Java Events
(and how to handle them)

Clicking on any of the four different colors below will cause an event

A different sound is triggered when you press the mouse button down
within the borders of a color region.

We trigger events in Java in many ways. The sections on sound and animation used the start and stop events that are automatically triggered when you enter or leave a page that contains an applet. The mouse can be pressed to cause an event called mouseDown. When you release the mouse button you trigger a mouseUp event. Mouse actions and movements (mouseDrag, mouseEnter, mouseExit & mouseMove) , keystrokes on the keyboard (keyDown & keyUp) all are examples of events we can use to trigger actions by handling the event when it is triggered. This particular applet uses the mouseDown event and handles it in the code below.

 

import java.awt.Graphics;
import java.awt.Color;
import java.awt.Event;
import java.applet.AudioClip;

public class events1 
extends java.applet.Applet {
We want the graphics, color and event handling
facilities of the Java Abstract Windowing Toolkit

We also want to use the audio capabilities.
And create an Applet called events1
public void paint(Graphics g)
{
for (int i=0 ; i <= 4 ; i++) {
g.setColor(
new Color(63*i,100,(255-(63*i))));
g.fillRect((i-1)*100,0,i*100,80);
}
}
 The paint method will

create four ....
different colors **

and draw rectangles filled with those colors
public boolean mouseDown(Event evt, int x, int y)


{
if (x < 100)
play(getCodeBase(),"bark.au");
if ((x > 100)&(x < 200))
play(getCodeBase(),"honk.au");
if ((x > 200)&(x < 300))
play(getCodeBase(),"cellular.au");
if (x > 300)
play(getCodeBase(),"feryhorn.au");
return true;
}
}		 This is the event handling code. The x and y
coordinates of the mouse pointer will be passed
to this routine when the event occurs.
We simply need to consider the x coordinate
if its less than 100 we have a click on the first
rectangle and play the bark.au sound
Between 100 and 200 ...
Play the honk.au sound
and so on



When finished processing this event return whatever
boolean value we want for the result.

** we can create custom colors by using 'new Color(red,green,blue)' where red,green and blue are integers from 0 to 255 that represent the level of the primary color in the custom composite color.

Java does have a switch statement that is similar to the one in C++.   Unfortunately it only handles discrete values and not ranges.  We could enter the following,

switch (x){
case 100:play(getCodeBase(),"bark.au");
break;
case 200:play(getCodeBase(),"honk.au");
break;
....}

It will handle only the values 100, 200, ... but there is no way to handle ranges of values.  It makes the switch statement somewhat limited in use.

The easiest way to copy the code for this example is to click here, copy the contents of the screen that loads in your browser , paste it in a text editor and save it in a file called "events1.java".

After compiling this program we need to create an HTML page with a reference to the class file. It does not need a graphic window unless you are running it with an image or animation.

Save the HTML code in the same directory as the class file.

When you load the HTML file in your browser the applet will execute as below.

View of the applet's output

If you try really hard, you will be able to click on the border between two rectangles and not get any sound. Notice in the code that there is no option to handle the event if x = 100, 200 or 300. We can choose to ignore or handle any event as we see fit.
Other events are handled in a similar fashion. The components of the AWT covered in the next section can also trigger events and allow us to program true Windows style GUI Applets and Applications.

Go to the next topic: AWT Components

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