}

  @Override
  protected SortedMap<String, Integer> makeEmptyMap() {
    return new SimpleForwardingSortedMap<>(
        new TreeMap<String, Integer>(Ordering.<String>natural().nullsFirst()));
  }

  @Override
  protected SortedMap<String, Integer> makePopulatedMap() {
    final SortedMap<String, Integer> sortedMap = makeEmptyMap();
    sortedMap.put("one", 1);
    sortedMap.put("two", 2);

  }

  @Override
  protected SortedMap<String, Integer> makeEmptyMap() {
    return new SimpleForwardingSortedMap<>(
        new TreeMap<String, Integer>(Ordering.<String>natural().nullsFirst()));
  }

  @Override
  protected SortedMap<String, Integer> makePopulatedMap() {
    final SortedMap<String, Integer> sortedMap = makeEmptyMap();
    sortedMap.put("one", 1);
    sortedMap.put("two", 2);