java 卡片布局管理器的一点小疑点

import java.awt.*;import java.awt.event.*;public class CardLayoutDemo extends Frame implements MouseListener{    private Button first=new Button("A");    private Button second=new Button("B");    private Button third=new Button("C");    private CardLayout cl=new CardLayout();    private Panel cards=new Panel();    public CardLayoutDemo(String string){        super(string);        init();    //调用init方法    }        public void init(){        setLayout(new BorderLayout());        setSize(500, 500);                Panel p=new Panel();        p.setLayout(new FlowLayout());                first.addMouseListener(this);        second.addMouseListener(this);        third.addMouseListener(this);                p.add(first);        p.add(second);        p.add(third);                add("North",p);        cards.setLayout(cl);        cards.add("First cards",new Button("AA"));        cards.add("second Button",new Button("BB"));        cards.add("third cards",new Button("CC"));        add("Center",cards);                addWindowListener(new WindowAdapter(){            public void WindowClosing(WindowEvent evt){                setVisible(false);                dispose();                System.exit(0);            }        });                setVisible(true);    }        public static void main(String[] args) {        new CardLayoutDemo("ABCD");   //创建CardLayoutDemo类型变量            }    public void mouseClicked(MouseEvent evt){    if(evt.getSource()==first){    cl.first(cards);}else if(evt.getSource()==second){    cl.first(cards);    cl.next(cards);}else if(evt.getSource()==third){    cl.last(cards);}}    public void mouseEntered(MouseEvent arg0){    }    public void mouseExited(MouseEvent arg0){    }    public void mousePressed(MouseEvent arg0){    }    public void mouseReleased(MouseEvent arg0){    }    }