问答题

本题程序的功能是用复选框来控制鼠标右键的弹出菜单是否弹出。窗口中有一个复选框“弹出菜单”，勾选该复选框后，鼠标置于窗口上，右击会弹出一个菜单，单击菜单中的命令后，后台会输出所单击的菜单项。如果取消勾选该复选框，右击就不能弹出菜单。请将下述程序补充完整(注意：不得改动程序的结构，不得增行或删行)。
import java.awt.*;
import java.awt.event.*;
class CanvasWithPopup extends Canvas

______;
CanvasWithPopup(PopupMenu popup)

enableEvents(AWTEvent.MOUSE EVENT MASK);
this.popup = popup;

void addPopup()

add(popup);

void removePopup()

remove(popup);

protected void processMouseEvent(MouseEvent evt)

if (popup.getParent() ! = null ＆＆ evt.isPopupTrigger())

popup.show(evt.getComponent(),evt.getX(),evt.getY());

super.processMouseEvent(evt);


public class advance extends Frame implements ItemListener,ActionListener

Checkbox cb = new Checkbox("弹出菜单",false);
CanvasWithPopup canvas;
advance()

super("advance");
addWindowListener(new WindowAdapter()
public void windowClosing(WindowEvent e)

System.exit(0);

);
add(cb,BorderLayout.NORTH);
cb.addItemListener(this);
PopupMenu popup = new PopupMenu("Button Control");
popup.add("item1");
popup.add("item2");
popup.addActionListener(this);
canvas = new CanvasWithPopup(popup);
add(canvas,BorderLayout.CENTER);
setSize(100,200);
show();

public void itemStateChanged(ItemEvent evt)

______

case ItemEvent.SELECTED:
canvas.addPopup(); break;
case ItemEvent.DESELECTED:
canvas.removePopup(); break;


public void actionPerformed(ActionEvent evt)

______;

static public void main(String[] args)

new advance ();