Java如何确定是否显示JComboBox菜单？

本示例说明如何创建一个实现，PopupMenuListener以侦听JComboBox菜单可见，不可见或取消时的菜单。

package org.nhooo.example.swing;

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

public class ComboBoxPopupMenuListener extends JFrame {
    public ComboBoxPopupMenuListener() {
        initialize();
    }

    private void initialize() {
        setSize(300, 300);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        setLayout(new FlowLayout(FlowLayout.LEFT));

        Integer[] years = new Integer[] {2005, 2006, 2007, 2008, 2009, 2010};
        JComboBox<Integer> comboBox = new JComboBox<>(years);
        comboBox.setEditable(true);

        // 添加一个PopupMenu侦听器，它将监听通知
        // 来自组合框弹出部分的消息。
        comboBox.addPopupMenuListener(new PopupMenuListener() {
            public void popupMenuWillBecomeVisible(PopupMenuEvent e) {
                // 在弹出菜单变为可见之前调用此方法。
                System.out.println("PopupMenuWillBecomeVisible");
            }

            public void popupMenuWillBecomeInvisible(PopupMenuEvent e) {
                // 在弹出菜单不可见之前调用此方法
                System.out.println("PopupMenuWillBecomeInvisible");
            }

            public void popupMenuCanceled(PopupMenuEvent e) {
                // 取消弹出菜单时调用此方法
                System.out.println("PopupMenuCanceled");
            }
        });

        getContentPane().add(comboBox);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new ComboBoxPopupMenuListener().setVisible(true);
            }
        });
    }
}