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

You can achieve that by using a bounded queue to store elements which flows through the stream (which is basing on the idea which I described in detail here: Is it possible to get next element in the Stream?

Belows example first defines instance of BoundedQueue class which will store elements going through the stream (if you don't like idea of extending the LinkedList, refer to link mentioned above for alternative and more generic approach). Later you just examine the two subsequent elements - thanks to the helper class:

public class Kata {
  public static void main(String[] args) {
    List input = new ArrayList(asList(10, 1, 15, 30, 2, 6));

    class BoundedQueue extends LinkedList {
      public BoundedQueue save(T curElem) {
        if (size() == 2) { // we need to know only two subsequent elements
          pollLast(); // remove last to keep only requested number of elements
        }

        offerFirst(curElem);
        return this;
      }

      public T getPrevious() {
        return (size() < 2) ? null : getLast();
      }

      public T getCurrent() {
        return (size() == 0) ? null : getFirst();
      }
    }

    BoundedQueue streamHistory = new BoundedQueue();

    final List answer = input.stream()
      .map(i -> streamHistory.save(i))
      .filter(e -> e.getPrevious() != null)
      .filter(e -> e.getCurrent() > e.getPrevious())
      .map(e -> e.getPrevious())
      .collect(Collectors.toList());

    answer.forEach(System.out::println);
  }
}