CODEWITHSUNDEEP
scala
chenhry
chenhry

Reputation: 526

Scala - why fail to override superclass's method

  class A
  class B extends A

  class D { def get: A = ??? }
  class E extends D { override def get: B = ??? } // OK

  class F { def set(b: B): Unit = ??? }
  class G extends F { override def set(a: A): Unit = ??? } // Compile Error, override nothing

my question is why G doesn't work given that: (A=>Unit) is subtype of (B=>Unit)

implicitly[(A => Unit) <:< (B => Unit)]

Upvotes: 3

Views: 82

Answers (1)

rsenna
rsenna

Reputation: 11963

The reason is simple: the JVM doesn't allow overrides based on contravariance over parameter types. It only allows overrides based on covariance of return type.

Take a look here for some discussion about the implications of that on Scala.

From Wikipedia:

Similarly, it is type safe to allow an overriding method to accept a more general argument than the method in the base class:

Ex:

class AnimalShelter {
    void putAnimal(Animal animal) {
        ...
    }
}

class CatShelter extends AnimalShelter {
    void putAnimal(Object animal) {
        ...
    }
}

Not many object oriented languages actually allow this — C++ and Java [and also Scala] would interpret this as an unrelated method with an overloaded name.

Upvotes: 5

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