FULL PRODUCT VERSION :


A DESCRIPTION OF THE PROBLEM :
stream.flatMap(f).peek(c).findAny() does not display the expected short-circuiting behaviour. The consumer c is executed for every element of the stream output by f, when the expectation is it should only be executed once for non-parallel streams. This affects not just peek, but all stream operations.

This is problematic when intermediate operations are side-effectful, as they will be executed more often than expected, and when the flatmap produces an infinite stream, as the stream execution will not terminate.

Further discussion and examples exist at https://stackoverflow.com/questions/29229373/why-filter-after-flatmap-is-not-completely-lazy-in-java-streams


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
    @Test
    public void flatMapStreamIsLazy() {
        AtomicInteger peeks = new AtomicInteger();
        Stream.of(1)
              .flatMap(n -> Stream.of(n, n + 1))
              .peek(n -> peeks.incrementAndGet())
              .findAny();
        assertEquals(1, peeks.get()); // FAILS!
    }
---------- END SOURCE ----------