Partition a Java 8 Stream

It's impossible to partition the arbitrary source stream to the fixed size batches, because this will screw up the parallel processing. When processing in parallel you may not know how many elements in the first sub-task after the split, so you cannot create the partitions for the next sub-task until the first is fully processed.

However it is possible to create the stream of partitions from the random access List. Such feature is available, for example, in my StreamEx library:

List<Type> input = Arrays.asList(...);

Stream<List<Type>> stream = StreamEx.ofSubLists(input, partitionSize);

Or if you really want the stream of streams:

Stream<Stream<Type>> stream = StreamEx.ofSubLists(input, partitionSize).map(List::stream);

If you don't want to depend on third-party libraries, you can implement such ofSubLists method manually:

public static <T> Stream<List<T>> ofSubLists(List<T> source, int length) {
    if (length <= 0)
        throw new IllegalArgumentException("length = " + length);
    int size = source.size();
    if (size <= 0)
        return Stream.empty();
    int fullChunks = (size - 1) / length;
    return IntStream.range(0, fullChunks + 1).mapToObj(
        n -> source.subList(n * length, n == fullChunks ? size : (n + 1) * length));
}

This implementation looks a little bit long, but it takes into account some corner cases like close-to-MAX_VALUE list size.

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If you want parallel-friendly solution for unordered stream (so you don't care which stream elements will be combined in single batch), you may use the collector like this (thanks to @sibnick for inspiration):

public static <T, A, R> Collector<T, ?, R> unorderedBatches(int batchSize, 
                   Collector<List<T>, A, R> downstream) {
    class Acc {
        List<T> cur = new ArrayList<>();
        A acc = downstream.supplier().get();
    }
    BiConsumer<Acc, T> accumulator = (acc, t) -> {
        acc.cur.add(t);
        if(acc.cur.size() == batchSize) {
            downstream.accumulator().accept(acc.acc, acc.cur);
            acc.cur = new ArrayList<>();
        }
    };
    return Collector.of(Acc::new, accumulator,
            (acc1, acc2) -> {
                acc1.acc = downstream.combiner().apply(acc1.acc, acc2.acc);
                for(T t : acc2.cur) accumulator.accept(acc1, t);
                return acc1;
            }, acc -> {
                if(!acc.cur.isEmpty())
                    downstream.accumulator().accept(acc.acc, acc.cur);
                return downstream.finisher().apply(acc.acc);
            }, Collector.Characteristics.UNORDERED);
}

Usage example:

List<List<Integer>> list = IntStream.range(0,20)
                                    .boxed().parallel()
                                    .collect(unorderedBatches(3, Collectors.toList()));

Result:

[[2, 3, 4], [7, 8, 9], [0, 1, 5], [12, 13, 14], [17, 18, 19], [10, 11, 15], [6, 16]]

Such collector is perfectly thread-safe and produces ordered batches for sequential stream.

If you want to apply an intermediate transformation for every batch, you may use the following version:

public static <T, AA, A, B, R> Collector<T, ?, R> unorderedBatches(int batchSize,
        Collector<T, AA, B> batchCollector,
        Collector<B, A, R> downstream) {
    return unorderedBatches(batchSize, 
            Collectors.mapping(list -> list.stream().collect(batchCollector), downstream));
}

For example, this way you can sum the numbers in every batch on the fly:

List<Integer> list = IntStream.range(0,20)
        .boxed().parallel()
        .collect(unorderedBatches(3, Collectors.summingInt(Integer::intValue), 
            Collectors.toList()));

It seem like, as Jon Skeet has shown in his comment, it's not possible to make partitions lazy. For non-lazy partitions, I already have this code:

public static <T> Stream<Stream<T>> partition(Stream<T> source, int size) {
    final Iterator<T> it = source.iterator();
    final Iterator<Stream<T>> partIt = Iterators.transform(Iterators.partition(it, size), List::stream);
    final Iterable<Stream<T>> iterable = () -> partIt;

    return StreamSupport.stream(iterable.spliterator(), false);
}

Provided you want to use the Stream sequentially, it is possible to partition a Stream (as well as perform related functions such as windowing - which I think is what you really want in this case). Two libraries that will support partitoning for standard Streams are cyclops-react (I am the author) and jOOλ which cyclops-react extends (to add functionality such as Windowing).

cyclops-streams has a collection of static functions StreamUtils for operating on Java Streams, and a series of functions such as splitAt, headAndTail, splitBy, partition for partitioning.

To window a Stream into a Stream of nested Streams of size 30 you can use the window method.

To the OPs point, in Streaming terms, splitting a Stream into multiple Streams of a given size is a Windowing operation (rather than a Partitioning operation).

  Stream<Streamable<Integer>> streamOfStreams = StreamUtils.window(stream,30);

There is a Stream extension class called ReactiveSeq that extends jool.Seq and adds Windowing functionality, that may make the code a little cleaner.

  ReactiveSeq<Integer> seq;
  ReactiveSeq<ListX<Integer>> streamOfLists = seq.grouped(30);

As Tagir points out above though, this isn't suitable for parallel Streams. If you want to window or batch a Stream you wish to executed in a multithreaded fashion. LazyFutureStream in cyclops-reactmight be useful (Windowing is on the to-do list, but plain old batching is available now).

In this case data will be passed from the multiple threads executing the Stream to a Multi-Producer/Single-Consumer wait-free Queue and the sequential data from that queue can be windowed before being distributed to threads again.

  Stream<List<Data>> batched = new LazyReact().range(0,1000)
                                              .grouped(30)
                                              .map(this::process);

I found an elegant solution: Iterable parts = Iterables.partition(stream::iterator, size)