static void onEnterInAllTextFields(JComponent c, O action) {
  if (action == null) ret;
  for (JTextField tf : childrenOfType(c, JTextField.class))
    onEnter(tf, action);
}

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