16.9

 1 import java.awt.*;
 2 import java.awt.event.*;
 3 import javax.swing.*;
 4 
 5 public class Test_16_9 extends JFrame{
 6     private JP JP1 = new JP();
 7     public Test_16_9(){        
 8         int i = JP1.getWidth();
 9         add(JP1);        
10         JP1.setFocusable(true);
11     }
12     
13     public static void main(String[] args) {
14         // TODO Auto-generated method stub
15         Test_16_9 t1 = new Test_16_9();
16         t1.setTitle("Test_16.9");
17         t1.setSize(300,300);
18         t1.setLocationRelativeTo(null);
19         t1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
20         t1.setVisible(true);
21     }
22     
23     static class JP extends JPanel{
24         private int x1 = 150;
25         private int y1 = 150;
26         private int x2 =x1,y2 =y1;        
27         
28         public JP(){            
29         addKeyListener(new KeyAdapter(){
30             
31             public void keyPressed(KeyEvent e) {
32                 // TODO Auto-generated method stub
33                 switch(e.getKeyCode()){
34                 case KeyEvent.VK_UP: update();y2 = y2 -20; break;
35                 case KeyEvent.VK_DOWN: update();y2 =y2 + 20;break;
36                 case KeyEvent.VK_LEFT: update();x2 =x2 - 20;break;
37                 case KeyEvent.VK_RIGHT: update();x2 =x2 + 20;break;
38                 }
39                 repaint();             
40             }            
41         });        
42         }    
43         protected void paintComponent(Graphics g){
44 //            super.paintComponent(g);
45             
46             g.drawLine(x1, y1, x2, y2);    
47             
48             System.out.println("("+x1+","+y1+") ("+x2+","+y2+")");
49         }
50         protected void update(){
51             x1 = x2; y1 = y2;
52         }
53     }
54 }

Test_16_9.java

效果图：

有一个小问题，暂时没解决。对于取得JPanel的getWidth()和getHeight()，无法在paintComponent()外面取得，在外面取得的值为0。而如果将x1 y1放入内部的话，又无法更新x1 y1的位置。