Map values in Collectors.groupingBy()

Question

For the sake of this example, let's assume I have a simple type Tuple with two attributes:

interface Tuple<T, U> {
    T getFirst();
    U getSecond();
}

Now I want to transform a collection of (first, second) tuples into a map which maps each first value to a set of all second values contained in tuples with that specific first value. The method groupSecondByFirst() shows a possible implementation doing what I want:

<T, U> Map<T, Set<U>> groupSecondByFirst(Set<Tuple<T, U>> tuples) {
    Map<T, Set<U>> result = new HashMap<>();

    for (Tuple<T, U> i : tuples) {
        result.computeIfAbsent(i.getFirst(), x -> new HashSet<>()).add(i.getSecond());
    }

    return result;
}

If the input was [(1, "one"), (1, "eins"), (1, "uno"), (2, "two"), (3, "three")] the output would be { 1 = ["one", "eins", "uno"], 2 = ["two"], 3 = ["three"] }

I would like to know whether and how I can implement this using the streams framework. The best I got is the following expression, which returns a map which contains the full tuple as values and not just their second elements:

Map<T, Set<Tuple<T, U>>> collect = tuples.stream().collect(
    Collectors.groupingBy(Tuple::getFirst, Collectors.toSet()));

Answer 1

I found a solution; It involves Collections.mapping(), which can wrap a collector and apply mapping function over stream to supply elements to the wrapped collector:

static <T, U> Map<T, Set<U>> groupSecondByFirst(Collection<Tuple<T, U>> tuples) {
    return tuples
        .stream()
        .collect(
            Collectors.groupingBy(
                Tuple::getFirst,
                Collectors.mapping(
                    Tuple::getSecond,
                    Collectors.toSet())));
}