How to declare and initialize a MutableSet in Kotlin?

I would use mutableSetOf:

val s = mutableSetOf(1, 2, 3)

which is a kotlin.collections.LinkedHashSet under the hood.

Kotlin doesn't have its own implementations of collection interfaces. You can use standard Java sets such as HashSet or TreeSet, or any other set implementation out there. HashSet is the most popular one, and the preferred way of creating a HashSet from given elements is using the hashSetOf function:

val set: MutableSet<Int> = hashSetOf(1, 2, 3)