Repaint() method 's not work but paint(getGraphics()) does, why?

Question

I've just read the following code ( in the Core Java 2 advance features vol 2 book):
  import java.awt.*;
  import java.awt.event.*;
   import java.awt.geom.*;
   import java.util.*;
   import javax.swing.*;
      Shows an animated bouncing ball.
  public class Bounce
     public static void main(String[] args)
        JFrame frame = new BounceFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.show();
     The frame with canvas and buttons.
  class BounceFrame extends JFrame
        Constructs the frame with the canvas for showing the
        bouncing ball and Start and Close buttons
     public BounceFrame()
        setSize(WIDTH, HEIGHT);
        setTitle("Bounce");
        Container contentPane = getContentPane();
        canvas = new BallCanvas();
        contentPane.add(canvas, BorderLayout.CENTER);
        JPanel buttonPanel = new JPanel();
        addButton(buttonPanel, "Start",
           new ActionListener()
                 public void actionPerformed(ActionEvent evt)
                    addBall();
        addButton(buttonPanel, "Close",
           new ActionListener()
                 public void actionPerformed(ActionEvent evt)
                   System.exit(0);
        contentPane.add(buttonPanel, BorderLayout.SOUTH);
        Adds a button to a container.
        @param c the container
        @param title the button title
        @param listener the action listener for the button
     public void addButton(Container c, String title,
        ActionListener listener)
        JButton button = new JButton(title);
        c.add(button);
        button.addActionListener(listener);
        Adds a bouncing ball to the canvas and makes
        it bounce 1,000 times.
     public void addBall()
        try
           Ball b = new Ball(canvas);
           canvas.add(b);
           for (int i = 1; i <= 1000; i++)
              b.move();
              Thread.sleep(5);
        catch (InterruptedException exception)
     private BallCanvas canvas;
     public static final int WIDTH = 450;
     public static final int HEIGHT = 350;
    The canvas that draws the balls.
class BallCanvas extends JPanel
        Add a ball to the canvas.
        @param b the ball to add
   public void add(Ball b)
       balls.add(b);
    public void update(Graphics g) {
         super.update(g);
         System.out.println("Test");
    public void paintComponent(Graphics g)
      super.paintComponent(g);
      Graphics2D g2 = (Graphics2D)g;
      for (int i = 0; i < balls.size(); i++)
         Ball b = (Ball)balls.get(i);
         b.draw(g2);
        // System.out.println("Test");
    private ArrayList balls = new ArrayList();
    A ball that moves and bounces off the edges of a
   component
class Ball
        Constructs a ball in the upper left corner
        @c the component in which the ball bounces
    public Ball(Component c) { canvas = c; }
       Draws the ball at its current position
       @param g2 the graphics context
    public void draw(Graphics2D g2)
       g2.fill(new Ellipse2D.Double(x, y, XSIZE, YSIZE));
      Moves the ball to the next position, reversing direction
      if it hits one of the edges
   public void move()
      x += dx;
      y += dy;
       if (x < 0)
          x = 0;
          dx = -dx;
     if (x + XSIZE >= canvas.getWidth())
         x = canvas.getWidth() - XSIZE;
          dx = -dx;
       if (y < 0)
         y = 0;
          dy = -dy;
       if (y + YSIZE >= canvas.getHeight())
          y = canvas.getHeight() - YSIZE;
          dy = -dy;
       //canvas.paint(canvas.getGraphics());//this would OK.
       canvas.repaint();//This not work, please tell me why?
    private Component canvas;
    private static final int XSIZE = 15;
   private static final int YSIZE = 15;
   private int x = 0;
   private int y = 0;
   private int dx = 2;
   private int dy = 2;
}this program create a GUI containing a "add ball" button to creat a ball and make it bounce inside the window. ( this seems to be stupid example but it is just an example of why we should use Thread for the purpose ).
Note: in the move() method, if i use canvas.repaint() then the ball is not redrawn after each movement. but if i use canvas.paint(canvas.getGraphics()) then everythings seem to be OK.
Another question: Still the above programe, but If I create the ball and let it bounce in a separate thread, then canvas.repaint() in the move method work OK.
Any one can tell me why? Thanks alot !!!

Answer 1

I don't know why the one method works. Based on my Swing knowledge neither method should work. Did you notice that the JButton wasn't repainted until after the ball stopped bouncing. That is because of the following code:
for (int i = 1; i <= 1000; i++)
    b.move();
    Thread.sleep(5);
}This code is attempting to move the ball and then sleep for 5 milliseconds. The problem with this code is that you are telling the Event Thread to sleep. Normally painting events are added to the end of the Event Thread to allow for multiple painting requests to be combined into one for painting efficiency. When you tell the Thread to sleep then the GUI doesn't get a chance to repaint itself.
More details about the EventThread can be found in the Swing tutorial:
http://java.sun.com/docs/books/tutorial/uiswing/misc/threads.html
This explains why the button isn't repainted, but it doesn't explain why the ball bounces. I don't know the answer to this for sure, but I do know that there is a method called paintImmediately(...) which causes the painting to be performed immediately without being added to the end of the Event Thread. This means the painting can be done before the Thread sleeps. Maybe the canvas.paint(....) is somehow invoking this method.
When you click on the button this code is executed in the EvWell in Swing, all GUI painting is done on the Event Thread