Mapping array indices to List with streams

Question

I have a boolean[] and I want to turn into List<Integer> . Each item in the list will be an index of the boolean[] where that index must be true . Right now I'm stuck with this code:

Stream.of(sieve)
        .filter(x->x.equals(true))
        .mapToInt(???)
        .collect(toList());

Any idea on how to accomplish this (with streams of course)?

Answer 1

You have to introduce the indices to your Stream pipeline by using IntStream::range .

boolean[] sieve = sieve();
int[] numbers = IntStream.range(0, sieve.length).filter(i -> sieve[i]).toArray();

Or if you want a List<Integer> as result:

List<Integer> = IntStream.range(0, sieve.length).filter(i -> sieve[i])
                         .boxed().collect(Collectors.toList());