static void onEnterInAllTextFields(JComponent c, O action) {
  if (action == null) ret;
  for (Component tf : allChildren(c))
    onEnterIfTextField(tf, action);
}

static void onEnterInAllTextFields(L c, O action) {
  for (O o : unnull(c))
    if (o instanceof JComponent)
      onEnterInAllTextFields((JComponent) o, action);
}