What are Java applets?

A Java applet is Java source code whose bytecodes will be executed as part of viewing a WWW page. The applet's author compiles the Java source code into bytecodes.
These bytecodes will be downloaded from their author's site by a WWW browser when the WWW page is visited. So, the browser needs a Java interpreter to interpret the bytecodes.
This is true for browsers that are Java-aware, e.g., Microsoft's Internet Explorer, Netscape's Navigator or Mozilla. The earlier versions of these browsers only understood JDK 1.0.2. Later versions of these browsers understand JDK 1.1.x or Java 2 SDK bytecode files that use the facilities of JDK 1.1.x. Examples are Version 4.0x (and later versions) of Netscape's Navigator (which supports most aspects of JDK 1.1.x) and Version 4.x (and later versions) of Microsoft's Internet Explorer (which supports some aspects of JDK 1.1.x).
Some versions of WWW browsers support Java 2 Platform v 1.2 and later. For more details see ITS Guide 108 Advanced Java.
Sun's Java 2 SDK (and the earlier JDKs) also include an appletviewer that can be used if you do not have a Java-aware browser.

Suppose the file HWLet.java contains the Java applet:

0006: import java.applet.Applet;                                     // HWLet.java
0007: import java.awt.Graphics;
0008: public class HWLet extends Applet {
0009:    public void paint(Graphics rGraphics) {
0010:       rGraphics.drawString("Hello World!", 50, 25);
0011:    }
0012: }

The applet can be compiled by using the Unix/MS-DOS command:
```
javac HWLet.java
```
This produces a file of bytecodes in the file HWLet.class.

When a browser reads the WWW page given below, it finds that it has to retrieve the file HWLet.class. When the bytecodes in this file arrive, the browser can interpret them.

0013: <HTML>
0014: <HEAD> 
0015: <TITLE> HWLet example </TITLE>
0016: </HEAD>
0017: <BODY>
0018: Before the output from the applet.
0019: <APPLET CODE="HWLet.class" WIDTH=150 HEIGHT=25>
0020: </APPLET>
0021: After the output from the applet.
0022: </BODY>
0023: </HTML>

You can access a WWW page containing the above HTML instructions by using the URL http://www.dur.ac.uk/barry.cornelius/papers/a.taste.of.java/code/HWLet.html.