Stream of boolean values, is any true?

Question

I want to parallelize the following code snipped using a parallelStream:

boolean anyTrue() {
  for (Element e : setOfE) {
    if (eval(e)) {
      return true;
    }
  }
  return false;
}

Will the following work on parallel streams and use regular short-circuit evaluation?

setOfE.parallelStream().map(e -> eval(e)).reduce(false, (a,b) -> a || b))

Answer 1

Streams API actually has first-class support for your requirement:

setOfE.parallelStream().anyMatch(e -> eval(e));

As opposed to your approach with reduce, this is guaranteed to have short-circuit evaluation and optimally leverage parallelism.

Answer 2

No, reduction does not support short-circuit evaluation. The reason is that reduce just receives an arbitrary BinaryOperator implementation and has no idea about the possibilities of short-circuiting the particular operation.

But you can perform the entire operation much simpler:

setOfE.parallelStream().filter(e -> eval(e)).findAny().isPresent()

This simply searches for an arbitrary item for which eval returns true and findAny allows to end the operation as soon as one thread has encountered a match. The resulting Optional can be queried for being empty as you are not interested in the particular matching Element.

Alternatively you can use as suggested by Marko Topolnik’s comment:

setOfE.parallelStream().anyMatch(e -> eval(e))