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Kotlin Generics and Variance (Compared to Java)

Kotlin Generics and Variance (Compared to Java)

This article covers the concepts of Generics and Variance in Kotlin and compares it to Java. Kotlin Generics differ from Java’s in how users can define their type of variance. As opposed to Java, Kotlin allows defining variance on declaration-site, whereas Java only knows use-site variance.

Kotlin Generics – What is Variance?

Many programming languages support the concept of subtyping, which allows implementing hierarchies that represent relationships like “A Cat IS-An Animal“. In Java, we can either use the extends keyword in order to change/expand behavior 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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