The result of stream().parallel().reduce() is strange [duplicate]
This is how the parallel reducing works. The identity is not guaranteed to be used exactly once like in the sequential processing as long as multiple threads are involved. There is not even guaranteed it would be used exactly n times (size of the list). The output can be even the following which would manifest with a larger list.
A0 A12 A3 A45 A6 A78 A9 A1011 ....