Covariance and Contravariance

Ported from my old blog

Dated - 2018-12-03

Design Guideline 2 - Covariant and Contravariance

Variance

Variance refers to how subtyping between more complex types relates to subtyping between their components.

Consider this class

	class AnimalShelter {
        Animal getAnimalForAdoption() {
          ...
        }
    
        void putAnimal(Animal animal) {
          ...
        }
    }

Covariant Return Type

This is present in Java, but not in C#, personally it made me to rethink my design, however the need for variance can be removed.

	class CatShelter extends AnimalShelter {
        Cat getAnimalForAdoption() {
    	    return new Cat();
        }
    }

Covariant method argument type

This is not present in Java and C#, this would allow to send the object as derived class directly instead of up-casting to base class. This is not type safe as by up-casting to Base class, you can send any other derived class. ie) Within the CatShelter one can add a Dog Object

	class CatShelter extends AnimalShelter {
        void putAnimal(Cat animal) {
           ...
        }
    }

Contravariant method argument type

It is type safe to allow an overriding method to accept a more general argument.

	class CatShelter extends AnimalShelter {
        void putAnimal(Object animal) {
           ...
        }
    }

Language	Argument type	Return type
C++ (since 1998), Java (since J2SE 5.0), Scala, D	Invariant	Covariant
C#	Contravariant	Covariant

Problem with variance

Since variance switches between languages, it is easy to forget which variance is present

Avoiding the need for covariant argument types by the use of generics

The following code will behave the same in both Java and C#

	class Shelter<T extends Animal> {
        T getAnimalForAdoption() {
          ...
        }
    
        void putAnimal(T animal) {
          ...
        }
    }
    
    
    class CatShelter extends Shelter<Cat> {
        Cat getAnimalForAdoption() {
          ...
        }
    
        void putAnimal(Cat animal) {
          ...
        }
    }

References

https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)

Zettle

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