16.9

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Test_16_9 extends JFrame{
    private JP JP1 = new JP();
    public Test_16_9(){        
        int i = JP1.getWidth();
        add(JP1);        
        JP1.setFocusable(true);
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Test_16_9 t1 = new Test_16_9();
        t1.setTitle("Test_16.9");
        t1.setSize(300,300);
        t1.setLocationRelativeTo(null);
        t1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        t1.setVisible(true);
    }
    
    static class JP extends JPanel{
        private int x1 = 150;
        private int y1 = 150;
        private int x2 =x1,y2 =y1;        
        
        public JP(){            
        addKeyListener(new KeyAdapter(){
            
            public void keyPressed(KeyEvent e) {
                // TODO Auto-generated method stub
                switch(e.getKeyCode()){
                case KeyEvent.VK_UP: update();y2 = y2 -20; break;
                case KeyEvent.VK_DOWN: update();y2 =y2 + 20;break;
                case KeyEvent.VK_LEFT: update();x2 =x2 - 20;break;
                case KeyEvent.VK_RIGHT: update();x2 =x2 + 20;break;
                }
                repaint();             
            }            
        });        
        }    
        protected void paintComponent(Graphics g){
//            super.paintComponent(g);
            
            g.drawLine(x1, y1, x2, y2);    
            
            System.out.println("("+x1+","+y1+") ("+x2+","+y2+")");
        }
        protected void update(){
            x1 = x2; y1 = y2;
        }
    }
}

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。