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# Scala – Covariance

• Last Updated : 03 Jul, 2020

Variance is the interconnection of Sub-Typing relationships which are either of complicated types or of their constituent types. Scala provides three types of variance:

• Covariant
• Contravariant
• Invariant

Covariance states that if there are two parameterized types such that S is a subtype of T, then List[S] is a subtype of List[T]. This is an inheritance relationship. So, this basically defines the relationship whether we can replace a type by its base type. Simply putting in words, if we take an example where Car is a subtype of Vehicle, then List[Car] is a subtype of List[Vehicle]. Therefore, we can replace List[Car] by List[Vehicle]. When we declare a type to be covariant, then its safe use at various positions becomes limited. In the case of immutable types, covariance is widely used.
Syntax:

`List[+T]`

Here, T is a type parameter and the “+” symbol represents Scala covariance.

Let us discuss this concept with the help of examples:
Example 1:

 `// Scala program to illustrate the concept of covariance`` ` `// Creating an abstract class ``// for Flower ``abstract` `class` `Flower ``{``    ``def` `name``:` `String``}`` ` `// Creating a sub-class Lily ``// of Flower ``case` `class` `Lily(name``:` `String) ``extends` `Flower`` ` `// Creating a sub-class Carnation``// of Flower ``case` `class` `Carnation(name``:` `String) ``extends` `Flower ``object` `Covariance ``extends` `App``{``     ` `    ``// Creating a method``    ``def` `FlowerNames(flowers``:` `List[Flower])``:` `Unit ``=``    ``{   ``        ``flowers.foreach ``        ``{``            ``flower ``=>` `println(flower.name)``        ``}``    ``}``     ` `    ``// Assigning names``    ``val` `lily``:` `List[Lily] ``=` `List(Lily(``"White Lily"``), ``                                ``Lily(``"Jersey Lily"``))``    ``val` `carnations``:` `List[Carnation] ``=` `List(Carnation(``"White carnations"``),``                                           ``Carnation(``"Pink carnations"``))`` ` `    ``// Print: names of lily ``    ``FlowerNames(lily)`` ` `    ``// Print: names of carnation ``    ``FlowerNames(carnations)``}`

Output:

```White Lily
Jersey Lily
White carnations
Pink carnations
```

Explanation: In the above example, Lily and Carnation are subtypes of Flower. So, it is evident that a List[Lily] is a List[Flower] and a List[Carnation] is also a List[Flower], and we can substitute any of them for a List[Flower]. In the later part of the code, there is a method FlowerNames that prints names of flowers and the acceptable argument is a list of flowers. If the two lists are covariant, only then the method calls will compile and the flower names will get printed respectively. So, as Lily and Carnation are a subtype of Flowers and the last two lines will execute due to covariance.
Note:

• Abstract class is utilized here to apply covariance as it has List[+T] with it where the type parameter T is covariant.
• A trait App is used here to speedily change objects into workable programs.

Example 2:

 `// Scala program to illustrate the concept of covariance`` ` `// Creating an abstract class ``// for Animal``abstract` `class` `Animal ``{``    ``def` `name``:` `String``}`` ` `// Creating a sub-class Mammal``// of Animal ``case` `class` `Mammal(name``:` `String) ``extends` `Animal`` ` `// Creating a sub-class Reptile``// of Animal ``case` `class` `Reptile(name``:` `String) ``extends` `Animal `` ` `object` `CovarianceExample ``extends` `App``{``     ` `    ``// Creating a method``    ``def` `SpecieNames(animals``:` `List[Animal])``:` `Unit ``=``    ``{   ``        ``animals.foreach ``        ``{ ``            ``animal ``=>``println(animal.name)``        ``}``    ``}``     ` `    ``// Assigning names``    ``val` `mammals``:` `List[Mammal] ``=` `List(Mammal(``"Zebra"``), ``                                     ``Mammal(``"Horse"``))``    ``val` `reptiles``:` `List[Reptile] ``=` `List(Reptile(``"Snake"``), ``                                       ``Reptile(``"Lizard"``))`` ` `    ``// Print: names of mammals``    ``SpecieNames(mammals)`` ` `    ``// Print : names of reptiles``    ``SpecieNames(reptiles)``}`

Output:

```Zebra
Horse
Snake
Lizard
```

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