Zipping streams using JDK8 with lambda (java.util.stream.Streams.zip)

In JDK 8 with lambda b93 there was a class java.util.stream.Streams.zip in b93 which could be used to zip streams (this is illustrated in the tutorial Exploring Java8 Lambdas. Part 1 by Dhananjay Nene). This function :

Creates a lazy and sequential combined Stream whose elements are the result of combining the elements of two streams.

However in b98 this has disappeared. Infact the Streams class is not even accessible in java.util.stream in b98.

Has this functionality been moved, and if so how do I zip streams concisely using b98?

The application I have in mind is in this java implementation of Shen, where I replaced the zip functionality in the

• static <T> boolean every(Collection<T> c1, Collection<T> c2, BiPredicate<T, T> pred)
• static <T> T find(Collection<T> c1, Collection<T> c2, BiPredicate<T, T> pred)

functions with rather verbose code (which doesn't use functionality from b98).

I needed this as well so I just took the source code from b93 and put it in a "util" class. I had to modify it slightly to work with the current API.

For reference here's the working code (take it at your own risk...):

public static<A, B, C> Stream<C> zip(Stream<? extends A> a,
                                     Stream<? extends B> b,
                                     BiFunction<? super A, ? super B, ? extends C> zipper) {
    Objects.requireNonNull(zipper);
    Spliterator<? extends A> aSpliterator = Objects.requireNonNull(a).spliterator();
    Spliterator<? extends B> bSpliterator = Objects.requireNonNull(b).spliterator();

    // Zipping looses DISTINCT and SORTED characteristics
    int characteristics = aSpliterator.characteristics() & bSpliterator.characteristics() &
            ~(Spliterator.DISTINCT | Spliterator.SORTED);

    long zipSize = ((characteristics & Spliterator.SIZED) != 0)
            ? Math.min(aSpliterator.getExactSizeIfKnown(), bSpliterator.getExactSizeIfKnown())
            : -1;

    Iterator<A> aIterator = Spliterators.iterator(aSpliterator);
    Iterator<B> bIterator = Spliterators.iterator(bSpliterator);
    Iterator<C> cIterator = new Iterator<C>() {
        @Override
        public boolean hasNext() {
            return aIterator.hasNext() && bIterator.hasNext();
        }

        @Override
        public C next() {
            return zipper.apply(aIterator.next(), bIterator.next());
        }
    };

    Spliterator<C> split = Spliterators.spliterator(cIterator, zipSize, characteristics);
    return (a.isParallel() || b.isParallel())
           ? StreamSupport.stream(split, true)
           : StreamSupport.stream(split, false);
}

zip is one of the functions provided by the protonpack library.

Stream<String> streamA = Stream.of("A", "B", "C");
Stream<String> streamB  = Stream.of("Apple", "Banana", "Carrot", "Doughnut");

List<String> zipped = StreamUtils.zip(streamA,
                                      streamB,
                                      (a, b) -> a + " is for " + b)
                                 .collect(Collectors.toList());

assertThat(zipped,
           contains("A is for Apple", "B is for Banana", "C is for Carrot"));

If you have Guava in your project, you can use the Streams.zip method (was added in Guava 21):

Returns a stream in which each element is the result of passing the corresponding element of each of streamA and streamB to function. The resulting stream will only be as long as the shorter of the two input streams; if one stream is longer, its extra elements will be ignored. The resulting stream is not efficiently splittable. This may harm parallel performance.

 public class Streams {
     ...

     public static <A, B, R> Stream<R> zip(Stream<A> streamA,
             Stream<B> streamB, BiFunction<? super A, ? super B, R> function) {
         ...
     }
 }

Zipping two streams using JDK8 with lambda (gist).

public 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);
}

public static <T> Stream<T> iteratorToFiniteStream(Iterator<T> iterator, boolean parallel) {
    final Iterable<T> iterable = () -> iterator;
    return StreamSupport.stream(iterable.spliterator(), parallel);
}

Since I can't conceive any use of zipping on collections other than indexed ones (Lists) and I am a big fan of simplicity, this would be my solution:

<A,B,C>  Stream<C> zipped(List<A> lista, List<B> listb, BiFunction<A,B,C> zipper){
     int shortestLength = Math.min(lista.size(),listb.size());
     return IntStream.range(0,shortestLength).mapToObj( i -> {
          return zipper.apply(lista.get(i), listb.get(i));
     });        
}