Stream stateful computation: cumulative sums

Question

Assuming I have a Java IntStream, is it possible to convert it to an IntStream with cumulative sums? For example, a stream starting with [4, 2, 6, ...] should be converted t

Answer 1

It's possible to do with a collector that then creates a new stream:

class Accumulator {
    public static void accept(List list, Integer value) {
        list.add(value + (list.isEmpty() ? 0 : list.get(list.size() - 1)));
    }

    public static List combine(List list1, List list2) {
        int total = list1.get(list1.size() - 1);
        list2.stream().map(n -> n + total).forEach(list1::add);
        return list1;
    }
}

This is used as:

myIntStream.parallel()
    .collect(ArrayList::new, Accumulator::accept, Accumulator::combine)
    .stream();

Hopefully you can see that the important attribute of this collector is that even if the stream is parallel as the Accumulator instances are combined it adjusts the totals.

This is obviously not as efficient as a map operation because it collects the whole stream and then produces a new stream. But that's not just an implementation detail: it's a necessary function of the fact that streams are intended to be potentially concurrently processed.

I have tested it with IntStream.range(0, 10000).parallel() and it functions correctly.