How to create an infinite stream with Java 8

Question

Is there a easy way to create a infinity stream using java-8 without external libraries?

For example in Scala:

Iterator.iterate(0)(_ + 2)

Answer 1

Yes, there is an easy way:

IntStream.iterate(0, i -> i + 2);

With as usecase:

IntStream.iterate(0, i -> i + 2)
         .limit(100)
         .forEach(System.out::println);

Which prints out 0 to 198 increasing in steps of 2.

The generic method is:

Stream.iterate(T seed, UnaryOperator<T> f);

The latter may be more uncommon in usage.

Answer 2

Here is an example:

PrimitiveIterator.OfInt it = new PrimitiveIterator.OfInt() {
    private int value = 0;

    @Override
    public int nextInt() {
        return value++;
    }

    @Override
    public boolean hasNext() {
        return true;
    }
};

Spliterator.OfInt spliterator = Spliterators.spliteratorUnknownSize(it,
    Spliterator.DISTINCT | Spliterator.IMMUTABLE |
    Spliterator.ORDERED | Spliterator.SORTED);

IntStream stream = StreamSupport.intStream(spliterator, false);

It's a bit verbose, as you see. To print the first 10 elements of this stream:

stream.limit(10).forEach(System.out::println);

You can ofcourse also transform the elements, like you do in your Scala example:

IntStream plusTwoStream = stream.map(n -> n + 2);

Note that there are built-in infinite streams such as java.util.Random.ints() which gives you an infinite stream of random integers.

Answer 3

There is another possible solution in Java 8:

AtomicInteger adder = new AtomicInteger();
IntStream stream = IntStream.generate(() -> adder.getAndAdd(2));

Important: an order of numbers is preserved only if the stream is sequential.

---

It's also worth noting that a new version of the IntStream.iterate has been added since Java 9:

static IntStream iterate​(int seed,
                         IntPredicate hasNext,
                         IntUnaryOperator next);

• seed - the initial element;
• hasNext - a predicate to apply to elements to determine when the stream must terminate;
• next - a function to be applied to the previous element to produce a new element.

Examples:

IntStream stream = IntStream.iterate(0, i -> i >= 0, i -> i + 2);

IntStream.iterate(0, i -> i < 10, i -> i + 2).forEach(System.out::println);