Task: Write an applet that will display "Hello World! (updated x times)" - Where x is the number of times that it has been repainted.

Solution: For this task, we wish to create an Applet. It should paint a string on the applet! and so the paint() method must be used. The steps that we must take are:

The code solution is as follows: (listing for HelloWorldCount.java). Here we have derived from the Applet class (using extends Applet) and then overwritten the paint() method with our own code.

 1 
 2   
 3  // HelloWorld Applet with added painting counter.
 4  import java.awt.Graphics;
 5  import java.applet.Applet;
 6 
 7  public class HelloWorldCount extends Applet
 8  {
 9    private int count = 0;
10 
11    public void paint(Graphics g)
12    {
13      count++;
14      g.drawString("Hello World! (updated " + count + " times)", 20, 20);
15    }
16  }
17   
18 

See it working here How does this work? The import statements are used to include the class library files that we need to implement the applet. This includes the Graphics class (to allow us to draw strings) and the applet library with the generic Applet class.

We create a subclass HelloWorldCount of the Applet class using the extends keyword.

We only added one method, the paint() method to the applet. This method is called by the browser (that passes the Graphics object g) whenever the applet needs to be updated. This Graphics object g that is passed is used to display "Hello World! (updated x times)", using its drawString() method. This method displays the string at the (x,y) location (20,20) relative to the top left hand corner of the applet.

We also need the count variable (state) in the class to allow us to remember how many times paint() was called by the virtual machine. Each time the paint() method is called the count is incremented by 1 using the count++ call. This can then be displayed in the string by simply using the + operator which concatenates the int variable with the string.

Compile this piece of code by typing: javac HelloWorldCount.java

NB: while DOS is not case sensitive, parameters are - You must use the exact same case!

This will create a Java bytecode file in the same directory called: HelloWorld.class

The last step that we must perform is to create a .html file that this applet can be displayed within. The code is quite straighforward: (listing for HelloWorldCount.html, again place in the same directory)

 1 
 2   
 3  <title> Test Applet Page </title>
 4  <hr>
 5  <applet code=HelloWorldCount.class width=200 height=200>
 6  </applet>
 7  <hr>
 8   
 9 

To view this example use the appletviewer, or your favorite Java enabled web browser. To use the appletviewer type:appletviewer HelloWorldCount.html

So to compile and execute your applet your Command Prompt should look like Figure 7.1, “HelloWorldCount Command Prompt”:

Figure 7.1. HelloWorldCount Command Prompt

Your Applet should look like (See Figure 7.2, “HelloWorldCount Applet”): -

Figure 7.2. HelloWorldCount Applet

So why is the number 21 given in this capture of the applet. I have minimised and then maximised the applet a few times, but the major cause of the increments was that I dragged the corner of the applet to resize it - you can also drag another window over your applet and then re-expose it. Try this yourself.

Possible problems that can occur when stepping through this section are: