java - java中的子菜单问题

Question

除了子菜单之外，我的代码工作正常。 “导入”按钮应该扩展到新闻源、书签和邮件。然而，该程序甚至不显示“导入”。它显示无法悬停的第一个子菜单条目(“新闻源”)。我做错了什么？

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

public class test extends JFrame{

private static final long serialVersionUID = 1L;

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

public test()
{
    initGUI();
}

public final void initGUI()
{
    JMenuBar menubar = new JMenuBar();
    ImageIcon exitIcon = new ImageIcon("icons/exit.png");
    ImageIcon openIcon = new ImageIcon("icons/open.png");
    ImageIcon newIcon = new ImageIcon("icons/new.png");
    ImageIcon saveIcon = new ImageIcon("icons/save.png");

    JMenu file = new JMenu("File");

    JMenuItem importMenu = new JMenuItem("Import");
    importMenu.setMnemonic(KeyEvent.VK_M);

    JMenuItem newsfeedMenu = new JMenuItem("Import newsfeed list...");
    JMenuItem bookmarksMenu = new JMenuItem("Import bookmarks...");
    JMenuItem mailMenu = new JMenuItem("Import mail...");

    importMenu.add(newsfeedMenu);
    importMenu.add(bookmarksMenu);
    importMenu.add(mailMenu);

    JMenuItem newMenu = new JMenuItem("New", newIcon);
    newMenu.setMnemonic(KeyEvent.VK_N);
    newMenu.setToolTipText("Start new document");

    JMenuItem openMenu = new JMenuItem("Open", openIcon);
    openMenu.setMnemonic(KeyEvent.VK_O);
    openMenu.setToolTipText("Open document");

    JMenuItem saveMenu = new JMenuItem("Save", saveIcon);
    saveMenu.setMnemonic(KeyEvent.VK_S);
    saveMenu.setToolTipText("Save document");

    JMenuItem exitMenu = new JMenuItem("Exit", exitIcon);
    exitMenu.setMnemonic(KeyEvent.VK_X);
    exitMenu.setToolTipText("Exit application");
    exitMenu.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent event){
            System.exit(0);
        }
    });
    exitMenu.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_W, 
            KeyEvent.CTRL_MASK));

    file.add(newMenu);
    file.add(openMenu);
    file.add(saveMenu);
    file.addSeparator();
    file.add(importMenu);
    file.addSeparator();
    file.add(exitMenu);

    menubar.add(file);
    setJMenuBar(menubar);

    JPanel panel = new JPanel();
    getContentPane().add(panel);
    panel.setLayout(null);

    JButton button = new JButton("Quit");
    button.setBounds(100,60,80,40);
    button.setToolTipText("Press");
    button.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event){
            System.exit(0);
        }
    });

    panel.add(button);

    setSize(300, 200);
    setTitle("testGUI");
    setLocationRelativeTo(null);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
}

}

Answer 1

将其更改为 JMenu

JMenu importMenu = new JMenu("Import");