Covariance and contravariance (computer science): Difference between revisions

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Revision as of 04:04, 17 December 2021

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== Inheritance in object-oriented languages ==

When a subclass [[Method overriding|overrides]] a method in a superclass, the compiler must check that the overriding method has the right type. While some languages require that the type exactly matches the type in the superclass (invariance{{Dubious|date=November 2021|reason=according to the definition above, 'invariant' means noncompatible, it doesn't mean "the same type" (bidirectionally compatible).}}), it is also type safe to allow the overriding method to have a "better" type. By the usual subtyping rule for function types, this means that the overriding method should return a more specific type (return type covariance), and accept a more general argument (parameter type contravariance). In [[Unified Modeling Language|UML]] notation, the possibilities are as follows:

<gallery perrow="5" heights="190" caption="Variance and method overriding: overview">