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

For every index: iterate from zero to that index, get each element, and get the sum
Box the ints to Integers
Collect to a list

IntStream.range(0, list1.size())
    .map(i -> IntStream.rangeClosed(0, i).map(list1::get).sum())
    .boxed()
    .collect(Collectors.toList());

You're adding every number together every time, rather than reusing the previous cumulative result, but streams do not lend themselves to looking at results from previous iterations.

You could write your own collector but at this point, honestly why are you even bothering with streams?

list1.stream()
    .collect(
        Collector.of(
            ArrayList::new,
            (a, b) -> a.add(a.isEmpty() ? b : b + a.get(a.size() - 1)),
            (a, b) -> { throw new UnsupportedOperationException(); }
        )
    );