属性否决有些问题

import java.awt.*;
import java.awt.event.*;
import java.beans.*;
import java.io.*;
import java.util.*;
import javax.swing.*;
import javax.swing.event.*;public class InternalFrameTest { public static void main(String[] args) {
JFrame frame = new DesktopFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true); }}class DesktopFrame extends JFrame
{
public DesktopFrame()
{
setTitle("InternalFrame");
setSize(DEFAULT_WIDTH,DEFAULT_HEIGHT);

desktop = new JDesktopPane();
add(desktop,BorderLayout.CENTER);

JMenuBar menuBar = new JMenuBar();
setJMenuBar(menuBar);
JMenu fileMenu =new JMenu("File");
menuBar.add(fileMenu);

JMenuItem openItem = new JMenuItem("Open");
openItem.addActionListener(new 
   ActionListener()
             {
            public void actionPerformed(ActionEvent event)
            {
             createInternalFrame (new JLabel(new ImageIcon(planets[counter]+".gif")),planets[counter]);
             counter= (counter+1)%planets.length;
            }
             });
fileMenu.add(openItem);

JMenuItem exitItem = new JMenuItem("Exit");
exitItem.addActionListener( new 
   ActionListener()
             {
                 public void actionPerformed(ActionEvent event)
                 {
                  System.exit(0);
                 }
             });
fileMenu.add(exitItem);

JMenu windowMenu = new JMenu("Window");
menuBar.add(windowMenu);
JMenuItem nextItem = new JMenuItem("Next");
nextItem.addActionListener( new 
    ActionListener()
              {
            public void actionPerformed(ActionEvent event)
            {
             selectNextWindow();
            }
              });
windowMenu.add(nextItem);

JMenuItem cascadeItem = new JMenuItem ("Cascade");
cascadeItem.addActionListener( new 
       ActionListener()
               {
                  public void actionPerformed(ActionEvent event)
                  {
                   cascadeWindows();
                  }
               });
windowMenu.add(cascadeItem);

JMenuItem tileItem = new JMenuItem("Tile");
tileItem.addActionListener(new
     ActionListener()
              {
              public void actionPerformed(ActionEvent event)
              {
               tileWindows();
              }
              });
windowMenu.add(tileItem);

final JCheckBoxMenuItem dragOutlineItem = new JCheckBoxMenuItem("Drage Outline");
dragOutlineItem.addActionListener(new 
          ActionListener()
                   {
                   public void actionPerformed (ActionEvent event )
                   {
                    desktop.setDragMode(dragOutlineItem.isSelected()? 
                    JDesktopPane.OUTLINE_DRAG_MODE:JDesktopPane.LIVE_DRAG_MODE);
                    
                   }
                   });
windowMenu.add(dragOutlineItem);
}
public void createInternalFrame(Component c,String t)
{
final JInternalFrame iframe = new JInternalFrame(t,
                                         true,
                                                 true,
                                                 true,
                                                 true);
iframe.add(c,BorderLayout.CENTER);
desktop.add(iframe);

iframe.setFrameIcon(new ImageIcon("document.gif"));

iframe.addVetoableChangeListener( new 
       VetoableChangeListener()
                {
               public void vetoableChange(PropertyChangeEvent event)
               {
                String name = event.getPropertyName();
                Object value = event.getNewValue();
                
                if(name.equals("closed")&&value.equals(true))
                {
                int result = JOptionPane.showInternalConfirmDialog(
                 iframe,"OK to close?","Select an Option",JOptionPane.YES_NO_OPTION);
                if(result!=JOptionPane.YES_OPTION)
                throw new java.beans.PropertyVetoException("User canceled close",event);
                }
               }
                });
int width= desktop.getWidth()/2;
int height = desktop.getHeight()/2;
iframe.reshape(nextFrameX, nextFrameY, width, height);
iframe.show();

try
{
iframe.setSelected(true);
}
catch(PropertyVetoException e)
{

}
frameDistance = iframe.getHeight()-iframe.getContentPane().getHeight();

nextFrameX+=frameDistance;
nextFrameY+=frameDistance;
if((nextFrameX+width)>desktop.getWidth()) nextFrameX=0;
if((nextFrameY+height)>desktop.getHeight()) nextFrameY=0;
}

public void cascadeWindows()
{
int x=0;
int y=0;
int width = desktop.getWidth()/2;
int height = desktop.getHeight()/2;

for(JInternalFrame frame: desktop.getAllFrames())
{
if(!frame.isIcon())
{
try
{
frame.setMaximum(false);
frame.reshape(x, y, width, height);

x+=frameDistance;
y+=frameDistance;

if(x+width>desktop.getWidth()) x=0;
if(y+height>desktop.getHeight()) y=0;
}
catch(PropertyVetoException e)
{}
}
}
}

public void tileWindows()
{
int frameCount =0;
for(JInternalFrame frame: desktop.getAllFrames())
if(!frame.isIcon()) frameCount++;
if(frameCount==0) return;

int rows = (int) Math.sqrt(frameCount);
int cols = frameCount/rows;
int extra = frameCount%rows;

int width = desktop.getWidth()/cols;
int height = desktop.getHeight()/rows;
int r=0;
int c=0;

for(JInternalFrame frame: desktop.getAllFrames())
{
if(!frame.isIcon())
{
try
{
frame.setMaximum(false);
frame.reshape(c*width, r*height, width, height);
r++;
if(r==rows)
{
r=0;
c++;
if(c==cols-extra)
{
rows++;
height=desktop.getHeight()/rows;
}
}
}
catch(PropertyVetoException e)
{}
}
}
}
public  void selectNextWindow()
{
JInternalFrame[] frames = desktop.getAllFrames();
for(int i =0;i<frames.length;i++)
{
if(frames[i].isSelected())
{
int next = (i+1)%frames.length;
while(next!=i)
{
if(!frames[next].isIcon())
{
try
{
frames[next].setSelected(true);
frames[next].toFront();
frames[next].toBack();
return;
}
catch(PropertyVetoException e)
{}
}
next=(next+1)&frames.length;
}
}
}
  }

private JDesktopPane desktop;
private int nextFrameX;
private int nextFrameY;
private int frameDistance;
private int counter;
private static final String[] planets=
                                         {
                                        "Mercury",
                                        "Venus",
                                        "Earth",
                                        "Mars",
                                        "Jupiter",
                                        "Saturn",
                                        "Uranus",
                                        "Neptune",
                                        "Pluto"
                                         };
private static final int DEFAULT_WIDTH=300;
private static final int DEFAULT_HEIGHT =200;
}Exception in thread "AWT-EventQueue-0" java.lang.Error: Unresolved compilation problem: 
Unhandled exception type PropertyVetoException
我查看了，PropertyVetoException 是属于java.beans.*;里面类，为什么还是要报错了？