Create an IntStream and a Stream<Integer> from an Eclipse Collections IntList/IntIterable

Question

I have an Eclipse Collections IntList. How can I

1. Create a Java IntStream from this list
2. Create a Java Stream<Integer> from this list

without copying the elements?

Answer 1

With Eclipse Collections 10.0 you can now call primitiveStream directly on IntList.

IntStream intStream = IntLists.mutable.with(1, 2, 3, 4, 5).primitiveStream();

Stream<Integer> stream = intStream.boxed();

Answer 2

Edit: Holger's comment found a much clearer solution:

public static IntStream intListToIntStream(IntList intList) {
    return IntStream.range(0, intList.size()).map(intList::get);
}

After looking into the IntIterator code, it turns out the implementation is equivalent to this, so the below solutions are unnecessary. You can even make this more efficient using .parallel().

---

If you're on Java 9, you can use this method:

public static IntStream intListToIntStream(IntList intList) {
    IntIterator intIter = intList.intIterator();
    return IntStream.generate(() -> 0)
            .takeWhile(i -> intIter.hasNext())
            .map(i -> intIter.next());
}

Otherwise, I don't see a better solution than wrapping the IntIterator as a PrimitiveIterator.OfInt and building a stream out of that:

public static IntStream intListToIntStream(IntList intList) {
    IntIterator intIter = intList.intIterator();
    return StreamSupport.intStream(Spliterators.spliterator(new PrimitiveIterator.OfInt() {
        @Override
        public boolean hasNext() {
            return intIter.hasNext();
        }
        @Override
        public int nextInt() {
            return intIter.next();
        }
    }, intList.size(), Spliterator.ORDERED), false);
}

Either way, you can just get a Stream<Integer> by calling IntStream.boxed().