我按照课本得例子上得程序，说MultiListener不是抽象得，且未覆盖java.awt.event.MouseListener中得抽象方法mouseExited(java.awt.event.MouseEvent),怎么出现这种情况啊，是哪边得问题啊？
import java.awt.*;
import java.awt.event.*;
public class MultiListener implements MouseListener,MouseMotionListener,WindowListener
{
    private Frame f;
    private TextField tf;
    public static void main(String[] args)
    {
        MultiListener ml=new MultiListener();
        ml.go();
    }
    public void go()
    {
        Frame f=new Frame();
        f.add(new Label("单击和拖动鼠标"),"north");
        tf=new TextField(30);
        f.add(tf,"south");
    f.addMouseListener(new MouseAdapter(){
        public void mouseEntered(MouseEvent e)
        {
            String s="Yhe Mouse Entered";
            tf.setText(s);
        }
    });
    f.addMouseMotionListener(new MouseMotionAdapter(){
        public void mouseDragged(MouseEvent e)
        {
            String s="Yhe Mouse Dragging";
            tf.setText(s);
        }
    });
    f.addWindowListener(new WindowAdapter(){
        public void windowClosing(WindowEvent e)
        {
            System.exit(0);
        }
    });
    f.resize(200,200);
    f.setVisible(true);
}
}