这就是将正确的方法和常量String暴露给外界，以允许类交换视图本身。例如，给您的第一堂课一个私有的CardLayout字段，称为cardlayout，一个私有的JPanel字段称为cards（持卡人JPanel），以及一些用于将您的Card
JPanels添加到cards容器的公共String常量。还给它一个公共方法，比如所谓的public void swapView(String key)，它允许外部类交换卡…类似这样：

// code corrected
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Registration extends JPanel {
   // use these same constants as button texts later
   private static final Dimension PREF_SIZE = new Dimension(450, 300);
   public static final String USER_AGREEMENT = "User Agreement";
   public static final String USER_INFO = "User Information";
   public static final String ENROLLMENT = "Enrollment";
   // we'll extract them from this array
   public static final String[] KEY_TEXTS = {USER_AGREEMENT, USER_INFO, ENROLLMENT};
   private CardLayout cardlayout = new CardLayout();
   private JPanel cards = new JPanel(cardlayout);

   public Registration() {
      cards.add(createUserAgreePanel(), USER_AGREEMENT);
      cards.add(createUserInfoPanel(), USER_INFO);
      cards.add(createEnrollmentPanel(), ENROLLMENT);
      setLayout(new BorderLayout());
      add(cards, BorderLayout.CENTER);
   }

   @Override
   public Dimension getPreferredSize() {
      return PREF_SIZE;
   }

   private JPanel createEnrollmentPanel() {
      JPanel enrol = new JPanel();
      enrol.add(new JLabel("Enrollment"));
      return enrol;
   }

   private JPanel createUserAgreePanel() {
      JPanel userAgree = new JPanel();
      userAgree.add(new JLabel("User Agreement"));
      return userAgree;
   }

   private JPanel createUserInfoPanel() {
      JPanel userInfo = new JPanel();
      userInfo.add(new JLabel("User Information"));
      return userInfo;
   }

   public void swapView(String key) {
      cardlayout.show(cards, key);
   }

}

然后，外部类可以简单地通过在此类的可视化实例上调用swapView来交换视图，并传入适当的键String（例如，在本例中为CardTest.USER_INFO）以显示用户信息JPanel。

现在，您在这段代码中遇到了问题，我在注释中指出：

    jButtonAgree.addActionListener(new java.awt.event.ActionListener() {
        public void actionPerformed(java.awt.event.ActionEvent e) {
            Registration reg = new Registration(); // **** HERE *****
            LayoutManager cards = reg.getCards().getLayout();
            ((CardLayout) cards).show(reg.getCards(),"step1");
        }
    });

在这一行上，您正在创建一个新的Registration对象，该对象可能与在GUI上可视化的对象完全不相关，因此，对该新对象的调用方法对当前查看的gui绝对没有影响。相反，您可能需要获取对已查看的Registration对象的引用，这可能是通过给此类提供一个getRegistration方法，然后调用其方法，如下所示：

class OutsideClass {
   private Registration registration;
   private JButton jButtonAgree = new JButton("Agree");

   public OutsideClass() {
      jButtonAgree.addActionListener(new ActionListener() {
         public void actionPerformed(ActionEvent e) {
            // make sure registration reference has been obtained first!
            if (registration != null) { 
               registration.swapView(Registration.USER_AGREEMENT);
            }
         }
      });
   }

   // here I allow the calling class to pass a reference to the visualized
   // Registration instance.
   public void setRegistration(Registration registration) {
      this.registration = registration;
   }
}

例如：

@SuppressWarnings("serial")
class ButtonPanel extends JPanel {
   private Registration registration;

   public ButtonPanel() {
      setLayout(new GridLayout(1, 0, 10, 0));
      // go through String array making buttons
      for (final String keyText : Registration.KEY_TEXTS) {
         JButton btn = new JButton(keyText);
         btn.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
               if (registration != null) {
                  registration.swapView(keyText);
               }
            }
         });
         add(btn);
      }
   }

   public void setRegistration(Registration registration) {
      this.registration = registration;
   }
}

和驱动这一切的MainClass

class MainClass extends JPanel {
   public MainClass() {
      Registration registration = new Registration();
      ButtonPanel buttonPanel = new ButtonPanel();
      buttonPanel.setRegistration(registration);

      buttonPanel.setBorder(BorderFactory.createTitledBorder("Button Panel"));
      registration.setBorder(BorderFactory.createTitledBorder("Registration Panel"));

      setLayout(new BorderLayout());
      add(registration, BorderLayout.CENTER);
      add(buttonPanel, BorderLayout.SOUTH);
   }

   private static void createAndShowUI() {
      JFrame frame = new JFrame("Registration");
      frame.getContentPane().add(new MainClass());
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      java.awt.EventQueue.invokeLater(new Runnable() {
         public void run() {
            createAndShowUI();
         }
      });
   }
}