The word variance, often referred to in mathematics as well, is used to describe how sub-typing 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. In Java, we can either use the extends keyword to change/expand behavior of an existing class (inheritance) or use implements to provide implementations for an interface.Īccording to Liskov’s substitution principle, every instance of a class A can be substituted by instances of its subtype B. Many programming languages support the concept of sub-typing, which allows implementing hierarchies that represent relationships like "A cat IS-an animal". Kotlin Generics differ from Java's in how users can define how they behave in sub-typing relationships.Īs opposed to Java, Kotlin allows defining variance on declaration-site, whereas Java only knows use-site variance. This article covers the concepts of Generics and Variance in Kotlin and compares it to Java.
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