upper-type-bounds.md

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In Scala, type parameters and abstract type members may be constrained by a type bound. Such type bounds limit the concrete values of the type variables and possibly reveal more information about the members of such types. An upper type bound T <: A declares that type variable T refers to a subtype of type A. Here is an example that demonstrates upper type bound for a type parameter of class PetContainer:

{% tabs upper-type-bounds class=tabs-scala-version %} {% tab 'Scala 2' for=upper-type-bounds %}

abstract class Animal {
 def name: String
}

abstract class Pet extends Animal {}

class Cat extends Pet {
  override def name: String = "Cat"
}

class Dog extends Pet {
  override def name: String = "Dog"
}

class Lion extends Animal {
  override def name: String = "Lion"
}

class PetContainer[P <: Pet](p: P) {
  def pet: P = p
}

val dogContainer = new PetContainer[Dog](new Dog)
val catContainer = new PetContainer[Cat](new Cat)

{% endtab %} {% tab 'Scala 3' for=upper-type-bounds %}

abstract class Animal:
  def name: String

abstract class Pet extends Animal

class Cat extends Pet:
  override def name: String = "Cat"

class Dog extends Pet:
  override def name: String = "Dog"

class Lion extends Animal:
  override def name: String = "Lion"

class PetContainer[P <: Pet](p: P):
  def pet: P = p

val dogContainer = PetContainer[Dog](Dog())
val catContainer = PetContainer[Cat](Cat())

{% endtab %} {% endtabs %}

{% tabs upper-type-bounds_error class=tabs-scala-version %} {% tab 'Scala 2' for=upper-type-bounds_error %}

// this would not compile
val lionContainer = new PetContainer[Lion](new Lion)

{% endtab %} {% tab 'Scala 3' for=upper-type-bounds_error %}

// this would not compile
val lionContainer = PetContainer[Lion](Lion())

{% endtab %} {% endtabs %}

The class PetContainer takes a type parameter P which must be a subtype of Pet. Dog and Cat are subtypes of Pet so we can create a new PetContainer[Dog] and PetContainer[Cat]. However, if we tried to create a PetContainer[Lion], we would get the following Error:

type arguments [Lion] do not conform to class PetContainer's type parameter bounds [P <: Pet]

This is because Lion is not a subtype of Pet.