Java 8 generate stream of integer based on last value

Question

I need to generate stream of integers that each value is based on the value before according to some math function.

For example - lets say I want to take last number and add 10: [1, 11, 21, 31, 41, ...]

of course the real function is much more complex.

I tried taking the fibonaci example but couldn't make it work:

Stream.iterate(new long[]{ 1, 1 }, p->new long[]{ p[1], p[0]+p[1] })
      .limit(92).forEach(p->System.out.println(p[0]));

I can only start at 1.

This is what I tried doing:

Stream.iterate(new long[]{ 1 }, p-> {p[0], p[0] + 10})
.limit(4).forEach(p->System.out.println(p[0]));

Answer 1

According to Stream#iterate method docs:

Returns an infinite sequential ordered Stream produced by iterative application of a function f to an initial element seed, producing a Stream consisting of seed, f(seed), f(f(seed)), etc.

The first element (position 0) in the Stream will be the provided seed. For n > 0, the element at position n, will be the result of applying the function f to the element at position n - 1.

So, for your example, it should work as follows:

Stream.iterate(1L, x -> x + 10L)
    .limit(4)
    .forEach(System.out::println); // 1 11 21 31

If your function is too complex, you can abstract it to a method:

private long complexFunction(long value) {
    return <very_complex_calculation with value>;
}

long N = 4L;

Stream.iterate(1L, this::complexFunction)
    .limit(N)
    .forEach(System.out::println);

Answer 2

You can use an AtomicLong to keep another variable when iterating. For the Fibonacci sequence where you would keep the largest of the 2 numbers and in the AtomicLong and the iteration variable would be the smallest. E.g.

AtomicLong fibonacci = new AtomicLong(1);
Stream.iterate(1L, x -> fibonacci.getAndAdd(x))
    .limit(10)
    .forEach(i -> System.out.println(fibonacci.get()));