Stream.flatMap reference ambiguity

[David Hartveld]

Hi all,

I just ran into the following problem with flatMap: the compiler
(lambda-b78) is not able to infer the correct overload for flatMap.

The code:

List<String> ss = new ArrayList<>();
ss.stream().flatMap((x, sink) -> {
for (String s : x.split(",")) {
 sink.accept(s);
}
});

The compiler output:

FlatMapTest.java:[38,28] reference to flatMap is ambiguous
both method flatMap(java.util.stream.FlatMapper.ToLong<? super T>) in
java.util.stream.Stream and method
flatMap(java.util.stream.FlatMapper.ToDouble<? super T>) in
java.util.stream.Stream match
FlatMapTest.java:[40,37] method accept in interface
java.util.function.LongConsumer cannot be applied to given types;
required: long
found: java.lang.String
reason: argument mismatch; java.lang.String cannot be converted to long

This can be solved by typing the lambda parameters (actually, exactly that
is done in the default implementation of Stream.flatMap(Function<T,
Stream<? extends R>>).

Is this a bug/unimplemented inference situation? If not, I would prefer a
rename of the flatMap overloads that return a primitive stream (these cause
the ambiguity, right?). I think it is confusing that, when using the Stream
API (or any other coherent API), you have to specify types in some lambdas,
and not in others - I find this confusing, and it probably is even more so
for less advanced programmers.

Thanks in advance for a great-looking update to the JDK!

Regards,
David Hartveld