Java 8 Streams – collect vs reduce

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reduce is a “fold” operation, it applies a binary operator to each element in the stream where the first argument to the operator is the return value of the previous application and the second argument is the current stream element.

collect is an aggregation operation where a “collection” is created and each element is “added” to that collection. Collections in different parts of the stream are then added together.

The document you linked gives the reason for having two different approaches:

If we wanted to take a stream of strings and concatenate them into a
single long string, we could achieve this with ordinary reduction:

 String concatenated = strings.reduce("", String::concat)

We would get the desired result, and it would even work in parallel.
However, we might not be happy about the performance! Such an
implementation would do a great deal of string copying, and the run
time would be O(n^2) in the number of characters. A more performant
approach would be to accumulate the results into a StringBuilder,
which is a mutable container for accumulating strings. We can use the
same technique to parallelize mutable reduction as we do with ordinary
reduction.

So the point is that the parallelisation is the same in both cases but in the reduce case we apply the function to the stream elements themselves. In the collect case we apply the function to a mutable container.

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