1 package com.java7.myballfalldown.main; 2  3 import java.awt.Graphics; 4  5 import javax.swing.JFrame; 6 import javax.swing.JPanel; 7  8 public class MyBallFallDown { 9     public static void main(String[] args) {10         JFrame w = new JFrame();11         w.setSize(300, 400);12         13         MyPanel mp = new MyPanel();14         w.add(mp);15         16         Thread t = new Thread(mp);17         t.start();18         19         w.setVisible(true);20     }21 }22 23 class MyPanel extends JPanel implements Runnable {24     int x = 30;25     int y = 30;26     27     public void paint(Graphics g) {28         super.paint(g);29         g.fillOval(x, y, 20, 20);30     }31     public void run() {32         while(true) {33             y++;34             if(y > 400) {35                 y = 0;36             }37             try {38                 Thread.sleep(20);39             } catch(Exception e) {40                 41             }42             repaint();43         }44     }45 }