/** Example from "Java Certification Exam Guide" by Barry Boone p 253
    Creating threads by subclassing Thread
    Bounces a ball up and down on the screen
*/

import java.awt.*;
import java.applet.Applet;

public class UpDown extends Applet {
    static int RADIUS = 20;
    static int X = 30;
    public int y = 30;
    
    public void init(){
        new BounceThread(this).start();
    }
    
    public void paint(Graphics g) { 
        g.setColor(Color.blue);
        g.drawOval(X - RADIUS, y - RADIUS, 2*RADIUS, 2*RADIUS);
    }

}

class BounceThread extends Thread {
    UpDown applet;
    int yDir = +1;
    int incr = 10;
    int sleepFor = 100;
    
    BounceThread(UpDown a) {
        this.applet = a;        
    }
    
    public void run(){
        while(true) {
            applet.y += (incr * yDir);
            applet.repaint();
            
            if(applet.y - UpDown.RADIUS < incr ||
               applet.y + UpDown.RADIUS + incr > 
               applet.getSize().height)
               yDir *= -1;
               
            try{
                sleep(sleepFor);
            }catch(InterruptedException e) {}
        } // end while
    }
}