Java compilation fail with method reference but works with lambda

Question

We're trying to add an overloaded collect method to our extended Java Stream API which we're currently defining as:

interface ExtendedStream<R> extends Stream<R> {

   <R1> R1 collect(SerializableSupplier<Collector<? super R, ?, R1>> supplier);

}

SerializableSupplier is define as just:

interface SerializableSupplier<T> extends Serializable, Supplier<T> {
}

Calling this collect method with a lambda works fine, but calling it with a method reference fails to compile with errors such as:

Error:(50, 72) java: incompatible types: cannot infer type-variable(s) R1,capture#1 of ?,T
    (argument mismatch; bad return type in method reference
      java.util.stream.Collector<T,capture#1 of ?,java.util.List<T>> cannot be converted to java.util.stream.Collector<? super java.util.Map.Entry<java.lang.Integer,java.lang.String>,?,R1>)

I've created a standalone Java class here which you can load into your compiler to try it out.

I'm currently using Java version:

java version "1.8.0_144"
Java(TM) SE Runtime Environment (build 1.8.0_144-b01)
Java HotSpot(TM) 64-Bit Server VM (build 25.144-b01, mixed mode)

Is this is a bug or some error in how we're defining things?

NOTE: I get this compilation error both from IntelliJ and when compiling with Maven. I don't use Eclipse.

Cheers, Galder

Answer 1

This is a bug in javac 8 as it compiles just fine with javac-9 (with the latest build) - I just can't find it and I really really tried to.

Almost as usual adding more type information fixes such things... Also notice that I've changed your Comparator to Comparator.comparingInt...:

 return entrySet.stream().sorted(
            Comparator.comparingInt(Entry::getKey)).collect(
                    Collectors::<Map.Entry<Integer, String>> toList);