这个是一个简单的练习程序，窗口上有个小字母会根据键盘输入的上下左右移动。
然而我发现移动的同时，原先的字母没有被擦去？
网上说paint方法需要调用父方法才不会这样，可是我已经调用了啊？

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class KeyEventDemo extends JFrame
{
    private KeypboardPanel keyboardPanel=new KeypboardPanel();
    public KeyEventDemo() 
    {
        add(keyboardPanel);
        keyboardPanel.setFocusable(true);
    }
    public static void main(String[] args) 
    {
        JFrame frame=new KeyEventDemo();
        frame.setTitle("KeyEventDemo");
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
        frame.setSize(300,300);
        frame.setVisible(true);
    }
    static class KeypboardPanel extends JPanel
    {
        private int x=100,y=100;
        private char keyChar='A';
        public KeypboardPanel() 
        {
            addKeyListener(
                new KeyAdapter()
                {
                    public void keyPressed(KeyEvent e)
                    {
                        switch(e.getKeyCode())
                        {
                            case KeyEvent.VK_DOWN:y+=10;break;
                            case KeyEvent.VK_UP:y-=10;break;
                            case KeyEvent.VK_LEFT:x-=10;break;
                            case KeyEvent.VK_RIGHT:x+=10;break;
                            default:keyChar=e.getKeyChar();
                        }
                        repaint();
                    }
                });
        }
        protected void paintComponent(Graphics g)
        {
            super.paintComponents(g);
            g.setFont(new Font("TimesRoman",Font.PLAIN,24));
            g.drawString(String.valueOf(keyChar),x,y);
        }
    }
}