Scala Programming Projects

Covariance in collections

Most collection types in the SDK are covariant. For instance, if you open the Vector class, you will see the following:

final class Vector[+A] (...)

This allows us to assign Vector[B] to a variable of Vector[A] if B <:<A type, as shown in the following code:

val cats: Vector[Cat] = Vector(Cat("Max"))val animals: Vector[Animal] = cats

Now here is a bit of magic:

val catsAndDogs = cats :+ Dog("Medor")// catsAndDogs: Vector[Product with Serializable with Animal] = // Vector(Cat(Max), Dog(Medor))

Scala not only allows us to add Dog to Vector[Cat], but it also automatically infers the new collection to be of a Vector[Product with Serializable with Animal] type.

We saw earlier that the parameters of a function are in contravariant ...