Training and Consulting

Cleaning the Screen

(c) Alexander Hristov

We had a monster overload in our previous step which we must take care of. As a first attempt at solving our problem, we can try erasing the window manually before drawing the monster. The only lines that change are the ones inside the paint()method

           . . .  
76      

77      public void paint(Graphics g) {
78        g.setColor(getBackground());
79        g.fillRect(0,0,getWidth(),getHeight());

80        g.drawImage(getSprite("bicho.gif"), posX, posY,this);
           . . .  

If now we run the program, we see that the critter changes its position so fast that it's almost impossible to see it. It's clear that we must insert an artificial delay inside the main loop, to give the user some time to see the world before it changes again. Our main loop now becomes

1. Update the "state of the world" or "stage"
2. Refresh the screen (redraw the world)
3. Wait some time
4. Go back to point 1

The wainting period will be a global constant and that will be initially set at 10 ms:

           . . .  
24    
25    public class Invaders extends Canvas {
26      public static final int WIDTH = 640;
27      public static final int HEIGHT = 480;
28      public static final int SPEED = 10;
29      
           . . .  
87      
88      public void game() {
89        while (isVisible()) {
90          updateWorld();
91          paint(getGraphics());

92          try { 
93             Thread.sleep(SPEED);
94          } catch (InterruptedException e) {}

95        }
96      }
           . . .