java 多线程

 package server;
import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.Container;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Rectangle2D;
import java.util.ArrayList;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Bounce {
 public static void main(String[] args) {
  JFrame frame = new BounceFrame();
  frame.setDefaultCloseOperation((JFrame.EXIT_ON_CLOSE));
  frame.setVisible(true);
  System.out.format("%d",2);
 }
 
}
class Ball
{
 
 private static final int XSIZE = 15;
 private static final int YSIZE = 15;
 private double x = 0;
 private double y = 0;
 private double dx = 1;
 private double dy = 1;
 
 
 
 public void move(Rectangle2D bounds)
 {
  x += dx;
  y += dy;
  if (x < bounds.getMinX())
  {
   x = bounds.getMinX();
   dx = -dx;
  }
  if (x + XSIZE >= bounds.getMaxX())
  {
   x = bounds.getMaxX() - XSIZE;
   dx = -dx;
  }
  if (y < bounds.getMinY())
  {
   y = bounds.getMinY();
   dy = -dy;
  }
  if (y + YSIZE >= bounds.getMaxY())
  {
   y = bounds.getMaxY() - YSIZE;
   dy = -dy;
  }
 }
 
 public Ellipse2D getShape()
 {
  return new Ellipse2D.Double(x,y,XSIZE,YSIZE);
 }
}
class BallPanel extends JPanel
{
 private ArrayList<Ball> balls = new ArrayList<Ball>();
 
 public void add(Ball b)
 {
  balls.add(b);
 }
 
&nbs