How to simply mimic a moving ball in an applet

Question

Hi,
I'm new to Java and trying to understand some basics. I have a small problem; all i want to do is to draw a circle within an applet window, make it move slightly and then repaint it (setting a start and end parameter).
Now, i can get the circle to appear in the window, but it won't redraw it to the next part of the screen. The code, hopefully, will make things clearer. Any help appreciated.
//import the relevant libraries
import java.applet.Applet;
import java.awt.*;
import javax.swing.*;
public class J11Applet extends Applet
     public void init()
          setBackground(Color.yellow);               
     class Ball                                        //class ball
          int x;
          int y;
          int dx=1;
          Ball(int setX, int setY)
               x=setX;
               y=setY;               
          public void paint(Graphics g)          //draws an oval, i.e. the ball.
               g.setColor(Color.blue);
               g.drawOval(x,y,50,50);
          void move()                                   //makes the ball move direction
               if (x>=800-120)
                    dx=-dx;
               if (x<=0)
                    dx=-dx;
               x=x+dx;
     public void start()
     public void paint(Graphics g)               //paints on the applet window
          Ball myBall = new Ball(5,5);
          myBall.paint(g);
          myBall.move();
          repaint();
     public void stop()
     public void destroy()
}

Answer 1

If you want something doing something by itself, it needs to have a thread driving it, either implicitly (by creating a Thread) or implicitly (by using something like Timer/TimerTask, which creates its own thread).
Or I suppose you could do something hideous, like hijack the GUI update thread to update object state, but that's unreliable and can mess up GUI responsiveness.
So in an application you need threads as well (even if you do it implicitly or hideously as described above). I don't know what you were doing before to make a ball move in an application, but if it were doing it by itself (not just clicking a button over and over) then it involved a thread somehow.