java问题，请大虾们帮忙解决一下，急急！！！

import java.awt.*;
import java.awt.event.*;
import javax.swing.JPopupMenu;
import javax.swing.JMenu;
import javax.swing.JMenuItem;
import javax.swing.JCheckBoxMenuItem;
import javax.swing.JRadioButtonMenuItem;
import javax.swing.ButtonGroup;
import javax.swing.JMenuBar;
import javax.swing.KeyStroke;
import javax.swing.ImageIcon;

import javax.swing.JTextArea;
import javax.swing.JScrollPane;
import javax.swing.JFrame;

/*
* This class adds popup menus to MenuDemo.
*/
public class PopupMenuDemo extends JFrame
implements ActionListener, ItemListener {
JTextArea output;
JScrollPane scrollPane;
String newline = " ";
JPopupMenu popup;

public PopupMenuDemo() {
JMenuBar menuBar;
JMenu menu, submenu;
JMenuItem menuItem;
JRadioButtonMenuItem rbMenuItem;
JCheckBoxMenuItem cbMenuItem;

addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});

//Add regular components to the window, using the default BorderLayout.
Container contentPane = getContentPane();
output = new JTextArea(5, 30);
output.setEditable(false);
scrollPane = new JScrollPane(output);
contentPane.add(scrollPane, BorderLayout.CENTER);

//Create the menu bar.
menuBar = new JMenuBar();
setJMenuBar(menuBar);

//Build the first menu.
menu = new JMenu("A Menu");
menu.setMnemonic(KeyEvent.VK_A);
menu.getAccessibleContext().setAccessibleDescription(
"The only menu in this program that has menu items");
menuBar.add(menu);

//a group of JMenuItems
menuItem = new JMenuItem("A text-only menu item",
KeyEvent.VK_T);
//menuItem.setMnemonic(KeyEvent.VK_T); //used constructor instead
menuItem.setAccelerator(KeyStroke.getKeyStroke(
KeyEvent.VK_1, ActionEvent.ALT_MASK));
menuItem.getAccessibleContext().setAccessibleDescription(
"This doesn"t really do anything");
menuItem.addActionListener(this);
menu.add(menuItem);
menuItem = new JMenuItem("Both text and icon",
new ImageIcon("images/middle.gif"));
menuItem.setMnemonic(KeyEvent.VK_B);
menuItem.addActionListener(this);
menu.add(menuItem);
menuItem = new JMenuItem(new ImageIcon("images/middle.gif"));
menuItem.setMnemonic(KeyEvent.VK_D);
menuItem.addActionListener(this);
menu.add(menuItem);

//a group of radio button menu items
menu.addSeparator();
ButtonGroup group = new ButtonGroup();
rbMenuItem = new JRadioButtonMenuItem("A radio button menu item");
rbMenuItem.setSelected(true);
rbMenuItem.setMnemonic(KeyEvent.VK_R);
group.add(rbMenuItem);
rbMenuItem.addActionListener(this);
menu.add(rbMenuItem);
rbMenuItem = new JRadioButtonMenuItem("Another one");
rbMenuItem.setMnemonic(KeyEvent.VK_O);
group.add(rbMenuItem);
rbMenuItem.addActionListener(this);
menu.add(rbMenuItem);

//a group of check box menu items
menu.addSeparator();
cbMenuItem = new JCheckBoxMenuItem("A check box menu item");
cbMenuItem.setMnemonic(KeyEvent.VK_C);
cbMenuItem.addItemListener(this);
menu.add(cbMenuItem);
cbMenuItem = new JCheckBoxMenuItem("Another one");
cbMenuItem.setMnemonic(KeyEvent.VK_H);
cbMenuItem.addItemListener(this);
menu.add(cbMenuItem);

//a submenu
menu.addSeparator();
submenu = new JMenu("A submenu");
submenu.setMnemonic(KeyEvent.VK_S);

menuItem = new JMenuItem("An item in the submenu");
menuItem.setAccelerator(KeyStroke.getKeyStroke(
KeyEvent.VK_2, ActionEvent.ALT_MASK));
menuItem.addActionListener(this);
submenu.add(menuItem);

menuItem = new JMenuItem("Another item");
menuItem.addActionListener(this);
submenu.add(menuItem);
menu.add(submenu);

//Build second menu in the menu bar.
menu = new JMenu("Another Menu");
menu.setMnemonic(KeyEvent.VK_N);
menu.getAccessibleContext().setAccessibleDescription(
"This menu does nothing");
menuBar.add(menu);

//Create the popup menu.
popup = new JPopupMenu();
menuItem = new JMenuItem("A popup menu item");
menuItem.addActionListener(this);
popup.add(menuItem);
menuItem = new JMenuItem("Another popup menu item");
menuItem.addActionListener(this);
popup.add(menuItem);

//Add listener to components that can bring up popup menus.
MouseListener popupListener = new PopupListener();
output.addMouseListener(popupListener);
scrollPane.addMouseListener(popupListener);
menuBar.addMouseListener(popupListener);
}

public void actionPerformed(ActionEvent e) {
JMenuItem source = (JMenuItem)(e.getSource());
String s = "Action event detected."
+ newline
+ " Event source: " + source.getText()
+ " (an instance of " + getClassName(source) + ")";
output.append(s + newline);
}

public void itemStateChanged(ItemEvent e) {
JMenuItem source = (JMenuItem)(e.getSource());
String s = "Item event detected."
+ newline
+ " Event source: " + source.getText()
+ " (an instance of " + getClassName(source) + ")"
+ newline
+ " New state: "
+ ((e.getStateChange() == ItemEvent.SELECTED) ?
"selected":"unselected");
output.append(s + newline);
}

// Returns just the class name -- no package info.
protected String getClassName(Object o) {
String classString = o.getClass().getName();
int dotIndex = classString.lastIndexOf(".");
return classString.substring(dotIndex+1);
}

public static void main(String[] args) {
PopupMenuDemo window = new PopupMenuDemo();

window.setTitle("PopupMenuDemo");
window.setSize(450, 260);
window.setVisible(true);
}

class PopupListener extends MouseAdapter {
public void mousePressed(MouseEvent e) {
maybeShowPopup(e);
}

public void mouseReleased(MouseEvent e) {
maybeShowPopup(e);
}

private void maybeShowPopup(MouseEvent e) {
if (e.isPopupTrigger()) {
popup.show(e.getComponent(),
e.getX(), e.getY());
}
}
}
}

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