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Generic Types and Variance in Kotlin compared to Java

Generic Types and Variance in Kotlin compared to Java

Basics – What is Variance?

Many programming languages support the concept of subtyping, which allows us to implement hierarchies like “A Cat IS-An Animal“. In Java we can either use the extends keyword in order to change/expand behaviour of an existing class (inheritance) or use implements to provide implementations for an interface. According to Liskov’s substitution principle, every instance of a class A can be substituted by instances of its subtype B. The word variance, often referred to in mathematics as well, is used to describe how subtyping in complex aspects like method return types, type declarations, generic types or arrays relates to the direction of inheritance of the involved classes. There are three terms we need to take into account: Covariance, Contravariance and Invariance.

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