A mutable collection provides us with the ability to modify a collection by either adding, removing, or replacing an element. Kotlin's collections give us the ability to achieve a lot with just a little code-unlike in Java, which seems to need a lot of code to achieve a little! Kotlin has two variants of collections: mutable and immutable.

(If you want a refresher on arrays in Kotlin, kindly visit the first tutorial in this series.) In this section, we'll learn about the List, Set, and Map collections in Kotlin. To really understand the collections API in Kotlin, you'll need to be familiar with these basic classes and interfaces in Java. You can't see the implementation source code in Kotlin because the collections are actually implemented by the standard Java Collections such as ArrayList, Maps, HashMap, Sets, HashSet, List, and so on. You should note that these interfaces are linked to their implementation at compile time. (We'll discuss interfaces in Kotlin in a future post.) Kotlin provides its collections API as a standard library built on top of the Java Collections API.

We can retrieve, store, or organize the objects in a collection. Collections are used to store groups of related objects in memory.

Kotlin empty mutable list