如何实现Undo和Redo操作（在线等待）

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.util.LinkedList;
import java.util.Iterator;
import javax.swing.*;
import javax.swing.undo.*;/**
 * A very simple applet demonstrating usage of javax.swing.undo package.
 */
public class UndoTester extends JApplet
{
JButton linebutton;
JButton circlebutton;
JButton undobutton;
JButton redobutton;
JPanel buttonpanel;
JGraphPanel graphpanel;
LineListener linelistener;
CircleListener circlelistener;
UndoListener undolistener;
RedoListener redolistener;
LinkedList shapes;
UndoManager undomgr; public void init()
{ // Force SwingApplet to come up in the System L&F
String laf = UIManager.getSystemLookAndFeelClassName();
try
{
//UIManager.setLookAndFeel(laf);
// If you want the Cross Platform L&F instead, comment out the above line and
// uncomment the following:
UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());
}
catch (UnsupportedLookAndFeelException exc)
{
System.err.println("Warning: UnsupportedLookAndFeel: " + laf);
}
catch (Exception exc)
{
System.err.println("Error loading " + laf + ": " + exc);
} getContentPane().setLayout(new BorderLayout());
linebutton = new JButton("Draw Line");
circlebutton = new JButton("Draw Circle");
undobutton = new JButton("Undo");
redobutton = new JButton("Redo");
graphpanel = new JGraphPanel(false);
graphpanel.setPreferredSize(new Dimension(300, 300));
buttonpanel = new JPanel(false);
buttonpanel.setLayout(new FlowLayout());
buttonpanel.add(linebutton);
buttonpanel.add(circlebutton);
buttonpanel.add(undobutton);
buttonpanel.add(redobutton);
getContentPane().add(buttonpanel, BorderLayout.SOUTH);
getContentPane().add(graphpanel, BorderLayout.NORTH);
linelistener = new LineListener();
linebutton.addActionListener(linelistener);
circlelistener = new CircleListener();
circlebutton.addActionListener(circlelistener);
undolistener = new UndoListener();
undobutton.addActionListener(undolistener);
redolistener = new RedoListener();
redobutton.addActionListener(redolistener);
shapes = new LinkedList();
undomgr = new UndoManager();
} public void stop()
{ }

class JGraphPanel extends JPanel
{
public JGraphPanel(boolean doublebuffer)
{
super(doublebuffer);
}

public void paintComponent(Graphics g)
{
Graphics2D g2 = (Graphics2D) g;
g2.setColor(Color.white);
g2.fill3DRect(0, 0, getWidth(), getHeight(), true);
Iterator it;
Shape shape;
g2.setColor(Color.black);

for (it = shapes.iterator(); it.hasNext();)
{
shape = (Shape) it.next();
g2.draw(shape);
}
}
}

class LineListener implements ActionListener
{
Shape temp;

public void actionPerformed(ActionEvent e)
{
temp =
new Line2D.Double(
0.0,
0.0,
Math.random() * 100.0,
Math.random() * 100.0);
shapes.add(temp);
repaint();
UndoableEdit edit = new graphEdit(temp);
undomgr.addEdit(edit);
}
}

class CircleListener implements ActionListener
{
Shape temp;

public void actionPerformed(ActionEvent e)
{
temp =
new Ellipse2D.Double(
0.0,
0.0,
Math.random() * 100.0,
Math.random() * 100.0);
shapes.add(temp);
repaint();
UndoableEdit edit = new graphEdit(temp);
undomgr.addEdit(edit);
}
}

class UndoListener implements ActionListener
{
public void actionPerformed(ActionEvent e)
{
try
{
undomgr.undo();
}
catch (CannotUndoException ex)
{
System.err.println("Can't Undo More");
}
}
}

class RedoListener implements ActionListener
{
public void actionPerformed(ActionEvent e)
{
try
{
undomgr.redo();
}
catch (CannotRedoException ex)
{
System.err.println("Can't Redo More");
}
}
} class graphEdit extends AbstractUndoableEdit
{
Shape shape;

public graphEdit(Shape _shape)
{
shape = _shape;
}

public void undo()
{
shapes.remove(shape);
repaint();
System.out.println("undo draw line");
}

public void redo()
{
shapes.add(shape);
repaint();
System.out.println("redo draw line");
}
}
}