Java JTextArea redo undo action

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;

import javax.swing.AbstractAction;
import javax.swing.Action;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.KeyStroke;
import javax.swing.event.UndoableEditEvent;
import javax.swing.event.UndoableEditListener;
import javax.swing.text.AbstractDocument;
import javax.swing.undo.CannotRedoException;
import javax.swing.undo.CannotUndoException;
import javax.swing.undo.UndoManager;

public class Main extends JPanel {

  AbstractDocument doc;/*from w  ww  .  j  a  v a2s.  c o  m*/
  protected UndoManager undo = new UndoManager();
  protected UndoAction undoAction;
  protected RedoAction redoAction;

  public Main() {
    setLayout(new BorderLayout(3, 3));

    JTextArea textArea = new JTextArea();
    JScrollPane sp = new JScrollPane(textArea);
    doc = (AbstractDocument) textArea.getDocument();
    undoAction = new UndoAction(undo);
    redoAction = new RedoAction(undo);

    // connect both
    redoAction.setUndoAction(undoAction);
    undoAction.setRedoAction(redoAction);
    doc.addUndoableEditListener(new MyUndoableEditListener());
    add(sp, BorderLayout.CENTER);

  }

  public void apply(final JFrame frame) {
    JMenuBar menuBar = new JMenuBar();

    JMenu editMenu = new JMenu("Edit");

    JMenuItem undoItem = new JMenuItem("Undo", null);
    undoItem.setMnemonic(KeyEvent.VK_Z);
    undoItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_Z, ActionEvent.CTRL_MASK));
    undoItem.setAction(undoAction);
    editMenu.add(undoItem);

    JMenuItem redoItem = new JMenuItem("Redo", null);
    redoItem.setMnemonic(KeyEvent.VK_Y);
    redoItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_Y, ActionEvent.CTRL_MASK));
    redoItem.setAction(redoAction);
    editMenu.add(redoItem);
    menuBar.add(editMenu);

    frame.setJMenuBar(menuBar);
  }

  protected class MyUndoableEditListener implements UndoableEditListener {

    public void undoableEditHappened(UndoableEditEvent e) {
      undo.addEdit(e.getEdit());
      undoAction.updateState();
      redoAction.updateState();
    }
  }

  public static void main(String[] args) {
    final Main c = new Main();
    c.setPreferredSize(new Dimension(433, 312));
    JFrame frame = new JFrame("java2s.com");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    frame.add(c);
    
    c.apply(frame);
    frame.setSize(300, 250);
    frame.setVisible(true);
  }
}

class UndoAction extends AbstractAction {

  private UndoManager undo = null;
  private RedoAction redoAction = null;

  public UndoAction(UndoManager undo) {
    super("Undo");
    putValue(ACCELERATOR_KEY, KeyStroke.getKeyStroke(KeyEvent.VK_Z, ActionEvent.CTRL_MASK));
    putValue(MNEMONIC_KEY, KeyEvent.VK_Z);

    setEnabled(false);
    this.undo = undo;
  }

  public void setRedoAction(RedoAction redoAction) {
    this.redoAction = redoAction;
  }

  public void actionPerformed(ActionEvent e) {
    try {
      undo.undo();
    } catch (CannotUndoException ex) {
      ex.printStackTrace();
    }
    updateState();
    redoAction.updateState();
  }

  protected void updateState() {
    if (undo.canUndo()) {
      setEnabled(true);
      putValue(Action.NAME, undo.getUndoPresentationName());
    } else {
      setEnabled(false);
      putValue(Action.NAME, "Undo");
    }
  }
}

class RedoAction extends AbstractAction {

  private UndoManager undo = null;
  private UndoAction undoAction = null;

  public RedoAction(UndoManager undo) {
    super("Redo");
    putValue(ACCELERATOR_KEY, KeyStroke.getKeyStroke(KeyEvent.VK_Y, ActionEvent.CTRL_MASK));
    putValue(MNEMONIC_KEY, KeyEvent.VK_Y);

    setEnabled(false);
    this.undo = undo;
  }

  public void setUndoAction(UndoAction undoAction) {
    this.undoAction = undoAction;
  }

  public void actionPerformed(ActionEvent e) {
    try {
      undo.redo();
    } catch (CannotRedoException ex) {
      ex.printStackTrace();
    }
    updateState();
    undoAction.updateState();
  }

  protected void updateState() {
    if (undo.canRedo()) {
      setEnabled(true);
      putValue(Action.NAME, undo.getRedoPresentationName());
    } else {
      setEnabled(false);
      putValue(Action.NAME, "Redo");
    }
  }
}