When collecting elements from a stream it is not possible for the stream pipeline to inform the collecting supplier of known size when requesting a new mutable result container.

For example, if the exact size of the stream pipeline is known and results are collected concurrently into a concurrent hash map, then that map can be pre-sized thus ensuring concurrent updates will likely be more efficient as there is less chance of resizes occurring.

The new method could be:

    default IntFunction<A> sizedSupplier() {
        return i -> supplier().get();
    }

A -1 can be passed to the sized supplying function if the exact size is not known.