我在用卡片布局显示下一张卡片时遇到了问题，我在这里阅读了文档和各种线程。代码是“工作”，但如上所述，不显示下一张卡。这三张卡分别工作，但通过next（）方法访问时不可见
代码如下：

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.JButton;
import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class Start {

public static void main(String[] args) {
    new Start().createFrame();
}

public void createFrame() {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            new Card().createMainGui();
        }
    });
}

class Card1 extends Card {

    public Card1() {
        createGui();
    }

    private void createGui() {
        /*
         * Create buttons and add them to the Card1 Panel
         */

        final JButton button = new JButton("Next");
        button.addActionListener(this);

        add(button);
    }
}

class Card2 extends Card {

    public Card2() {
        createGui();
    }

    private void createGui() {
        /*
         * Create label and add it to the Card2 Panel
         */

        JLabel label = new JLabel("Card2");
        add(label);
    }
}

class Card extends JPanel implements ActionListener {

    JFrame frame;
    final JPanel cards = new JPanel(new CardLayout(20, 20));

    void createMainGui() {
        /*
         * create main frame
         */
        frame = new JFrame("Testframe");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        /*
         * add the different cards to the Card Panel (Card Layout)
         */

        Card1 card1 = new Card1();
        cards.add(card1, "card1");

        Card2 card2 = new Card2();
        cards.add(card2, "card2");

        // Card3 card3 = new Card3();
        // cards.add(card3, "card3");

        /*
         * add the Card panel to the frame, pack it and make it visible
         */
        frame.add(cards, BorderLayout.CENTER);
        frame.pack();
        frame.setVisible(true);

    }

    @Override
    public void actionPerformed(ActionEvent e) {
        CardLayout cl = (CardLayout) cards.getLayout();
        cl.next(cards);
    }
}
}