What are Covariance, Invariance, and Contravariance in Plain English?

Understanding Covariance, Invariance, and Contravariance in Plain English

Covariance, invariance, and contravariance are terms that describe how typing relationships behave under certain transformations. In programming, these transformations can be type conversions, method overriding, or inheritance.

Covariance

Covariance means that a subtype of a type will also be a subtype of the transformed type. For example, if class A is a subtype of class B, and we have a function f(T) that takes a type T and returns a transformed type T'', then if A is a subtype of B, f(A) will also be a subtype of f(B).

Contravariance

Contravariance is the opposite of covariance. In contravariance, a subtype of a type will map to a supertype of the transformed type. If the same function f(T) from the covariance example above were contravariant, then if A is a subtype of B, f(B) would be a subtype of f(A).

Invariance

Invariance means that neither covariance nor contravariance applies to the transformation. That is, the subtype relationship is unaffected by the transformation.

Examples

Consider the following examples:

Java Generics

Generics in Java are invariant. This means that List, for example, is neither a subtype nor a supertype of List