3月18日，CardLayoutDemo，每日20行。

import java.awt.*;
public class CardLayoutDemo {

    public static void main(String[] args) {
        Frame f = new Frame("CardLayout Sample");
        CardLayout card = new CardLayout();
        f.setLayout(card);
        f.setSize(400,300);
        f.add(new Button("Button1"),"c1");    //用字符串c1作为本层的索引标识
        f.add(new Button("Button2"),"c2");    //用字符串c2作为本层的索引标识
        f.add(new Button("Button3"),"c3");    //用字符串c3作为本层的索引标识
        card.show(f,"c3");
        f.setVisible(true);
    }

}