JAVA弹出式菜单JPopupMenu怎么改才会正确的出？？

public class Demo {

JMenu men;
JPopupMenu pop;
JMenuItem pen, close, exit;
JRadioButtonMenuItem copy, past;
ButtonGroup button;
JFrame frame;

public Demo() {
pop = new JPopupMenu();
button = new ButtonGroup();
men = new JMenu("文件");
pen = new JMenuItem("打开");
close = new JMenuItem("关闭");
men.add(pen);
men.add(close);
pop.add(men);
pop.addSeparator();
copy = new JRadioButtonMenuItem("复制");
past = new JRadioButtonMenuItem("黏贴");
button.add(copy);
button.add(past);
pop.add(copy);
pop.add(past);
pop.addSeparator();
exit = new JMenuItem("退出");
pop.add(exit);
pop.setVisible(true);
JFrame frame = new J