Problems with jumping ball in Java applet

Question

The ball is supposed to jump to the same height from which it fell down, someway it always gets higher and higher. My question is why?
import java.applet.*;
import java.awt.*;
public class Bug extends Applet implements Runnable
   private int y_pos = 200;
   private int y_speed = 0;
   private int radius = 20;
   private int appletSize_x = 400;
   private int appletSize_y = 400;
   private Image dbImage;
   private Graphics dbg;
   public void run(){
       while(true){
           y_speed++;
           if ((y_pos > appletSize_y)&& (y_speed > 0))
                y_speed = (-1)*(y_speed);
           y_pos += y_speed; 
           repaint();
           //controls the time
           try{
               Thread.sleep(20);
          catch (InterruptedException ex){}
    public void update (Graphics g){
        if (dbImage == null)
            dbImage = createImage (this.getSize().width, this.getSize().height);
            dbg = dbImage.getGraphics ();
        dbg.setColor (getBackground ());
        dbg.fillRect (0, 0, this.getSize().width, this.getSize().height);//erase this line to understand better
        dbg.setColor(getForeground());
        paint(dbg);
        g.drawImage (dbImage, 0, 0, this);
    public void stop(){}
    public void destroy(){}
    public void start(){
        Thread th = new Thread(this);
        th.start();
    public void paint(Graphics g){
     g.setColor(Color.green);
     g.fillOval (30 - radius, y_pos - radius, 2 * radius, 2 * radius);
}

Answer 1

Shurik007.5 wrote:
The ball is supposed to jump to the same height from which it fell down, someway it always gets higher and higher. My question is why?
g.fillOval (30 - radius, y_pos - radius, 2 * radius, 2 * radius);There are variables here involved in what gets drawn, no? And do (some of) these variables change during the lifetime? Yes. So, there should be no question about why the display changes.
Edit: I guess your question is more about "why doesn't y_pos keep cycling back and forth between the same two values?" I'd suggest you do some debugging, to make the computer tell you what the values are at each step until you understand what is going on. The simple answer is "It did what you told (programmed) it to do", of course.