code: public static <T, R> Map<R, T> mapBy(Collection<T> collection,
              Function<T, R> classifier) {
          return mapBy(collection.stream(), classifier);
      }
docs: Map by map.
  
  @param <T>        the type parameter
  @param <R>        the type parameter
  @param collection the collection
  @param classifier the classifier
  @return the map