Copy/paste this code into a file TestApplet.java: 

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

// this is a simple Java applet

// all applets are extentions of the class Applet
public class TestApplet extends Applet {

    // method paint displays the graphics of the applet
    public void paint(Graphics g) {
        // set the Color for drawing (default is black):
	g.setColor(Color.green);
        // draw a rectangle:
	g.drawRect(100, 100, 200, 50);
    }
}
Copy/paste this into a file TestApplet.html: 

<HTML>
<APPLET CODE="TestApplet.class" HEIGHT=300 WIDTH=400></APPLET>
</HTML>
To run (on Windows):
  1. Open Command Prompt window. Go to the folder where your programs are: cd C:\myprograms (replace myprograms by the folder where you copied the files).
  2. Compile the code: javac TestApplet.java in the Command Prompt window.
  3. Run it using appletviewer: appletviewer TestApplet.html.
This is an example from CSci 1211 course.