Undo/Redo in JTextArea using UndoableEditListener

Question

I tried to implement undo/redo functionality in JTextArea using UndoableEditListener
interface, but it throws java.lang.NullPointerException. I really not sure
how the UndoableEditListener works?? I mean when the user type text in JTextArea,
it already listens the event?? But when I go to menu bar and select "undo item,"
how can they communicate??
Here's my attempts. Thanks a lot!!
private UndoManager undoManager;
UndoableEditSupport edit = new UndoableEditSupport();
//Registers an UndoableEditListener.
edit.addUndoableEditListener(this);
public void undoableEditHappened(UndoableEditEvent evt)
{     undoManager = (UndoManager)evt.getEdit();
     //AbstractUndoableEdit edit = evt.getEdit();
if (undoManager.canRedo())
     System.out.println("can redo...");
if (undoManager.canUndo())
     System.out.println("can undo...");
public void actionPerformed(ActionEvent evt)
Object source = evt.getSource();
     if (source == undoItem)
          String s = undoManager.getUndoPresentationName();
          System.out.println("undo item = " + s);
}

Answer 1

Hi
Sinceraly never used it in JTextArea components but since it also is an JTextComponent (as is JTextField) try to use this method:
    * add UNDO&REDO commands to a given textfield
  private void addUndoRedo(JTextField tf) {
    final UndoManager undo = new UndoManager();
    Document doc = tf.getDocument();
    doc.addUndoableEditListener(
      new UndoableEditListener() {
        public void undoableEditHappened(UndoableEditEvent evt) {
          undo.addEdit(evt.getEdit());
    tf.getActionMap().put(EDIT_CONTROLS[6][0], // UNDO
      new AbstractAction(EDIT_CONTROLS[6][0]) {
        public void actionPerformed(ActionEvent evt) {
          try {
            if (undo.canUndo()) { undo.undo(); }
          } catch (CannotUndoException e) {}
    tf.getInputMap().put(KeyStroke.getKeyStroke(EDIT_CONTROLS[6][1]), EDIT_CONTROLS[6][0]);
    tf.getActionMap().put(EDIT_CONTROLS[7][0], // REDO
      new AbstractAction(EDIT_CONTROLS[7][0]) {
        public void actionPerformed(ActionEvent evt) {
          try {
            if (undo.canRedo()) { undo.redo(); }
          } catch (CannotRedoException e) {}
    tf.getInputMap().put(KeyStroke.getKeyStroke(EDIT_CONTROLS[7][1]), EDIT_CONTROLS[7][0]);
  }I believe it'll work fine, please reply as possible.
Regards.