Why is using reduce on a parallel stream producing a larger value than on a sequential stream?

Question

Can someone tell me why this is happening and if it's expected behaviour or a bug

List<Integer> a = Arrays.asList(1,1,3,3);

a.parallelStream().filter(Objects::nonNull)
        .filter(value -> value > 2)
        .reduce(1,Integer::sum)

Answer: 10

But if we use stream instead of parallelStream I'm getting the right & expected answer 7

Answer 1

The first argument to reduce is called "identity" and not "initialValue".

1 is no identity according to addition. 1 is identity for multiplication.

Though you need to provide 0 if you want to sum the elements.

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Java uses "identity" instead of "initialValue" because this little trick allows to parallelize reduce easily.

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In parallel execution, each thread will run the reduce on a part of the stream, and when the threads are done, they will be combined using the very same reduce function.

Though it will look something like this:

mainThread:
  start thread1;
  start thread2;
  wait till both are finished;

thread1:
  return sum(1, 3); // your reduce function applied to a part of the stream

thread2:
  return sum(1, 3);

// when thread1 and thread2 are finished:
mainThread:
  return sum(sum(1, resultOfThread1), sum(1, resultOfThread2));
  = sum(sum(1, 4), sum(1, 4))
  = sum(5, 5)
  = 10

I hope you can see, what happens and why the result is not what you expected.