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:
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.
Here, T is a type parameter and the “+” symbol represents Scala covariance.
Let us discuss this concept with the help of examples:
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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.
- 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.
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