Using Scala to Override a Java method that uses "? extends"

Question

So I have a Java method:

public abstract List<Class<? extends MyClass>> getListOfClasses();

and I need to override it in Scala. Here is how I am currently doing it:

override def getListOfClasses: java.util.List[Class[_ <: MyClass[_]]] = { null }

However, this does not compile, and I get the error: method getListOfClasses has incompatible type

What am I doing wrong?

EDIT: MyClass is defined in Java like this:

abstract class MyClass[T] extends MyOtherClass[T] {...}

FINAL EDIT: As suggested by @AlexeyRomanov below, changing my Java method return type fixed the problem:

public abstract List<Class<? extends MyClass<?>>> getListOfClasses();

Shot in the dark, but do you need to add a pair of parens `()` to the end of the Scala `def`? — Alec, Dec 15 '16 at 01:09
For me it is compiling or am I doing something wrong. Take a look on this project: https://github.com/paweln1986/SOFIssue — PawelN, Dec 15 '16 at 06:45
The Java signature is already problematic, since it uses a raw type. Can you try changing it to `List>>` (and Scala signature as in the question)? — Alexey Romanov, Dec 15 '16 at 14:46
See http://stackoverflow.com/questions/2770321/what-is-a-raw-type-and-why-shouldnt-we-use-it for more on raw types. — Alexey Romanov, Dec 15 '16 at 18:22

score 0 · Answer 1 · answered Dec 15 '16 at 11:16

It's ugly but asInstanceOf works. This ugliness comes from Java's inability to express covariance and contra-variance.

Interface:

import java.util.List;
public abstract class Game2J {
  public interface MyClass {}

  public abstract List<Class<? extends MyClass>> getListOfClasses();
}

Scala code:

import java.util

import Game2J.MyClass
import scala.collection.JavaConverters._

class Game2S extends Game2J {
  class MyClass1 extends MyClass
  class MyClass2 extends MyClass

  override def getListOfClasses: util.List[Class[_ <: MyClass]] =
    List(classOf[MyClass1], classOf[MyClass2]).asJava.asInstanceOf[util.List[Class[_ <: MyClass]]]
}

Working example:

public class Game2 {
  public static void main(String[] args) {
    new Game2S().getListOfClasses().stream().forEach((cl) -> System.out.println(cl.getSimpleName()));
  }
}

Using Scala to Override a Java method that uses "? extends"

1 Answers1