undo/redo

I’m trying to put undo/redo facility to JEditorPne using the example code I found here.
http://javaalmanac.com/egs/javax.swing.undo/UndoText.html


However it does not do the job and it seems to be that the if condition in actionPerformed() always seems to return false.

if (undo.canUndo()) { //always return false;
                        undo.undo();
                    }

Can any one tell me what correction needs to be done in order it to work?
Here is my code
  JEditorPane editor = new JEditorPane();

          //undo redo         
           final UndoManager undo = new UndoManager();
    Document doc = editor.getDocument();
   
    // Listen for undo and redo events
    doc.addUndoableEditListener(new UndoableEditListener() {
        public void undoableEditHappened(UndoableEditEvent evt) {
            undo.addEdit(evt.getEdit());
        }
    });
   
    // Create an undo action and add it to the text component
    editor.getActionMap().put("Undo",
        new AbstractAction("Undo") {
            public void actionPerformed(ActionEvent evt) {
                try {
                             System.out.println("undo()");
                  //  if (undo.canUndo()) {
                        undo.undo();
                       
                             
                 
                 //   }
                } catch (CannotUndoException e) {
                      e.printStackTrace();
                }
            }
       });
       
          // Bind the undo action to ctl-Z
    editor.getInputMap().put(KeyStroke.getKeyStroke(KeyEvent.VK_Z, InputEvent.CTRL_MASK), "Undo");
   
    // Create a redo action and add it to the text component
    editor.getActionMap().put("Redo",
        new AbstractAction("Redo") {
            public void actionPerformed(ActionEvent evt) {
                try {
                          
                        System.out.println("redo()");
                //    if (undo.canRedo()) {
                        undo.redo();
                       
                   
                 //   }
                } catch (CannotRedoException e) {
                      e.printStackTrace();
                }
            }
        });

   
// Bind the redo action to ctl-Y
    editor.getInputMap().put(KeyStroke.getKeyStroke(KeyEvent.VK_Y, InputEvent.CTRL_MASK), "Redo");