public class cardlayout extends Frame implements ActionListener{
    //去掉Panel num;
    CardLayout lay=new CardLayout();
    
    public cardlayout(){
        setLayout(new BorderLayout());
        
        Panel k=new Panel();
        Button button=new Button("切换");
        button.addActionListener(this);
        k.add(button);
        add("North",k);
        k.setBackground(Color.blue);
        
        
        Panel p1=new Panel();
        p1.setLayout(new GridLayout());
        p1.add(new Button("老婆"));
        p1.add(new Button("爱你"));
        p1.add(new Button("亲亲"));
        p1.add(new Button("xiaojiji"));
        p1.setBackground(Color.yellow);
        
        
        Panel p2=new Panel();
        p2.add(new TextField("nainia",20));
        p2.setBackground(Color.red);
        
        Panel p3=new Panel();
        p3.add(new TextField("kanjiji",20));
        p3.setBackground(Color.black);
        
                
               Panel num=new Panel();报错
        num.setLayout(lay);
        num.add("see me",p1);
        num.add("nee",p2);
        num.add("kan",p3);
        
        add("Center",num);
    }
    
    public void actionPerformed(ActionEvent e){
        lay.next(num);
    }
    
    
    public static void main(String args[]){
        cardlayout window=new cardlayout();
        window.pack();
        window.show();
        window.setBackground(Color.green);
    }
}
去掉上面的，怎么会报错呢