Kotlin gives preference to immutability wherever possible, but leaves the choice to the developer whether or when to use it. This power of choice makes the language even more powerful. Unlike most languages, where they have either only mutable (like Java, C#, and so on) or only immutable collections (like F#, Haskell, Clojure, and so on), Kotlin has both and distinguishes between them, leaving the developer with the freedom to choose whether to use an immutable or mutable one.
Kotlin has two interfaces for collection objects—Collection<out E> and MutableCollection<out E>; all the collection classes (for example, List, Set, or Map) implement either of them. As the name suggests, the two interfaces are designed to serve immutable and mutable collections respectively. Let us have an example:
fun main(args: Array<String>) {
...
