Question

How can I force base methods to take in the same specific subclass instance when overriden by a subclass?

i.e.:

abstract class Animal {
  def mateWith(that: Animal)
}

class Cow extends Animal {
  override def mateWith...?
}

Logically, a Cow should only be able to mateWith another Cow. However, if I do override def mateWith(that: Cow), this doesn’t actually override the base class method (which I want it to, since I want to enforce its existence in the subclass).

I could check to make sure the other instance is of type Cow, and throw an exception if it isn’t – is this my best option? What if I have more animals? I would have to repeat the exception-throwing code.

Answer 1

abstract class Animal[T <: Animal[T]] {
  def mateWith(that: T)
}

class Cow extends Animal[Cow] {
  override def mateWith(that: Cow) { println("cow") }
}

class Dog extends Animal[Dog] {
  override def mateWith(that: Dog) { println("dog") }
}

And use it like this:

scala> (new Cow).mateWith(new Cow)
cow

scala> (new Cow).mateWith(new Dog)
<console>:17: error: type mismatch;
 found   : Dog
 required: Cow
              (new Cow).mateWith(new Dog)
                                 ^

No exception-throwing code needed; the type system handles it for you at compile-time!