How to use Java 8 streams to find all values preceding a larger value?

Question

Use Case

Through some coding Katas posted at work, I stumbled on this problem that I\'m not sure how to solve.

Using Java 8 Streams, given a

Answer 1

It's not a one-liner (it's a two-liner), but this works:

List result = new ArrayList<>();
values.stream().reduce((a,b) -> {if (a < b) result.add(a); return b;});

Rather than solving it by "looking at the next element", this solves it by "looking at the previous element, which reduce() give you for free. I have bent its intended usage by injecting a code fragment that populates the list based on the comparison of previous and current elements, then returns the current so the next iteration will see it as its previous element.

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Some test code:

List result = new ArrayList<>();
IntStream.of(10, 1, 15, 30, 2, 6).reduce((a,b) -> {if (a < b) result.add(a); return b;});
System.out.println(result);

Output:

[1, 15, 2]