Group sequences of values

Question

I'm wondering if there's in any nifty way to use the new Stream APIs to "group" sequences of values.

e.g. split a series of integers, into groups of integers where each group is an ascending number sequence:

IntStream seq = IntStream.of(1, 2, 3, -1, -1, 1, 2, 1, 2);
IntFunction next = i -> i + 1;

// DESIRED OUTPUT: [[1,2,3], [-1], [-1], [1,2], [1,2]]

Answer 1

Unfortunately, the Stream API is not very well suited to tackle problems that involve dependant operations on the Stream element, like this one.

However, you can use the StreamEx library for this:

public static void main(String[] args) {
    IntStream seq = IntStream.of(1, 2, 3, -1, -1, 1, 2, 1, 2);
    IntUnaryOperator next = i -> i + 1;

    List<List<Integer>> result = 
        IntStreamEx.of(seq).boxed().groupRuns((i1, i2) -> next.applyAsInt(i1) == i2).toList();

    System.out.println(result); // prints "[[1, 2, 3], [-1], [-1], [1, 2], [1, 2]]"
}

This groups into a List all consecutive integers where the second one is equal to the next function applied to the first one. Finally, this Stream is collected into a List.