Scala: Can't subclass abstract generic superclass

Question

I am trying to return a subclass of an abstract generic class in Scala but it won't compile. I get Expression of type A doesn't conform to to expected type B. here is the code I am using:

abstract class AA[T](val var1: String){
  def doSomething(): T;
}
class BB(override val var1: String) extends AA[Int](var1){
  override def doSomething(): Int = {
    return 5
  }
}

object Factory {
  def create(v: String) : AA[Any] = {
  return new BB("5") // this is the error
  }
}

What should be the signature of create()?

Thanks

unrelated to your issue, but note that you don't need `return` and don't need semicolons — Seth Tisue, Dec 14 '16 at 15:37

score 1 · Accepted Answer · edited Dec 14 '16 at 15:36

1

BB extends AA[Int] but Factory.create claims to return an AA[Any]. You can fix this by making AA covariant in T:

abstract class AA[+T](val var1: String){
  def doSomething(): T
}

edited Dec 14 '16 at 15:36

Seth Tisue

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answered Dec 14 '16 at 14:57

Lee

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Scala: Can't subclass abstract generic superclass

1 Answers1