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

I've got two ideas on this topic. First one is getting the indices like in this answer and group it accordingly.

The second idea - if you already got a Stream a custom Collector should be used (similar to the other solutions, though using Deque):

private Collector> squashTuples() {
  return new Collector, List>() {
    @Override
    public Supplier> supplier() {
      return ArrayDeque::new;
    }

    @Override
    public BiConsumer, Tuple> accumulator() {
      return (acc, e) -> {
        Objects.requireNonNull(e);
        if (!acc.isEmpty() && acc.peekLast().getDirection() == e.getDirection()) {
          acc.offerLast(acc.pollLast().merge(e));
        } else {
          acc.offerLast(e);
        }
      };
    }

    @Override
    public BinaryOperator> combiner() {
      return (left, right) -> {
        if (!left.isEmpty() && !right.isEmpty() && left.peekLast().getDirection() == right.peekFirst().getDirection()) {
          left.offerLast(left.pollLast().merge(right.pollFirst()));
        }
        left.addAll(right);
        return left;
      };
    }

    @Override
    public Function, List> finisher() {
      return ArrayList::new;
    }

    @Override
    public Set characteristics() {
      return EnumSet.noneOf(Characteristics.class);
    }
  };
}