在Java中的CardLayout中显示一个面板

Question

我编写了以下代码，用于显示三个面板，具体取决于点击底部的三个按钮中的哪一个。

import java.awt.BorderLayout;
import java.awt.Button;
import java.awt.CardLayout;
import java.awt.Color;
import java.awt.Frame;
import java.awt.Panel;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

public class cart extends Frame implements ActionListener {

Panel cardPanel;
Panel firstP, secondP, thirdP;
Panel buttonP;

Button b1, b2, b3;

CardLayout cLayout;

cart() {
    cardPanel = new Panel();
    cLayout = new CardLayout();
    cardPanel.setLayout(cLayout);

    firstP = new Panel();
    firstP.setBackground(Color.blue);

    secondP = new Panel();
    secondP.setBackground(Color.red);

    thirdP = new Panel();
    thirdP.setBackground(Color.yellow);

    b1 = new Button("first");
    b2 = new Button("Second");
    b3 = new Button("Third");

    b1.addActionListener(this);
    b2.addActionListener(this);
    b3.addActionListener(this);

    buttonP = new Panel();
    buttonP.add(b1);
    buttonP.add(b2);
    buttonP.add(b3);

    add(buttonP, BorderLayout.SOUTH);
    add(cardPanel, BorderLayout.CENTER);

    cardPanel.add("first", firstP);
    cardPanel.add("second", secondP);
    cardPanel.add("third", thirdP);

    setSize(400, 500);
    setVisible(true);

    addWindowListener(new WindowAdapter() {
        public void WindowClosing(WindowEvent e) {
            System.exit(0);
        }

    });

}// constructor ends here

public void actionPerformed(ActionEvent e) {

    if (e.getSource() == b1) {
        cLayout.show(cardPanel, "firstP");

        System.out.print("first");
    }

    else if (e.getSource() == b2) {
        cLayout.show(cardPanel,"secondP");

        System.out.print("second");
    }

    else if (e.getSource() == b3) {
        cLayout.show(cardPanel, "thirdP");

    }

}

public static void main(String args[]) {
    new cart();

}

 }

处理这三个按钮的方法被调用，并且它们也会打印到终端点击哪个按钮但面板不会改变。它只是可见的第一个按钮。