What Extension Functions Are Not

External implementations of interfaces, multi-methods, and what you can do instead

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THE CURRENT VERSION OF THIS ARTICLE IS PUBLISHED HERE.

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Tags: #FUNDAMENTAL CONCEPT

This article is part of the Kotlin Primer, an opinionated guide to the Kotlin language, which is indented to help facilitate Kotlin adoption inside Java-centric organizations. It was originally written as an organizational learning resource for Etnetera a.s. and I would like to express my sincere gratitude for their support.

It is recommended to read the Introduction before moving on. Check out the Table of Contents for all articles.

We’ve spent the previous few articles talking about what extension functions are, but it’s also important to understand what they are not. So let’s take a look at a couple of things that cannot be achieved using extension functions.

Implementing Interfaces Outside of a Class Definition

When discussing the motivation for extension functions, we mentioned the problem of not…