Why does it come out as a line instead of a moving ball?

Question

I wish to have a moving ball, but got a growing line, anybody can let me know how to solve it?
Thanks
import java.awt.event.*;
import java.awt.*;
import java.util.*;
import javax.swing.*;
import java.lang.*;
public class Bouncer extends javax.swing.JApplet implements Runnable {
    /** Creates a new instance of Bouncer */
    public Bouncer() {
    int rectW, rectH, rectPosX, rectPosY;
    int ballW, ballH, ballPosX, ballPosY, dx, dy;
    Thread ballThread;
    public void init()
        rectW = 5000;
        rectH =5000;
        rectPosX = 0;
        rectPosY = 0;
        ballW =10;
        ballH = 10;
        ballPosX =20;
        ballPosY = 20;
        ballThread= new Thread(this);
        ballThread.start();
    public void paint(Graphics g)
     g.drawRect(rectPosX, rectPosY, rectW, rectH);
     g.drawOval(ballPosX, ballPosY, ballW, ballH);
     g.setColor(Color.black);
     g.fillOval(ballPosX, ballPosY, ballW, ballH);
     g.dispose();
    public void move()
       dx =2;
       dy=2;
      ballPosX += dx;
      ballPosY += dy;
      repaint();
    public void run() {
        while((ballPosX < rectPosX + rectW) && (ballPosY < rectPosY + rectH))
            try
                ballThread.sleep(100);
            catch(InterruptedException e)
            {e.getMessage();}
            move();
}

Answer 1

OK, were you trying to use drawRect() to erase the ball? That won't work since the new ball might not be in the same rectangle as the old one. Try this:public void paint(Graphics g)
    // Erase old ball
    // Note: bkgdColor should have already been set
    // to your background color (probably white)
    g.setColor(bkgdColor);
    g.fillOval(ballOldPosX, ballOldPosY, ballW, ballH);
    // Draw new ball
    // Note: fgndColor should have already been set
    // to your foreground color (probably black)
    g.setColor(fgndColor);
    g.fillOval(ballPosX, ballPosY, ballW, ballH);
    g.dispose();
}