@Override
  public @Nullable K lowerKey(@ParametricNullness K key) {
    return Maps.keyOrNull(lowerEntry(key));
  }

  @Override
  public @Nullable K floorKey(@ParametricNullness K key) {
    return Maps.keyOrNull(floorEntry(key));
  }

  @Override
  public @Nullable K ceilingKey(@ParametricNullness K key) {
    return Maps.keyOrNull(ceilingEntry(key));
  }

  @Override