code: public static <T, R> Map<R, T> mapBy(Stream<T> stream,
              Function<T, R> classifier) {
          return stream
                  .collect(toMap(classifier, identity(), (o, o2) -> o2));
      }
docs: Map by map.
  
  @param <T>        the type parameter
  @param <R>        the type parameter
  @param stream     the stream
  @param classifier the classifier
  @return the map