java - CardLayout 无法正常工作

Question

我用 Splitpane、Combobox 和其他一些包含按钮和标签的面板编写了这个简单的 Cardlayout 示例。

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.Container;
import java.awt.EventQueue;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;

import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JSplitPane;
import javax.swing.JTextField;
import javax.swing.border.EmptyBorder;

public class splitpane_test extends JFrame implements ItemListener {

    private JPanel contentPane;
    final static String BUTTONPANEL = "Card with JButtons";
    final static String TEXTPANEL = "Card with JTextField";
    JPanel cards;

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            public void run() {
                try {
                    splitpane_test frame = new splitpane_test();
                    //frame.addComponentToPane(frame.getContentPane());
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    public splitpane_test() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 450, 300);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        contentPane.setLayout(new BorderLayout(0, 0));
        setContentPane(contentPane);

        JSplitPane splitPane = new JSplitPane();
        contentPane.add(splitPane, BorderLayout.CENTER);


        JPanel comboBoxPane = new JPanel();
        String comboBoxItems[] = {BUTTONPANEL, TEXTPANEL};
        JComboBox cb = new JComboBox(comboBoxItems);
        cb.setEditable(false);
        cb.addItemListener(this);
        comboBoxPane.add(cb);
        splitPane.setLeftComponent(comboBoxPane);

        //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".
        cards = new JPanel();
        cards.add(card1, BUTTONPANEL);
        cards.add(card2, TEXTPANEL);
        splitPane.setRightComponent(cards);
        cards.setLayout(new CardLayout(0, 0));
    }

    @Override
    public void itemStateChanged(ItemEvent e) {
        // TODO Auto-generated method stub
        System.out.print("Event Triggered \n");
        CardLayout cl = (CardLayout) (cards.getLayout());
        cl.show(cards, TEXTPANEL);
    }
}

我可以在左侧看到带有组合框的拆分 Pane ，在右侧看到其他卡片布局面板。 当我更改组合框项目时，没有任何事情发生在正确的大小上。 为了验证我是否击中了卡片，我使用了 System.out.print("事件触发\n"); 但令我惊讶的是，它为每个组合框项目显示两次，就像它调用两次一样

Event Triggered
Event Triggered

能否请您告诉我我在这里做错了什么以及为什么事件触发被击中两次。 感谢您的宝贵时间和帮助。

Answer 1

Can you please suggest me what I am doing wrong here and why event triggered is getting hit twice.

如果查看 ItemEvent，您会看到一个项目被DESELECTED，另一个被SELECTED。相反，监听 ActionEvent，如 here 所示，并相应地选择正确的卡片。

附录:如果您在@Michael Brewer-Davis 的 answer 中实现有用的更改, 那么合适的 ActionListener 就特别简单了:

@Override
public void actionPerformed(ActionEvent e) {
    JComboBox jcb = (JComboBox) e.getSource();
    CardLayout cl = (CardLayout) cards.getLayout();
    cl.show(cards, jcb.getSelectedItem().toString());
}