Java 8 stream are lazy. Or not?

README.md

I've always learned that Java 8 streams are lazy. As such, I would expect that:

Optional<Integer> intOpt = Stream
  .of(1, 2, 3)
  .flatMap(i -> Stream.of(4, 5, 6))
  .filter(i -> {System.out.println(i); return true;})
  .findAny();

applies the filter only once. However, the above program outputs:

4
5
6

and the value of intOpt is an Optional[4]. If the filter performs some expensive function, then the above code will perform this expensive function more times than necessary.

Streams are not so lazy after all!

A quick Google search shows that I'm not the only one surprised by this semantics.