Wsrtjtyk

 Stream<String> a = Stream.of("one", "three", "five");

 Stream<String> b = Stream.of("two", "four", "six");

What do I need to do for the output to be the below?

// one

// two

// three

// four

// five

// six

I looked into concat but as the javadoc explains, it just appends one after the other, it does not interleave / intersperse.

Stream<String> out = Stream.concat(a, b);

out.forEach(System.out::println);

Creates a lazily concatenated stream whose elements are all the
elements of the first stream followed by all the elements of the
second stream.

Wrongly gives

 // one

 // three

 // five

 // two

 // four

 // six

Could do it if I collected them and iterated, but was hoping for something more Java8-y, Streamy :-)

Note

I don't want to zip the streams

“zip” operation will take an element from each collection and combine them.

the result of a zip operation would be something like this: (unwanted)

 // onetwo

 // threefour

 // fivesix

I’d use something like this:

public static <T> Stream<T> interleave(Stream<T> a, Stream<T> b) {

    Spliterator<T> spA = a.spliterator(), spB = b.spliterator();

    long s = spA.estimateSize() + spB.estimateSize();

    if(s < 0) s = Long.MAX_VALUE;

    int ch = spA.characteristics() & spB.characteristics()

           & (Spliterator.NONNULL|Spliterator.SIZED);

    ch |= Spliterator.ORDERED;



    return StreamSupport.stream(new Spliterators.AbstractSpliterator<T>(s, ch) {

        Spliterator<T> sp1 = spA, sp2 = spB;



        @Override

        public boolean tryAdvance(Consumer<? super T> action) {

            Spliterator<T> sp = sp1;

            if(sp.tryAdvance(action)) {

                sp1 = sp2;

                sp2 = sp;

                return true;

            }

            return sp2.tryAdvance(action);

        }

    }, false);

}

It retains the characteristics of the input streams as far as possible, which allows certain optimizations (e.g. for count()and toArray()). Further, it adds the ORDERED even when the input streams might be unordered, to reflect the interleaving.

When one stream has more elements than the other, the remaining elements will appear at the end.

A much dumber solution than Holger did, but may be it would fit your requirements:

private static <T> Stream<T> interleave(Stream<T> left, Stream<T> right) {

    Spliterator<T> splLeft = left.spliterator();

    Spliterator<T> splRight = right.spliterator();



    T single = (T) new Object[1];



    Stream.Builder<T> builder = Stream.builder();



    while (splRight.tryAdvance(x -> single[0] = x) && splLeft.tryAdvance(builder)) {

        builder.add(single[0]);

    }



    return builder.build();

}

As you can see from the question comments, I gave this a go using zip:

Stream<String> a = Stream.of("one", "three", "five");

Stream<String> b = Stream.of("two", "four", "six");



Stream<String> out = interleave(a, b);





    public static <T> Stream<T> interleave(Stream<T> streamA, Stream<T> streamB) {

        return zip(streamA, streamB, (o1, o2) -> Stream.of(o1, o2)).flatMap(s -> s);

    }



    /**

    * https://stackoverflow.com/questions/17640754/zipping-streams-using-jdk8-with-lambda-java-util-stream-streams-zip

    **/

    private static <A, B, C> Stream<C> zip(Stream<A> streamA, Stream<B> streamB, BiFunction<A, B, C> zipper) {

        final Iterator<A> iteratorA = streamA.iterator();

        final Iterator<B> iteratorB = streamB.iterator();

        final Iterator<C> iteratorC = new Iterator<C>() {

            @Override

            public boolean hasNext() {

                return iteratorA.hasNext() && iteratorB.hasNext();

            }



            @Override

            public C next() {

                return zipper.apply(iteratorA.next(), iteratorB.next());

            }

        };

        final boolean parallel = streamA.isParallel() || streamB.isParallel();

        return iteratorToFiniteStream(iteratorC, parallel);

    }



    private static <T> Stream<T> iteratorToFiniteStream(Iterator<T> iterator, boolean parallel) {

        final Iterable<T> iterable = () -> iterator;

        return StreamSupport.stream(iterable.spliterator(), parallel);

    }

This may not be a good answer because

(1) it collects to map, which you don't want to do I guess and

(2) it is not completely stateless as it uses AtomicIntegers.

Still adding it because

(1) it is readable and

(2) community can get an idea from this and try to improve it.

Stream<String> a = Stream.of("one", "three", "five");

Stream<String> b = Stream.of("two", "four", "six");



AtomicInteger i = new AtomicInteger(0);

AtomicInteger j = new AtomicInteger(1);



Stream.of(a.collect(Collectors.toMap(o -> i.addAndGet(2), Function.identity())),

        b.collect(Collectors.toMap(o -> j.addAndGet(2), Function.identity())))

        .flatMap(m -> m.entrySet().stream())

        .sorted(Comparator.comparing(Map.Entry::getKey))

        .forEach(e -> System.out.println(e.getValue())); // or collect

Output

one

two

three

four

five

six

@Holger's edit

Stream.concat(a.map(o -> new AbstractMap.SimpleEntry<>(i.addAndGet(2), o)),

        b.map(o -> new AbstractMap.SimpleEntry<>(j.addAndGet(2), o)))

        .sorted(Map.Entry.comparingByKey())

        .forEach(e -> System.out.println(e.getValue())); // or collect