How to implement Java method returning a generic type parametrized with array in Scala?

Question

I have an interface with the following method in Java:

<E> Encoder1<E[]> arrayArg(Encoder1<? super E> elemEncoder);

I'm trying to implement it in Scala with the following declaration:

override def arrayArg[E](elemEncoder: Encoder1[_ >: E]): Encoder1[Array[E]]

I get the following error:

Error:(38, 16) overriding method arrayArg in trait Encoders of type [E](elemEncoder: redradishes.encoder.expr.Encoder1[_ >: E])redradishes.encoder.expr.Encoder1[Array[E with Object]];
 method arrayArg has incompatible type
  override def arrayArg[E](elemEncoder: Encoder1[_ >: E]): Encoder1[Array[E]] = ???
               ^

What is wrong with my Scala method signature?

Answer 1

Please see this: https://issues.scala-lang.org/browse/SI-4390

I'm not sure I fully understand why, but you need to specify E with Object in return value for it to compile. The error informs you about that. Probably this is because in java you cannot use primitive types ( AnyVal ) as the generic parameter, but in scala you can so you need to specify that E extends Object to align it with Java rules. Still I'm not sure why this situation is so special.

If you are uncomfortable with Object , E with AnyRef also seem to work.