Collection Types in Kotlin
From the Kotlin cheat sheet · Collections · verified Jul 2026
Collection Types
Lists, Sets, and Maps
kotlin
// Immutable collections
val list = listOf(1, 2, 3)
val set = setOf("a", "b", "c")
val map = mapOf("key1" to "value1", "key2" to "value2")
// Mutable collections
val mutableList = mutableListOf(1, 2, 3)
mutableList.add(4)
mutableList.removeAt(0)
val mutableSet = mutableSetOf("a", "b")
mutableSet.add("c")
val mutableMap = mutableMapOf<String, Int>()
mutableMap["one"] = 1
mutableMap["two"] = 2
// Array types
val intArray = intArrayOf(1, 2, 3)
val stringArray = arrayOf("a", "b", "c")
val nullArray = arrayOfNulls<String>(5)💡 Default collections are immutable, use mutable* for mutability
⚡ to infix function creates Pair for map entries
📌 buildList/buildMap efficiently create immutable collections
🟢 Arrays have fixed size, Lists can be resized (if mutable)
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