Java 8 Stream API - Java 9 Collectors.flatMapping rewritten in Java 8

Question

I got in touch with a new feature since java-9 called Collectors.flatMapping that takes place as a downstream of grouping or partitioning. Such as (example taken from here):

Answer 1

For just this particular case, I guess this would be a simpler version:

Map> map =
        list.stream()
            .collect(Collectors.toMap(
                Collection::size,
                x -> x.stream().filter(y -> y % 2 == 0).collect(Collectors.toList())
            ));

If there would be merging involved (two collections that would have the same size), I would add a merge function that is pretty trivial:

 Map> map =
        list.stream()
            .collect(Collectors.toMap(
                Collection::size,
                x -> x.stream().filter(y -> y % 2 == 0).collect(Collectors.toCollection(ArrayList::new)),
                (left, right) -> {
                    left.addAll(right);
                    return left;
                }
            ));

Otherwise, I agree with Michael in this comment, this is not that hard to back-port to java-8.