Applets

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.
 

Applet Support

• Most modern browsers support Java 1.4 if they have the appropriate plugin.
• In the PC labs, Internet Explorer 5.5 has been updated, but Netscape has not.
• The best support isn’t a browser, but the standalone program appletviewer.
• 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.
• 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

java.lang.Object
|
+—-java.awt.Component
|
+—-java.awt.Container
|
+—-java.awt.Panel
|
+—-java.applet.Applet
 

The simplest possible applet

TrivialApplet.java
import java.applet.Applet;
public class TrivialApplet extends Applet { }
 
TrivialApplet.html
<applet
code=”TrivialApplet.class”
width=150 height=100>
</applet>

 

The simplest reasonable applet

import java.awt.*;
import java.applet.Applet;
public class HelloWorld extends Applet {
public void paint( Graphics g ) {
g.drawString( “Hello World!”, 30, 30 );
}
}
 

Applet methods

• public void init ()
• public void start ()
• public void stop ()
• public void destroy ()
• public void paint (Graphics)
• public void repaint()
• public void update (Graphics)
• public void showStatus(String)
• public String getParameter(String)

 

Why an applet works

• You write an applet by extending the class Applet.
• Applet defines methods init( ), start( ), stop( ), paint(Graphics), destroy( ).
• These methods do nothing–they are stubs.
• You make the applet do something by overriding these methods.
• When you create an applet in BlueJ, it automatically creates sample versions of these methods for you.

public void init ( )

• This is the first method to execute.
• It is an ideal place to initialize variables.
• It is the best place to define the GUI Components (buttons, text fields, scrollbars, etc.), lay them out, and add listeners to them.
• Almost every applet you ever write will have an init( ) method.

 

public void start ( )

• Not always needed.
• Called after init( ).
• Called each time the page is loaded and restarted.
• Used mostly in conjunction with stop( ).
• start() and stop( ) are used when the Applet is doing time-consuming calculations that you don’t want to continue when the page is not in front.

public void stop( )

• Not always needed.
• Called when the browser leaves the page.
• Called just before destroy( ).
• Use stop( ) if the applet is doing heavy computation that you don’t want to continue when the browser is on some other page.
• Used mostly in conjunction with start().

public void destroy( )

• Seldom needed.
• Called after stop( ).
• Use to explicitly release system resources (like threads).
• System resources are usually released automatically.

Methods are called in this order

 

 

• init and destroy are only called once each.
• start and stop are called whenever the browser enters and leaves the page.
• do some work is code called by your listeners.
• paint is called when the applet needs to be repainted.

public void paint(Graphics g)

• Needed if you do any drawing or painting other than just using standard GUI Components.
• Any painting you want to do should be done here, or in a method you call from here.
• Painting that you do in other methods may or may not happen.
• Never call paint(Graphics), call repaint( ).

repaint( )

• Call repaint( ) when you have changed something and want your changes to show up on the screen.
• repaint( ) is a request–it might not happen.
• When you call repaint( ), Java schedules a call to update(Graphics g).

update( )

• When you call repaint( ), Java schedules a call to update(Graphics g)
• Here’s what update does:

public void update(Graphics g) {
    // Fills applet with background color, then
    paint(g);
}
 

Sample Graphics methods

A Graphics is something you can paint on

• g.drawString(“Hello”, 20, 20);
• g.drawRect(x, y, width, height);
• g.fillRect(x, y, width, height);
• g.drawOval(x, y, width, height);
• g.fillOval(x, y, width, height);
• g.setColor(Color.red);

Output

 

Painting at the right time is hard

• Rule #1: Never call paint(Graphics g), call repaint( ).
• Rule #2: Do all your painting in paint, or in a method that you call from paint.
• Rule #3: If you paint on any Graphics other than the Applet’s, call its update method from the Applet’s paint method.
• Rule #4. Do your painting in a separate Thread.
• These rules aren’t perfect, but they should help.

Other useful Applet methods

System.out.println(String s)

• Works from appletviewer, not from browsers
• Automatically opens an output window.

showStatus(String) displays the String in the applet’s status line.

• Each call overwrites the previous call.
• You have to allow time to read the line.

Applets are not magic

• Anything you can do in an applet, you can do in an application.
• You can do some things in an application that you can’t do in an applet.
• If you want to access files from an applet, it must be a “trusted” applet.
• Trusted applets are beyond the scope of this course.

Structure of an HTML page

• Most HTML tags are containers.
• A container is <tag> to </tag>

 

HTML

<html>
   <head>
       <title> Hi World Applet </title>
   </head>
   <body>
       <applet code=”HiWorld.class”
         width=300 height=200>
            <param name=”arraysize” value=”10″>
       </applet>
   </body>
</html>
 
<param name=”arraysize” value=”10″>

• public String getParameter(String name)
• String s = getParameter(“arraysize”);
• try { size = Integer.parseInt (s) }
  catch (NumberFormatException e) {…}