package secondpack;import java.awt.*;import java.awt.event.*;public class MyBall {    public static void main(String args[]) {        Frame w = new Frame();        w.setSize(800, 700);                MyPanel mp = new MyPanel();        w.add(mp);                w.addKeyListener(mp);        mp.addKeyListener(mp);                w.setBackground(Color.black);        w.setVisible(true);    }}class MyPanel extends Panel implements KeyListener {    int x = 30;    int y = 30;    public void paint(Graphics g) {        g.setColor(Color.red);        g.fillOval(x, y, 20, 20);    }    @Override    public void keyTyped(KeyEvent e) {        // TODO Auto-generated method stub            }    @Override    public void keyPressed(KeyEvent e) {        System.out.println(e.getKeyCode());//通过打印内容可以得到“上、下、左、右”键的值        if(e.getKeyCode() == 37) {            x--;        }        if(e.getKeyCode() == 38) {            y--;        }        if(e.getKeyCode() == 39) {            x++;        }        if(e.getKeyCode() == 40) {            y++;        }        repaint();//重画，这个很重要，没有这条命令，小球是不会有反应的    }    @Override    public void keyReleased(KeyEvent e) {        // TODO Auto-generated method stub            }}