Notice that the accumulator function multiplies the square roots of two elements, but the combiner multiplies the partial results. Thus, the two functions differ. (link)
Understand, of course, that even with a parallel stream, parallelism will be achieved only if the environment supports it.
Once a parallel (link)
By using reduce( ), you can return a value from a stream based on any arbitrary criteria. By definition, all reduction operations are terminal operations (link)