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
Contravariance
Contravariance is the opposite of covariance. It indicates that the subtype relation is reversed by the type transformation. If A is a subtype of B, then f(B) is a subtype of f(A). For instance, consider the transformation f(A) = A[]. The transformation reverses the subtype relation between arrays: Object[] is a subtype of String[], but String[] is not a subtype of Object[]. This makes f contravariant.
Invariance
Invariance occurs when neither covariance nor contravariance holds. In other words, the subtype relation is not preserved or reversed by the type transformation. For example, consider the transformation f(A) = Map
Applications in Language Features
class List<T> { ... }Understanding covariance, invariance, and contravariance is essential for comprehending the behavior of polymorphic operations and ensuring type safety in programming.
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