Undoable Drawing Panel

[Java Tip]

import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.Container;
import java.awt.Graphics;
import java.awt.Polygon;
import java.awt.event.ActionEvent;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.util.Hashtable;

import javax.swing.AbstractAction;
import javax.swing.Action;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JToolBar;
import javax.swing.event.UndoableEditListener;
import javax.swing.undo.CannotRedoException;
import javax.swing.undo.CannotUndoException;
import javax.swing.undo.StateEdit;
import javax.swing.undo.StateEditable;
import javax.swing.undo.UndoManager;
import javax.swing.undo.UndoableEditSupport;

public class UndoableDrawingPanel2 extends JPanel implements StateEditable {
  private static String POLYGON_KEY = "Polygon";

  UndoableEditSupport undoableEditSupport = new UndoableEditSupport(this);

  Polygon polygon = new Polygon();

  public UndoableDrawingPanel2() {
    MouseListener mouseListener = new MouseAdapter() {
      public void mouseReleased(MouseEvent mouseEvent) {
        StateEdit stateEdit = new StateEdit(UndoableDrawingPanel2.this);
        polygon.addPoint(mouseEvent.getX(), mouseEvent.getY());
        stateEdit.end();
        undoableEditSupport.postEdit(stateEdit);
        repaint();
      }
    };
    addMouseListener(mouseListener);
  }

  public void addUndoableEditListener(
      UndoableEditListener undoableEditListener) {
    undoableEditSupport.addUndoableEditListener(undoableEditListener);
  }

  public void removeUndoableEditListener(
      UndoableEditListener undoableEditListener) {
    undoableEditSupport.removeUndoableEditListener(undoableEditListener);
  }

  public void storeState(Hashtable state) {
    state.put(POLYGON_KEY, getPolygon());
  }

  public void restoreState(Hashtable state) {
    Polygon polygon = (Polygon) state.get(POLYGON_KEY);
    if (polygon != null) {
      setPolygon(polygon);
    }
  }

  public void setPolygon(Polygon newValue) {
    polygon = newValue;
    repaint();
  }

  public Polygon getPolygon() {
    Polygon returnValue;
    if (polygon.npoints == 0) {
      returnValue = new Polygon();
    } else {
      returnValue = new Polygon(polygon.xpoints, polygon.ypoints,
          polygon.npoints);
    }
    return returnValue;
  }

  protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.drawPolygon(polygon);
  }

  public static void main(String args[]) {
    JFrame frame = new JFrame("Drawing Sample2");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    UndoableDrawingPanel2 drawingPanel = new UndoableDrawingPanel2();

    UndoManager manager = new UndoManager();
    drawingPanel.addUndoableEditListener(manager);

    JToolBar toolbar = new JToolBar();
    toolbar.add(UndoManagerHelper.getUndoAction(manager));
    toolbar.add(UndoManagerHelper.getRedoAction(manager));

    Container content = frame.getContentPane();
    content.add(toolbar, BorderLayout.NORTH);
    content.add(drawingPanel, BorderLayout.CENTER);
    frame.setSize(300, 150);
    frame.setVisible(true);
  }
}

class UndoManagerHelper {

  public static Action getUndoAction(UndoManager manager, String label) {
    return new UndoAction(manager, label);
  }

  public static Action getUndoAction(UndoManager manager) {
    return new UndoAction(manager, "Undo");
  }

  public static Action getRedoAction(UndoManager manager, String label) {
    return new RedoAction(manager, label);
  }

  public static Action getRedoAction(UndoManager manager) {
    return new RedoAction(manager, "Redo");
  }

  private abstract static class UndoRedoAction extends AbstractAction {
    UndoManager undoManager = new UndoManager();

    String errorMessage = "Cannot undo";

    String errorTitle = "Undo Problem";

    protected UndoRedoAction(UndoManager manager, String name) {
      super(name);
      undoManager = manager;
    }

    public void setErrorMessage(String newValue) {
      errorMessage = newValue;
    }

    public void setErrorTitle(String newValue) {
      errorTitle = newValue;
    }

    protected void showMessage(Object source) {
      if (source instanceof Component) {
        JOptionPane.showMessageDialog((Component) source, errorMessage,
            errorTitle, JOptionPane.WARNING_MESSAGE);
      } else {
        System.err.println(errorMessage);
      }
    }
  }

  public static class UndoAction extends UndoRedoAction {
    public UndoAction(UndoManager manager, String name) {
      super(manager, name);
      setErrorMessage("Cannot undo");
      setErrorTitle("Undo Problem");
    }

    public void actionPerformed(ActionEvent actionEvent) {
      try {
        undoManager.undo();
      } catch (CannotUndoException cannotUndoException) {
        showMessage(actionEvent.getSource());
      }
    }
  }

  public static class RedoAction extends UndoRedoAction {
    String errorMessage = "Cannot redo";

    String errorTitle = "Redo Problem";

    public RedoAction(UndoManager manager, String name) {
      super(manager, name);
      setErrorMessage("Cannot redo");
      setErrorTitle("Redo Problem");
    }

    public void actionPerformed(ActionEvent actionEvent) {
      try {
        undoManager.redo();
      } catch (CannotRedoException cannotRedoException) {
        showMessage(actionEvent.getSource());
      }
    }
  }
}