Cumulative Sum using Java 8 stream API

Question

I have a List of Integer say list1, and I want to get another list list2 which will contain the cumulative sum up until the current index from start. How can I do this using Str

Answer 1

You can just use Stream.collect() for that:

List list1 = Arrays.asList(1, 2, 3, 4);
List list2 = list1.stream()
        .collect(ArrayList::new, (sums, number) -> {
            if (sums.isEmpty()) {
                sums.add(number);
            } else {
                sums.add(sums.get(sums.size() - 1) + number);
            }
        }, (sums1, sums2) -> {
            if (!sums1.isEmpty()) {
                int sum = sums1.get(sums1.size() - 1);
                sums2.replaceAll(num -> sum + num);
            }
            sums1.addAll(sums2);
        });

This solution also works for parallel streams. Use list1.parallelStream() or list1.stream().parallel() instead of list1.stream().

The result in both cases is: [1, 3, 6, 10]