Is there a way to coalesce repeated numbers in a list using streams in Java 8?

Question

e.g.#1 [1, 1, 1, 2, 22, 35, 35, 120, 320] ==>> [3, 2, 22, 70, 120, 320]

note how repeated consecutive 1\'s and 35\'s are coalesced to 3 and 70

Answer 1

Stream.of(1, 1, 1, 2, 22, 35, 35, 120, 320)
          .collect(Collectors.toMap(
              Function.identity(),
              Function.identity(),
              Integer::sum,
              LinkedHashMap::new
          ))
          .values()
          .forEach(System.out::println);

In the case the you posted a comment, you would need a custom collector, actually:

static class Custom implements Collector, List> {

    private Integer match;

    @Override
    public Supplier> supplier() {
        return ArrayList::new;
    }

    @Override
    public BiConsumer, Integer> accumulator() {
        return (list, x) -> {
            int lastIndex = list.size() - 1;
            if (match != null && match.equals(x)) {
                list.set(lastIndex, list.get(lastIndex) + x);
            } else {
                match = x;
                list.add(x);
            }
        };
    }

    @Override
    public BinaryOperator> combiner() {
        return (left, right) -> {
            throw new RuntimeException("Not for parallel");
        };
    }

    @Override
    public Function, List> finisher() {
        return Function.identity();
    }

    @Override
    public Set characteristics() {
        return Set.of();
    }
}

And usage would be:

public static void main(String[] args) {
    Stream.of(1, 1, 3, 1, 1)
          .collect(new Custom())
          .forEach(System.out::println);
}