Passing a collection using a reduce (3 parameters) function - streams java 8

Question

I am trying to calculate the multiplication of a value using the previous two values using java 8\'s stream. I want to call a function that will return an array/list/collect

Answer 1

Just for completeness, here is a solution which does not need an additional class.

List output = Stream.iterate(
    (ToIntFunction)f -> f.applyAsInt(1, 2),
    prev -> f -> prev.applyAsInt((a, b) -> f.applyAsInt(b, a*b) )
)
.limit(9).map(pair -> pair.applyAsInt((a, b)->a))
.collect(Collectors.toList());

This is a functional approach which doesn’t need an intermediate value storage. However, since Java is not a functional programming language and doesn’t have optimizations for such a recursive function definition, this is not recommended for larger streams.

Since for this example a larger stream would overflow numerically anyway and the calculation is cheap, this approach works. But for other use cases you will surely prefer a storage object when solving such a problem with plain Java (as in Stuart Marks’ answer)