How does reduce() method work with parallel streams in Java 8?

Question

I try to understand how reduce() method works exactly with parallel streams and I don\'t understand why the following code do not return the concatenation of th

Answer 1

The problem is you use Stream::reduce for Mutable reduction, which has been Stream::collect specifically designed for. It can be either used for the safe parallel storing. Read about the differences also at official Oracle documentation Streams: Reduction. Unlike the reduce method, which always creates new value when it processes an element, the collect method modifies or mutates an existing value.

Notice the differences between both methods from their JavaDoc:

• reduce(U identity, BiFunction accumulator, BinaryOperator combiner):

  Performs a reduction on the elements of this stream, using the provided identity, accumulation and combining functions.

• collect(Supplier supplier, BiConsumer accumulator, BiConsumer combiner):

  Performs a mutable reduction operation on the elements of this stream.

Therefore, I suggest you do the following:

StringBuilder concat = Arrays.stream(grades)
    .parallel()
    .collect(StringBuilder::new, StringBuilder::append, StringBuilder::append);