In Stream reduce method, must the identity always be 0 for sum and 1 for multiplication?

Question

I proceed with java 8 learning.

I have found an interesting behavior:

let\'s see code sample:

// identity value and accumulator and combiner         


        

Answer 1

The JavaDoc documentation for Stream.reduce specifically states that

The identity value must be an identity for the combiner function

1 is not an identity value for the addition operator, which is why you get unexpected results. If you used 0 (which is the addition operator's identity value) then you'd get the same result from serial and parallel streams.