Implementing an abstract, generic Java class with abstract member class in Scala

Question

I'm trying to implement an abstract, generic Java class in Scala, and I'm getting an error of the form

object creation impossible, since method B in class A of type (x$1: A<Concrete1, Concrete2>#C)Unit is not defined

The library classes that I'm implementing look like this:

public abstract class A<T, U> {
    public void B(C);

    public abstract class C {
        // elided
    }
}

and I want to implement an A to hand back to the library (which will call B, supplying a C). In Java, I can do:

new A<Concrete1, Concrete2>() {
    @Override
    public void B(C c) {
        // implementation
    }
}

In Scala, I'm trying

new A[Concrete1, Concrete2] {
    def B(c: C): Unit = {
        // implementation
    }
}

and I get the error message at the top. Using override, the compiler complains that method B overrides nothing. It seems that the Scala type system isn't recognizing the C I pass in as an A<Concrete1, Concrete2>#C, but I'm not sure what type it does think it is.

I've tried specifying self: A[Concrete1, Concrete2] =>, as well as a self-type on C: def B(c: self.C, but neither solves the problem. I also tried def B(c: A[Concrete1, Concrete2].C), but this raises a syntax error.

Any suggestions?

mammothbane · Accepted Answer · 2016-04-22 21:45:09Z

1

Solved: def B(c: A[Concrete1, Concrete2]#C): Unit. Needed the generic type information for this to be a valid override, but I wasn't aware of the # operator.

answered Apr 22, 2016 at 21:45

mammothbane

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