Java Applet

From CS315

A Java applet is a type of software program that Java or browsers that include a Java Virtual Machine (JVM) can download and run, and it was developed by a team at Sun Microsystems. Sun Microsystems also developed a stand alone tool for developing and testing java applets, called AppletViewer, it is also obviously able to run java applets. Java Applets are usually used to add features to web pages that HTML or JavaScript cannot. They are most commonly written in the Java Programming Language, but can also be written in other languages that compile to Java bytecode.

History

Java, first bound for the cable TV business, didn’t get the feedback it hoped from the executives at the cable companies, so, it was introduced somewhere else, the Internet. It was first introduced in early 1995 at the Technology, Entertainment and Design Conference in Monterey by John Gage, director of Sun’s Science Office and James Gosling, one of the original creators of Java. At first, the internet was a completely text based, un-interactive world. With the introduction of Java, it could bring the internet to life with moving images and neat animations, needless to say it was a hit at the conference. This made people rethink the potential of the internet and played a big part in making the internet what it is today.

Code Examples

This is a typical “Hello World” program example done in an applet form. When this program executes, it simply displays the text “Hello World”. As you can see, in addition to the standards of a normal Java program, it requires import statements and special methods rather than the usual “Main” method, which is non-existent. Some of those special methods that are used within applets are below in the Technical Information section.

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

public class HelloWorld extends Applet
{
     public void paint(Graphics g)
     {
          g.drawString("Hello World", 20, 20);
     } 
}

There are also some much more complex code examples provided through Sun Microsystems at the website below.

http://java.sun.com/applets/jdk/1.4/index.html

Technical Information

The code of the java applet is downloaded from a web server and the browser either embeds the applet into a web page or opens a new window to display the applet. Java applets can be displayed within a web page by using the applet tags or the object tags. This specifies the applet's source and the applet's location.

As you can see from the example, Java applets extend the Applet class. The class must override methods from the extended class in order to set up a user interface inside itself. Below are a few methods and their responsibilities.

• public void init()
  • This method will be automatically called when the applet is started, nothing is required to go inside, but this is a good place to specify size, and other constructor type conditions.

• public void stop()
  • This method gets called when the applet is terminated, for example, when the user goes to another page or exits out of their browser.

• public void paint(Graphics g)
  • This is the method that is used primarily to "paint" on the applet window, this is also the method I used in the Hello World example. This overrides the empty Applet method, you can't call it "display" for example.

I have provided a link below to the Graphics class, this will give you an idea of what you can do inside the paint method, because you are given a Graphics object as a parameter.

http://java.sun.com/j2se/1.4.2/docs/api/java/awt/Graphics.html

Below is the link to the Applet Class, which includes the methods that you gain access to, and can perform when writing your own applet code.

http://java.sun.com/j2se/1.5.0/docs/api/java/applet/Applet.html

Tutorials

Here I’ve provided a few links for those of you interested in learning how to use java applets.