Why the ball won't come out?

Question

hi, in the following code there are two lines that each creates a Ball instance. But the line commented with "ball one" won't display a ball on the frame, however the line commented with "another ball" can do.
Totally confused!
import java.awt.event.*;
import java.awt.*;
import java.util.*;
import javax.swing.*;
import java.lang.*;
public class BouncerFrame extends JFrame {
       JPanel canvas;
    public BouncerFrame()
       setSize(200,300); 
       setVisible(true);
       Container contentPane = getContentPane();
       canvas = new  JPanel();
       contentPane.add(canvas);
       JPanel p = new JPanel();
      Ball b = new Ball(canvas);// ball one
      JButton button = new JButton("start");
      p.add(button);
      button.addActionListener(new ActionListener()
                              { public void actionPerformed(ActionEvent ae)
                                  Ball ball = new Ball(canvas);// another ball
     contentPane.add(p,"South");
    public static void main (String [] args)
        BouncerFrame bf = new BouncerFrame();
        bf.show();
    class Ball
       private JPanel box;
       private static final int XSIZE = 10;
       private static final int YSIZE = 10;
       private int x = 10;
       private int y = 10;
       private int dx = 2;
       private int dy = 2;
        public Ball(JPanel b)
            box = b;
            Graphics g = box.getGraphics();
            g.setColor(Color.black);
            g.fillOval(x, y, XSIZE, YSIZE);
            g.dispose();
   

Answer 1

Don't know exactly why but this should work:
import java.awt.event.*;
import java.awt.*;
import java.util.*;
import javax.swing.*;
import java.lang.*;
public class BouncerFrame extends JFrame {
JPanel c;
public BouncerFrame()
setSize(200,300);
setVisible(true);
Container contentPane = getContentPane();
c = new JPanel();
contentPane.add(c);
JPanel p = new JPanel();
JButton button = new JButton("start");
p.add(button);
button.addActionListener(new ActionListener()
{ public void actionPerformed(ActionEvent ae)
Ball ball = new Ball(c, 20, 20);// another ball
contentPane.add(p,"South");
public static void main (String [] args)
BouncerFrame bf = new BouncerFrame();
bf.show();
Ball ball1 = new Ball(bf.c);// first ball
class Ball
private JPanel box;
private static final int XSIZE = 10;
private static final int YSIZE = 10;
private int x = 10;
private int y = 10;
private int dx = 2;
private int dy = 2;
public Ball(JPanel b)
box = b;
Graphics g = box.getGraphics();
g.setColor(Color.black);
g.fillOval(30, 30, XSIZE, YSIZE);
g.dispose();
public Ball(JPanel b, int x, int y)
box = b;
Graphics g = box.getGraphics();
g.setColor(Color.red);
g.fillOval(x, y, XSIZE, YSIZE);
g.dispose();