code: public void replaceValues(String key, Object value) {
  //    fields.replaceValues(key, Collections.singletonList(value)); // unnecessarily slow
      List<Object> list = fields.get(key);
      list.clear(); 
      list.add(value);
    }
docs: Removes all values that are associated with the given key, and then associates the given value
  with the given key.