Under the current implementation, flatmap
is eager; like any other stateful intermediate operation (like sorted
and distinct
). And it’s very easy to prove :
int result = Stream.of(1)
.flatMap(x -> Stream.generate(() -> ThreadLocalRandom.current().nextInt()))
.findFirst()
.get();
System.out.println(result);
This never finishes as flatMap
is computed eagerly. For your example:
urls.stream()
.flatMap(url -> fetchDataFromInternet(url).stream())
.filter(...)
.findFirst()
.get();
It means that for each url
, the flatMap
will block all others operation that come after it, even if you care about a single one. So let’s suppose that from a single url
your fetchDataFromInternet(url)
generates 10_000
lines, well your findFirst
will have to wait for all 10_000 to be computed, even if you care about only one.
EDIT
This is fixed in Java 10, where we get our laziness back: see JDK-8075939
EDIT 2
This is fixed in Java 8 too (8u222): JDK-8225328