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Java Applets

Updated on December 31, 2016

Java is a fully machine independent language. This means that, the programs developed in java can virtually work on any machine in any part of the world without the need for rewritten or recompiling. This feature of java makes it one of the ideal languages to be used on the internet. Java program code can be embedded into the hypertext code of any web page to add sound, animation, multimedia effects and much, much more to the page. This page, could then be viewed by any internet user, anywhere in the world, on any machine.

An introduction to Applets

Programs in java can be of two types.

1. Applications

2.Applets

An application is a java program, that can be run on your computer, using the JVM you have installed on it.

A Java applet is a java program that can be executed by a web browser. The browser has an inbuilt JVM that helps in executing these applets. The applets can also be executed tested using a utility available as part of JDK - the applet viewer.

Though an applet is also a java program,it is one important feature that makes it very different from a java program. An applet is a java program that can be used in a web page. So when the web page is viewed, the Java applet code is also downloaded and executed along with the HTML code in the page.

My First Java Applet

import java.applet.Applet;
import java.awt.Graphics;
Public class applet1 extends Applet
{
public void paint (Graphics g)
{
g.drawString("My first applet", 20, 100);
}
}

The first line of the applet imports the java.applet.Applet.class. The class is a the superclass of all applets. That is every applet created must be defined as a subclass of the Applet class.

Unlike an application, which uses I/O classes for user interaction, an applet uses the AWT for user interaction. The AWT included several very sophisticated classes that provide graphical interface. One of them is the Graphics class, which isimported, in the second line.

The next line, Line 3,declares a class applet1 as a subclass of the superclass Applet.

Line 5, calls the method paint().This method is defined by AWT and is used to display the output of the window. The paint() method accepts one argument of type Graphics. In the applet shown above, this object is g.

The next line calls another method drawString(). This method belongs to the Graphics class and is used to display a string in the applet window. The method accepts 3 arguments - the String message to be displayed, the x and y coordinates of the location where it has to be displayed. In the above example, the drawString() method will display the String "My first applet" at the location 20, 100.


How to run the applet program?

To do so, first key in the applet using any text editor. Save the file like you save any other java program with the extension .java. In this case, the name of the applet will be applet1.java.

Compile it with javac.

javac applet1.java


Next create another text file with the following code,

<applet code ="applet1 width=200 height=200>

</applet>

Save the file with the extension .HTML, say test.html.

Once the applet has been successfully compiled and the corresponding HTML file created, it can be executed in two ways.

1. Using a web browser.

2. Using the appletviewer that is available as part of JDK.

To execute the applet using a web browser, start the web browser and open the file HTML file with File --> Open. When you do so, the browser executes the applet specified by the <applet>tag.

To execute using applet viewer, type the following command at the DOS prompt.

appletviewer test.html

There is a shortcut to executing applets using an appletview. Instead of creating a separate HTML file, the <applet> and </applet> tags can be included as comments in the applet code itself.

To execute this applet,

appletviewer applet.java

Note:

An applet does not need main() method.

AWT - Abstract window toolkit.

JVM - Java Virtual Machine.

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