code: protected int getListenerIndex (ChangeListener listener)
      {
          if (_listeners == null) {
              return -1;
          }
          for (int ii = 0, ll = _listeners.length; ii < ll; ii++) {
              Object olistener = _listeners[ii];
              if (olistener == listener || (olistener instanceof WeakReference<?> &&
                      ((WeakReference<?>)olistener).get() == listener)) {
                  return ii;
              }
          }
          return -1;
      }
docs: Returns the index of the identified listener, or -1 if not found.