this.transformer = checkNotNull(transformer);
    }

    Collection<V2> transform(@ParametricNullness K key, Collection<V1> values) {
      Function<? super V1, V2> function = Maps.asValueToValueFunction(transformer, key);
      if (values instanceof List) {
        return Lists.transform((List<V1>) values, function);
      } else {
        return Collections2.transform(values, function);
      }
    }