Java 8 collections streaming - Convert list to Set, transforming result

Question

Imaging object with following method:

class A { List<B> getIds(){...} }

Now I have an Collection of A as input; And I want to get set of unique Ids out of it, normally you would go for:

Set<B> ids = new HashSet<>();
for(A a : input){
  ids.addAll(a.getIds());
}

Is there a way to do the same in one line using stream API, like following

Set<List<B>> set = input.stream().map((a) -> a.getIds()).collect(Collectors.toSet());

but making flat set of B

Why is question marked as a duplicate? I know the question and answers are very similar, but a Set and List are not the same thing. — Kraagenskul, Jan 11 '18 at 15:03

score 9 · Accepted Answer · answered Jun 12 '17 at 12:52

9

You have to use flatMap

input.stream()
    .map(a -> a.getIds())
    .flatMap(ids -> ids.stream())
    .collect(Collectors.toSet());

This will produce flat Set.

answered Jun 12 '17 at 12:52

ByeBye

2

You can also use method reference: `.map(A::getIds).flatMap(List::stream)` – Stefan Warminski Jun 12 '17 at 12:54
Just a single step `.flatMap(a -> a.getIds().stream())`… – Holger Jun 14 '17 at 13:37
It is better to use four `map` then one `map` with `map(a -> a.b().c().d().e())` – ByeBye Jun 14 '17 at 13:38

1 Answers1