Kotlin mutableSetOf

In this guide, we will learn about the Kotlin mutableSetOf() function with lots of examples. 

What is mutableSetOf()? 

mutableSetOf() is a function in Kotlin's standard library that creates an instance of a mutable set. A set is a collection that doesn't allow duplicate elements. When we say "mutable," it means that the set can be changed after it's been created. You can add, remove, or modify elements in a mutable set.

Basic Syntax:

val mutableSet: MutableSet<Type> = mutableSetOf(element1, element2, ...)

Examples with Outputs

Constructing a Mutable Set

val animals = mutableSetOf("Cat", "Dog", "Bird")
println(animals)  // Output: [Cat, Dog, Bird]

Adding Elements to a Mutable Set

val veggies = mutableSetOf("Broccoli", "Carrot")
println(veggies)  // Output: [Broccoli, Carrot, Pepper]

Removing Elements from a Mutable Set

val fruits = mutableSetOf("Apple", "Banana", "Cherry")
println(fruits)  // Output: [Apple, Cherry]

Element Uniqueness in Mutable 

Sets Remember, sets inherently ensure element uniqueness:

val numbers = mutableSetOf(1, 2, 2, 3)
println(numbers)  // Output: [1, 2, 3]

Combining Two Mutable 

Sets Merging retains unique elements:

val setA = mutableSetOf(1, 2, 3)
val setB = mutableSetOf(3, 4, 5)
println(setA)  // Output: [1, 2, 3, 4, 5]

Checking for Elements 

Using Kotlin's in operator:

val colors = mutableSetOf("Red", "Green", "Blue")
println("Yellow" in colors)  // Output: false

Clearing All Elements

val letters = mutableSetOf('a', 'b', 'c')
println(letters)  // Output: []

Retaining Specific Elements 

Retain elements that are present in another collection:

val setOne = mutableSetOf(1, 2, 3, 4)
setOne.retainAll(listOf(3, 4, 5, 6))
println(setOne)  // Output: [3, 4]


In this guide, we learned everything about Kotlin mutableSetOf() with lots of examples. So mutableSetOf() is a function in Kotlin's standard library, tailored to instantiate a mutable set. Unlike its immutable counterpart created by setOf(), a mutable set allows additions, modifications, and removal of elements post-creation.