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) {…}