example

• The next example is a simple applet to draw a
  string.
• First is the html page containing the applet,
  then the applet code itself follows

8

• lthtmlgt
• ltapplet code "WelcomeApplet.class" width
  "300" height "45"gt
• lt/appletgt
• lt/htmlgt

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• // Fig. 20.6 WelcomeApplet.java
• // A first applet in Java.
• import java.awt.Graphics // program uses class
  Graphics
• import javax.swing.JApplet // program uses class
  JApplet
• public class WelcomeApplet extends JApplet
• // draw text on applet's background
• public void paint( Graphics g )
• // call superclass version of method paint
• super.paint( g )
• // draw a String at x-coordinate 25 and
  y-coordinate 25
• g.drawString( "Welcome to Java
  Programming!", 25, 25 )
• // end method paint
• // end class WelcomeApplet

10
Example

• The next example demonstrates the use of the init
  method.
• Here init is used to input some values from the
  user.
• The applet then prints a string containing the
  sum of the values

11

• lthtmlgt
• ltapplet code "AdditionApplet.class" width
  "300" height "50"gt
• lt/appletgt
• lt/htmlgt

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• // Fig. 20.10 AdditionApplet.java
• // Adding two floating-point numbers.
• import java.awt.Graphics // program uses
  class Graphics
• import javax.swing.JApplet // program uses
  class JApplet
• import javax.swing.JOptionPane // program uses
  class JOptionPane
• public class AdditionApplet extends JApplet
• private double sum // sum of values entered
  by user
• // initialize applet by obtaining values from
  user
• public void init()
• String firstNumber // first string
  entered by user
• String secondNumber // second string
  entered by user
• double number1 // first number to add
• double number2 // second number to add

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• // convert numbers from type String to type
  double
• number1 Double.parseDouble( firstNumber
  )
• number2 Double.parseDouble( secondNumber
  )
• sum number1 number2 // add numbers
• // end method init
• // draw results in a rectangle on applets
  background
• public void paint( Graphics g )
• super.paint( g ) // call superclass
  version of method paint
• // draw rectangle starting from (15, 10)
  that is 270
• // pixels wide and 20 pixels tall
• g.drawRect( 15, 10, 270, 20 )
• // draw results as a String at (25, 25)
• g.drawString( "The sum is " sum, 25, 25
  )
• // end method paint

14
Security

• Applets use a security model known as the Sandbox
  Security model.
• The idea here is that the applet is limited in
  what it can do, in essence, staying inside the
  sandbox.

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End of Slides