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 getFirst();
    U ge         


        

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())));
}