Set — setOf() / mutableSetOf()

// Create an immutable set (duplicates are removed automatically)
val fruits = setOf("apple", "banana", "apple", "orange")
// {apple, banana, orange} (1 duplicate removed)

// Create a mutable set
val tags = mutableSetOf("kotlin", "jvm")
tags.add("android")
tags.remove("jvm")

// Set operations
val a = setOf(1, 2, 3, 4)
val b = setOf(3, 4, 5, 6)
val union = a union b          // {1, 2, 3, 4, 5, 6}
val intersect = a intersect b  // {3, 4}
val subtract = a subtract b   // {1, 2}

fun main() {
    // Basic set (duplicates removed automatically)
    val fruits = setOf("apple", "banana", "apple", "orange", "banana")
    println("fruits: $fruits")  // Duplicates removed
    println("Size: ${fruits.size}")

    // Membership check
    println("banana exists: ${"banana" in fruits}")  // true
    println("melon exists: ${"melon" in fruits}")    // false

    println()

    // Mutable set
    val langs = mutableSetOf("Kotlin", "Java", "Python")
    langs.add("Swift")
    langs.add("Kotlin")   // Duplicate — ignored.
    langs.remove("Java")
    println("Languages: $langs")

    println()

    // Set operations
    val set1 = setOf(1, 2, 3, 4, 5)
    val set2 = setOf(3, 4, 5, 6, 7)

    println("Union: ${set1 union set2}")           // {1,2,3,4,5,6,7}
    println("Intersection: ${set1 intersect set2}") // {3,4,5}
    println("Difference: ${set1 subtract set2}")    // {1,2}
    println("Reverse difference: ${set2 subtract set1}") // {6,7}

    println()

    // Removing duplicates from a list
    val names = listOf("Alice", "Bob", "Alice", "Carol", "Bob", "Dave")
    println("Original list (${names.size} items): $names")

    val unique = names.toSet()
    println("Deduplicated (${unique.size} items): $unique")

    // distinct() — removes duplicates while preserving order.
    val distinctList = names.distinct()
    println("distinct (${distinctList.size} items): $distinctList")

    println()

    // Practical example: finding mutual friends.
    val aliceFriends = setOf("Bob", "Carol", "Dave", "Eve")
    val bobFriends = setOf("Alice", "Carol", "Frank", "Eve")
    val common = aliceFriends intersect bobFriends
    println("Mutual friends: $common")
}

A set is a collection designed for fast membership checks with no duplicates. Checking whether an element exists with contains() is O(n) for a list but O(1) for a set (HashSet). Sets are a good choice when you frequently need to check whether a large dataset contains a specific element.

Kotlin's setOf() uses LinkedHashSet by default, which preserves insertion order. If you do not need insertion order and want a pure HashSet, use hashSetOf() instead.

For map operations, see Maps — mapOf() / mutableMapOf(). For list basics, see Lists — listOf() / mutableListOf().