I have tried trapping the invalid entry using either a custom Key listener
or a Document.
The problem is that the data has already been commited to the
JFormattedTextField
before these edits are invoked.  For example, if I enter an invalid
character, the event
does not occur.  If I press the tab key to change focus, the event is then
fired.
Following is the code that I am using to trap the event.

class FolderKeyEvent extends KeyAdapter
   {
    /** Handle the key pressed event from the text field. */
 public void keyPressed(KeyEvent event) {

    int key = event.getKeyCode();
    char textchar = event.getKeyChar();
    String keytext = event.getKeyText(key);
 System.out.println("keytext = " + keytext);
    if (textchar != '0' &&
     textchar != '1' )
    {
     Toolkit.getDefaultToolkit().beep();
        JOptionPane.showMessageDialog(f, "Enter a \"0\" or \"1\" only.");
        event.consume();
        return;

    }

    }
 }
   /**
 * Provides a <Document> object to ensure that only a "0" or "1"
 * are entered
 */

   class FolderDocument extends PlainDocument
   {
    public void insertString(int pos, String text,
              AttributeSet attr) throws BadLocationException
    {
     System.out.println("\"" + text + "\"");
        for (int i=0; i<text.length(); i++)
        {
            if (text.charAt(i) != '0'
             && text.charAt(i) != '1'
             && text.charAt(i) != ' ')

            {
                Toolkit.getDefaultToolkit().beep();
                JOptionPane.showMessageDialog(f, "Enter a \"0\" or \"1\"
only.");
                return;
            }
        }
        System.out.println("text = " + text);
        super.insertString(pos,text,attr);
    }
}

Create an (anonymous) subclass of JFormattedTextField and override
invalidEdit().
<http://java.sun.com/j2se/1.5.0/docs/api/javax/swing/JFormattedTextField.html#inv
alidEdit()>