How to interleave (merge) two Java 8 Streams?

Question

 Stream a = Stream.of(\"one\", \"three\", \"five\");
 Stream b = Stream.of(\"two\", \"four\", \"six\");

What do I need

Answer 1

I’d use something like this:

public static  Stream interleave(Stream a, Stream b) {
    Spliterator 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(s, ch) {
        Spliterator sp1 = spA, sp2 = spB;

        @Override
        public boolean tryAdvance(Consumer action) {
            Spliterator 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.