static JTextField onEnter(JTextField tf, final O action) {
  if (action == null || tf == null) ret tf;
  tf.addActionListener(actionListener(action));
  ret tf;
}

static JButton onEnter(JButton btn, final O action) {
  if (action == null || btn == null) ret btn;
  btn.addActionListener(actionListener(action));
  ret btn;
}

static JList onEnter(JList list, O action) {
  list.addKeyListener(enterKeyListener(action));
  ret list;
}

static JComboBox onEnter(JComboBox cb, O action) {
  cb.addKeyListener(enterKeyListener(action));
  ret cb;
}