Event handling of key events
Handling key events in applets works very similar to the mouse event handling I talked about in the chapter before. There is one special method for every possible event (key down, key up). You have to overwrite the corrosponding method, if you want to handle, for example, a key down - event. In this chapter we'll have a short look at these two methods. Afterwards we will change our "moving ball" applet that way, so that one can change the direction of the ball movement by pressing the left or right cursor key and stop the ball movement by pressing the space bar. Be carefull! An applet is only able to handle key events once you have clicked in the applet window!
Here comes the explanation of the methods:
- public boolean keyDown (Event e, int key): This method listens to events that occur if a key is pressed down.
Every key has a value (ASCII). This value is given to the method with the help of the "key" variable. Space bar has a value of 32. If you want your applet to listen to Space bar pressed down, you only have to test, if the value of "key" is 32 (details later). Sometimes you don't know which value a certain key has. Then you can print out this value to the standard output by writing this line in your keyDown(...) - Method:
System.out.println ("Charakter: " + (char)key + " Integer Value: " + key);
Some frequently used keys have special variables. For example the cursor keys have Event.LEFT, Event.RIGHT, Event.UP, Event.DOWN. These values are field variables in the class Event. For more variables please read the API!
- public boolean keyDown (Event e, int key): This method reacts to key up - events. You can use this method exactly the same way as the keyDown - Method!
Now we want to modify our "moving ball" applet a little bit. The user should be able to change the direction of the ball movement by pressing the left or right cursor key. If he presses the space bar, the ball should stop its movement. To make this possible, we have to add the following lines to our applet:
// method to handle key - down events
public boolean keyDown (Event e, int key)
{
// user presses left cursor key
if (key == Event.LEFT)
{
// changing x - speed so that ball moves to the left side (x_speed negative)
x_speed = -1;
}
// user presses right cursor key
else if (key == Event.RIGHT)
{
// changing x - speed so that ball moves to the right side (x_speed positive)
x_speed = 1;
}
// user presses space bar (value = 32!)
else if (key == 32)
{
// Stop ball (x_speed = 0)
x_speed = 0;
}
else
{
/* Additionally the method prints out the ASCII - value if an other key is pressed. This is not necessary but a possibility for you to test which value a key has.*/
System.out.println ("Charakter: " + (char)key + " Integer Value: " + key);
}
// DON'T FORGET (although it has no meaning here)
return true;
}
Now you are able to program every essential thing used in a game. You can move objects, handle events and load sounds and images. In the next chapter we will program our first complete game (which was the first game I programmed in Java). To understand the next chapter you have to know something about classes, objects, calling methods of other classes... . And of course you have to know everything I talked about in the last chapters. By programming this game I will show you some new techniques; (how to "hit" a ball, changing mouse pointer, random ball movement...). After programming this game you should be able to program your own games. Well Ok have fun in the next chapter and now you can watch the applet you programmed in this chapter and download the source code!
Sourcecode download
Take a look at the applet
Next chapter
Our first game
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