Java stream reduce

Question

I have the following example data set that I want to transform / reduce using Java stream api based on direction\'s value

Direction    int[]
IN           1, 2
O         


        

Answer 1

This is an alternative approach that uses slightly different data structures.

If this is an option, changing from int[] to List allows for more flexibility (not to mention avoiding creating/copying arrays multiple times):

class Tuple {
    Direction direction;
    List data;
}

And the following function does the merging on a Deque collection:

private static List next(Deque t, Direction d) {
    if (!t.isEmpty() && t.peekLast().getDirection() == d) {
        return t.peekLast().getData();
    } else {
        Tuple next = new Tuple();
        next.direction = d;
        next.data = new ArrayList<>();
        t.addLast(next);
        return next.data;
    }
}

And with that, the stream can look as simple as:

Deque deq = new LinkedList<>(); //the final collection of tuples

tuples.stream()
.flatMap(tp -> tp.getData().stream()
                 .map(d -> Pair.of(tp.getDirection(), Integer.valueOf(d))))
.forEach(el -> next(deq, el.getLeft()).add(el.getRight()));