各位同仁，一个奇快的问题发生在如下程序中，现请高人指点：
问题是：file菜单可以弹出下拉菜单，但是无法点击下拉菜单中的选项；edit菜单弹不出下拉菜单。本程序编辑、运行无误！
/**
*
*/
/**
* @author yongjie66@126.com
*
*/
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
public class JMenuTest {
/**
* 定义控件
*/
JFrame frm=null;
JTextArea txta=null;
JMenu menuFile=null,menuEdit=null,openmenu=null;
JMenuItem open=null,close=null,exit=null,save=null,newf=null,openOld=null,openNew=null,copy=null,paste=null,cut=null;
public JMenuTest() {
// TODO Auto-generated constructor stub
/*
* 构造函数，初始化
*/
frm=new JFrame( "JMenu Test ");
Container cp=frm.getContentPane();
JMenuBar menuBar=new JMenuBar();
frm.setJMenuBar(menuBar);
menuFile=new JMenu( "file ");
menuEdit=new JMenu( "Edit ");
menuBar.add(menuFile);
menuBar.add(menuEdit);
open=new JMenuItem( "open File ", 'O ');//设置快捷键为 'O '
//open.setMnemonic( 'O ');//设置快捷键为 'O '方法二
open.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_O,InputEvent.ALT_MASK));//设置快捷键‘O’，屏蔽ALT键
open.setIcon(new ImageIcon( "2.gif "));//添加图标
newf=new JMenuItem( "new File ");
newf.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_N,InputEvent.ALT_MASK));
newf.setIcon(new ImageIcon( "4.gif "));
save =new JMenuItem( "save File ");
save.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_S,InputEvent.ALT_MASK));
save.setIcon(new ImageIcon( "6.gif "));
close =new JMenuItem( "close ");
close.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_L,InputEvent.ALT_MASK));
exit=new JMenuItem( "exit ");
exit.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_X,InputEvent.ALT_MASK));
menuFile.add(open);
menuFile.add(newf);
menuFile.add(save);
menuFile.add(close);
menuFile.addSeparator();
menuFile.add(exit);
openmenu=new JMenu();
openOld =new JMenuItem( "openOld ");
openNew =new JMenuItem( "openNew ");
openmenu.add(openOld);
openmenu.add(openNew);
open.add(openmenu);
menuEdit=new JMenu( "Edit ");
copy=new JMenuItem( "copy ");
paste=new JMenuItem( "paste ");
cut=new JMenuItem( "cut ");
menuEdit.add(copy);
menuEdit.add(paste);
menuEdit.add(cut);
/*
* 注册菜单的事件
*/
open.addActionListener(new ItemActionLis());
close.addActionListener(new ItemActionLis());
exit.addActionListener(new ItemActionLis());
save.addActionListener(new ItemActionLis());
newf.addActionListener(new ItemActionLis());
copy.addActionListener(new ItemActionLis());
openOld.addActionListener(new ItemActionLis());
openNew.addActionListener(new ItemActionLis());
paste.addActionListener(new ItemActionLis());
cut.addActionListener(new ItemActionLis());
txta=new JTextArea();
cp.add(txta);
frm.setBounds(300,300,500,400);