Our Hello World program was a Java application.  We will now look at creating some simple Java applets.  An applet is a Java program that can run within a Web browser, as opposed to a Java application, which is a stand-alone program that runs on your local machine.  Applets are a bit easier to work with than applications.

In our first Java applet, we will draw a cross on the screen.

Step 1 - Write your applet

Open Notepad and copy and paste the following code:

//Java applet to draw a cross on the screen

//created by M Meijers June2005

import java.awt.Graphics;

public class FirstApplet extends java.applet.Applet
{

public void paint(Graphics g)
      {
      g.drawLine(0,100, 200, 100);
      g.drawLine(100,0, 100, 200);
      }
}

Save it as FirstApplet.java 

 

Make sure you don't accidentally save it with a .txt file extension!

Java is case sensitive, so make sure you type uppercase wherever it is used

eg FirstApplet must have an upper case F and A

Step 2 - Compile it

Step 3 - Create a Web Page for your applet

Applets must live within a web page, so we will create one.

Open Notepad and type in the following html:

<html>
<body>
<applet code=FirstApplet.class width=200 height=200>
</applet>
</body>
</html>
 

This will insert your applet into a 200x200 pixel area on a web page.  Save it as cross.htm

Step 4 - View your page

Open your cross.htm file in your web browser

or alternatively, in your command window type: appletviewer cross.htm

Appletviewer is an application that comes with Java that enables you to quickly check your applets, without have to open your web browser.

 

 
  In our Java applet, the lines that do the work are:

      g.drawLine(0,100, 200, 100);
      g.drawLine(100,0, 100, 200);

We called a method called drawLine and we passed it parameters (the numbers that tell the computer where to start and finish the line)

The line begins with g., which means that we want to call the method named drawLine on the object named g (which you can see one line up is from the class Graphics

A method is simply a command - it tells the computer to do something. In this case, drawLine tells the computer to draw a line between the points specified: (0, 100) and (200, 100). You can think of the window as having its 0,0 coordinate in the upper left corner, with positive X and Y axes extending to the right and down.

 

More Methods

There are plenty of methods available for the Graphics class.  If you want to see all the methods available for the Graphics class, you can look in the Java documentation.  It should be located in the folder where Java is installed in docs\api\java\awt\Graphics.html

Here is another program for you to experiment with.  It uses the fillRect method which has 4 parameters.  The first two are the x and y coordinates of the top left corner, and the second two are the width and height of the rectangle in pixels.  The setColor method is used to set the colour.

//Java applet to draw coloured boxes
//created by M Meijers June2005
import java.awt.Graphics;
import java.awt.Color;

public class FirstApplet extends java.applet.Applet
{

public void paint(Graphics g)
    {
     g.setColor(Color.blue);
     g.fillRect(0, 0, 200, 200);
     g.setColor(Color.red);
     g.fillRect(25, 25, 150, 150);
     g.setColor(Color.green);
     g.fillRect(50, 50, 100, 100);
     g.setColor(Color.yellow);
     g.fillRect(75, 75, 50, 50);
     }
}

Some other methods you might like to try are:

drawOval(x, y, width, height)   (where x,y are the coordinates of the top left corner, and width and height in pixels)

fillOval(x, y, width, height)

drawString("Hello", x,y)  (where x, y are the coordinates where the string will start)

Write an applet that will draw a snowman or an animal on the screen. 
Create an applet that does a drawing by using loops.