How do Covariance, Invariance, and Contravariance Affect Type Relationships in Programming?

Covariance, Invariance, and Contravariance in Plain English: A Comprehensive Explanation

Covariance, invariance, and contravariance are programming concepts that describe how the relationship between types and subtypes is affected by type transformations. These concepts play a crucial role in understanding polymorphism and type safety in programming languages.

Covariance

A covariance indicates that the subtype relation is preserved by the type transformation. In other words, if type A is a subtype of type B, then the transformed type f(A) is also a subtype of f(B). For example, consider the transformation f(A) = List, where List is declared as:

class List<T> { ... }

Since List is a subtype of List