KeyTyped Event: funny behaviour

Question

This should be a very simple question... but I am stuck with it.
I want to implement the usual feature of enabling a button only if a text field is not empty. I suppose you all have seen this before.
I am catching the KeyTyped event and in the event handler I am checking for the length of the text. This is the code inside the handler:
String txt=myTextField.getText();
myButton.setEnabled(txt.length()>0);
So yes, it works, but with a funny behaviour: the button is enabled only after the second keystroke, that is, when the length of the text is >=2 and not >0. A quick explanation is that my event handler is being called before the event handler of the textfield itself so at that moment the text is still empty. But when pressing the "backspace" key... it works fine, so the theory is wrong!!
My workaround is to catch the KeyReleased event... but visual effect is not so satisfactory, and anyway, why is this happening?
And what is the "official" way of implementing this trick?
Thanks in advance,
Luis.
(P.S.: I am using a JTextField and a JButton, and have tested this with JDK 1.2 and 1.4 with the same results.)

Answer 1

You'd want to do something like this:
JTextField watchedField = new JTextField();
// construct the action
FieldWatcherAction watcher = new FieldWatcherAction( watchedField );
// make a button with the action
JButton b = new JButton( watcher );
// add a button to a toolbar with the action
jtoolBar.add( watcher );
// etc...
class FieldWatcherAction extends AbstractAction implements DocumentListener
  JTextField  field;
  public FieldWatcherAction( JTextField f )
    super( "", someIcon );
    setEnabled( false );
    field = f;
    field.getDocument().addDocumentListener( this );
  protected void updateState()
    setEnabled( field.getDocument().getLength() > 0 );
  public void insertUpdate( DocumentEvent evt )
    updateState();
  public void removeUpdate( DocumentEvent evt )
    updateState().;
  public void changedUpdate( DocumentEvent evt )
    updateState();
}The action would need to be fleshed out more, obviously. You'd probably want to add text for a label ( empty string in super call in constructor ), define the icon ( second arg in super call ). You can also set tooltip text and other things. All of this can be changed by editing various properties of the action. Relevant constants would be:
javax.swing.Action.ACCELERATOR_KEY  // key accelerator for JMenuItems
javax.swing.Action.MNEMONIC_KEY // mnemonic key for JMenuItems
javax.swing.Action.NAME // JButton text or JMenuItem text
javax.swing.Action.SHORT_DESCRIPTION // Tooltip text
javax.swing.Action.SMALL_ICON // image icon for JButton, JMenuItem and JToolBar buttons.