CODEWITHSUNDEEP
scala
laiboonh
laiboonh

Reputation: 1467

Why does this Scala code not compile?

I do not understand why this does not compile. It's complaining that friend which is of Mammal type does not have a talk method. But there is... 

class Animal {
  def talk:String = "i am animal"
}
class Mammal extends Animal {
  override def talk:String = "i am mammal"
}
class Cow extends Mammal {
  override def talk:String = "moo"
}

trait Farm[+A] {
  def set[AA >: A](friend: AA): String
}
class CowFarm extends Farm[Cow] {
  override def set[Mammal >: Cow](friend: Mammal): String = friend.talk
}

Upvotes: 0

Views: 72

Answers (1)

ZhekaKozlov
ZhekaKozlov

Reputation: 39654

In set Mammal is not the same Mammal that you declared a few lines above. It's a local generic type parameter that shadowed your existing Mammal. So your method signature is actually not different to:

override def set[AA >: Cow](friend: AA): String = friend.talk

Since AA is a supertype of Cow, it is not guaranteed to have a talk method. For example, it can be substituted with Any or AnyRef.

Upvotes: 4

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