Свертка элементов стрима

reduce

Optional<T> reduce(BinaryOperator<T> accumulator)

T reduce(T identity, BinaryOperator<T> accumulator)

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

collect

<R> R collect(Supplier<R> supplier, BiConsumer<R, ? super T> accumulator, BiConsumer<R, R> combiner)

<R,A> R collect(Collector<? super T,A,R> collector)

Collector

Collectors