Variance
Variance defines Inheritance relationships of Parameterized Types. Variance is all about Sub-Typing.
Advantage of Variance in Scala
The main advantage of Scala Variance is:
Variance makes Scala collections more Type-Safe.
Variance gives more flexible development.
Scala Variance gives us a technique to develop Reliable Applications.
Types of Variance in Scala
Scala supports the following three kinds of Variance.
Covariant
Invariant
Contravariant
Scala Covariance Syntax:
To represent Covariance relationship between two Parameterized Types, Scala uses the following syntax:
Prefixing Type Parameter with “+” symbol defines Covariance in Scala.
If we remove Variance Annotation in Animal class definition, like as shown below:
class Animal[T](val animial:T)
It wont compile. We will get the following compilation error message:
Type mismatch, expected: Animal[Dog], found: Animal[Puppy]
To solve these kind of problems, we should use Scala Covariance.
As per this example, we can say the following Scala Covariance:
“As Puppy is subtype of Dog, Animal[Puppy] is a subtype of Animal[Dog]. We can use Animal[Puppy] where we require Animal[Dog].” This is know as Scala Covariance.
Invariant in Scala
If “S” is subtype of “T” then List[S] and List[T] don’t have Inheritance Relationship or Sub-Typing. That means both are unrelated.
This kind of Relationship between two Parameterized Types is known as “Invariant or Non-Variant”
In Scala, by default Generic Types have Non-Variant relationship. If we define Parameterized Types without using “+’ or “-” symbols, then they are known as Invariants.
What is Variance Annotation in Scala?
Variance Annotation means defining “+” or “-” before Type Parameters.
Example:
+T and – T are know as Variance Annotations in Scala.
