问如何在JMenuBar中动态添加JMenus

EN

import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;

public class MenuBarProblem {

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setSize(new Dimension(300, 400));

        JMenu menu1 = new JMenu("First");
        JMenuItem item = new JMenuItem("Add menu");
        menu1.add(item);
        final JMenuBar mb = new JMenuBar();
        mb.add(menu1);
        frame.setJMenuBar(mb);
        item.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent arg0) {
                JMenu menu1 = new JMenu("First");
                JMenuItem item = new JMenuItem("Add menu");
                menu1.add(item);
                mb.add(menu1);
                System.out.println(mb.getMenuCount());
            }
        });
        frame.setVisible(true);
    }
}