如何使用Java中的子菜单创建弹出菜单

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

public class PopupMenuExample {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Java Popup Menu Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 300);

        JPanel panel = new JPanel();
        frame.add(panel);

        JPopupMenu popupMenu = new JPopupMenu("Popup Menu");
        JMenuItem item1 = new JMenuItem("Item 1");
        JMenuItem item2 = new JMenuItem("Item 2");
        JMenuItem item3 = new JMenuItem("Item 3");

        popupMenu.add(item1);
        popupMenu.add(item2);
        popupMenu.add(item3);

        JMenu subMenu = new JMenu("Sub Menu");
        JMenuItem subItem1 = new JMenuItem("Sub Item 1");
        JMenuItem subItem2 = new JMenuItem("Sub Item 2");
        JMenuItem subItem3 = new JMenuItem("Sub Item 3");

        subMenu.add(subItem1);
        subMenu.add(subItem2);
        subMenu.add(subItem3);

        popupMenu.add(subMenu);

        panel.addMouseListener(new MouseAdapter() {
            public void mousePressed(MouseEvent e) {
                if (e.isPopupTrigger()) {
                    showMenu(e);
                }
            }

            public void mouseReleased(MouseEvent e) {
                if (e.isPopupTrigger()) {
                    showMenu(e);
                }
            }

            private void showMenu(MouseEvent e) {
                popupMenu.show(e.getComponent(), e.getX(), e.getY());
            }
        });

        frame.setVisible(true);
    }
}