Java--用键盘控制小球

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

    }
}