A DESCRIPTION OF THE PROBLEM :
The accumulator in the following Stream methods
<R> R collect(Supplier<R> supplier,
              BiConsumer<R,? super T> accumulator,
              BiConsumer<R,R> combiner)

and
<U> U reduce(U identity,
             BiFunction<U,? super T,U> accumulator,
             BinaryOperator<U> combiner)

are marked as associative in the javadoc description . But they cannot be associative as their input parameters are of different type (or at least they can be).
This is a little bit confusing.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
accumulator - an non-interfering, stateless function for incorporating an additional element into a result
ACTUAL -
accumulator - an associative, non-interfering, stateless function for incorporating an additional element into a result

URL OF FAULTY DOCUMENTATION :
http://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html