16.18

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

public class Test_16_18 extends JFrame{
    private JP jp = new JP();
    public Test_16_18()
    {
        add(jp);
        jp.setFocusable(true); //必须在JPanel这里设置focusable
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Test_16_18 frame = new Test_16_18();        
        frame.setSize(300,300);
        frame.setTitle("Test_16_18");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null); // Center the frame        
        frame.setVisible(true); 
    }
    
    static class JP extends JPanel{
        private int x = 140, y = 140;
        public JP(){
            addKeyListener(new KeyAdapter(){
                public void keyPressed(KeyEvent e){
                    switch(e.getKeyCode()){
                    case KeyEvent.VK_UP: y -= 10; break;
                    case KeyEvent.VK_DOWN: y += 10; break;
                    case KeyEvent.VK_LEFT: x -= 10; break;
                    case KeyEvent.VK_RIGHT: x += 10; break;
                    }repaint();
                } 
            });
        }
        protected void paintComponent(Graphics g){
            super.paintComponent(g);
            g.drawOval(x, y, 30, 30);
        }
    }
}

需要注意的是，必须给JPanel设置焦点，否则键盘无用。