Question

这是我从Oracle的Java Tutorial网站获得的示例演示程序。最初方法itemStateChanged()的实现方式如下：

public void itemStateChanged(ItemEvent evt) {
    CardLayout cl = (CardLayout)(cards.getLayout());
    cl.show(cards, (String)evt.getItem());
}

我把它更改为：

public void itemStateChanged(ItemEvent evt) {
    CardLayout cl = (CardLayout)(cards.getLayout());
    cl.next(cards);
}

根据JavaDoc API，next（）方法应该翻转面板，但它没有发生。这是完整的代码：

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class CardLayoutDemo implements ItemListener {
    JPanel cards; //a panel that uses CardLayout
    final static String BUTTONPANEL = "Card with JButtons";
    final static String TEXTPANEL = "Card with JTextField";

    public void addComponentToPane(Container pane) {
        //Put the JComboBox in a JPanel to get a nicer look.
        JPanel comboBoxPane = new JPanel(); //use FlowLayout
        String comboBoxItems[] = { BUTTONPANEL, TEXTPANEL };
        JComboBox cb = new JComboBox(comboBoxItems);
        cb.setEditable(false);
        cb.addItemListener(this);
        comboBoxPane.add(cb);

        //Create the "cards".
        JPanel card1 = new JPanel();
        card1.add(new JButton("Button 1"));
        card1.add(new JButton("Button 2"));
        card1.add(new JButton("Button 3"));

        JPanel card2 = new JPanel();
        card2.add(new JTextField("TextField", 20));

        //Create the panel that contains the "cards".
        CardLayout cl = new CardLayout();
        cards = new JPanel(cl);
        cards.add(card1, BUTTONPANEL);
        cards.add(card2, TEXTPANEL);
        cl.show(cards, BUTTONPANEL);

        pane.add(comboBoxPane, BorderLayout.PAGE_START);
        pane.add(cards, BorderLayout.CENTER);
    }

    public void itemStateChanged(ItemEvent evt) {
        CardLayout cl = (CardLayout)(cards.getLayout());
        cl.next(cards);
    }

    /**
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event dispatch thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("CardLayoutDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Create and set up the content pane.
        CardLayoutDemo demo = new CardLayoutDemo();
        demo.addComponentToPane(frame.getContentPane());

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        /* Use an appropriate Look and Feel */
        try {
            //UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
            UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel");
        } catch (UnsupportedLookAndFeelException ex) {
            ex.printStackTrace();
        } catch (IllegalAccessException ex) {
            ex.printStackTrace();
        } catch (InstantiationException ex) {
            ex.printStackTrace();
        } catch (ClassNotFoundException ex) {
            ex.printStackTrace();
        }
        /* Turn off metal's use of bold fonts */
        UIManager.put("swing.boldMetal", Boolean.FALSE);

        //Schedule a job for the event dispatch thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

原始代码在此处：CardLayoutDemo.java

Answer 1

您的itemStateChanged事件实际上已被调用两次（这是正常行为），因此布局管理器将从您当前的窗格切换到下一个，然后切换到下一个（原始版本）。

您应该使用ActionListener代替。

调用next（）时，CardLayout不切换面板

1 个答案: