code: public static <T, R> Map<R, List<T>> groupBy(Stream<T> stream,
              Function<T, R> classifier) {
          return stream.collect(groupingBy(classifier));
      }
docs: Group by map.
  
  @param <T>        the type parameter
  @param <R>        the type parameter
  @param stream     the stream
  @param classifier the classifier
  @return the map