Why is a combiner needed for reduce method that converts type in java 8

Question

I\'m having trouble fully understanding the role that the combiner fulfils in Streams reduce method.

For example, the following code doesn

Answer 1

There is no reduce version that takes two different types without a combiner since it can't be executed in parallel (not sure why this is a requirement). The fact that accumulator must be associative makes this interface pretty much useless since:

list.stream().reduce(identity,
                     accumulator,
                     combiner);

Produces the same results as:

list.stream().map(i -> accumulator(identity, i))
             .reduce(identity,
                     combiner);