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Kotlin Academy · Lesson

listOf, setOf, mapOf: Read-Only Collections

Build immutable collections and understand their access patterns.

Lesson 1 of 413 steps

listOf, setOf, mapOf: Read-Only Collections is a free Kotlin Academy lesson on CoddyKit. This is lesson 1 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Kotlin Academy learning path, and your progress syncs across the web and the CoddyKit app. The Kotlin Academy course includes 4 lessons in total.

Three Builder Functions

Kotlin's stdlib offers listOf, setOf, and mapOf to construct read-only collections. Read-only means you cannot add or remove through the interface.

Building a List

listOf creates a read-only List<T>. Element type is inferred from arguments.

fun main() {
    val fruits = listOf("apple", "banana", "cherry")
    println(fruits)        // [apple, banana, cherry]
    println(fruits.size)   // 3
}

Building a Set

setOf returns a read-only Set<T>. Duplicate elements are collapsed automatically.

fun main() {
    val tags = setOf("kotlin", "java", "kotlin", "android")
    println(tags) // [kotlin, java, android]
    println(tags.size) // 3
}

Building a Map

mapOf takes key to value pairs and returns a read-only Map<K, V>.

fun main() {
    val ages = mapOf("Alice" to 30, "Bob" to 25)
    println(ages["Alice"]) // 30
    println(ages.keys)     // [Alice, Bob]
}

Type Annotations

If the inferred type is wrong (or the list is empty), provide explicit type parameters.

fun main() {
    val empty: List<String> = listOf()
    val numbers: List<Int> = listOf(1, 2, 3)
    val emptyMap: Map<String, Int> = mapOf()
    println("empty=${empty}, n=${numbers}, m=${emptyMap}")
}

Empty Variants

Use emptyList(), emptySet(), emptyMap() when you explicitly want an empty collection. Better than listOf() with a type parameter when intent matters.

fun main() {
    val xs = emptyList<String>()
    val ys = emptySet<Int>()
    val zs = emptyMap<String, Int>()
    println("${xs.size}, ${ys.size}, ${zs.size}") // 0, 0, 0
}

Read-Only Means... Read-Only

The List interface has no add. The compiler refuses to mutate.

fun main() {
    val items = listOf(1, 2, 3)
    // items.add(4) // ERROR: List has no add
    val bigger = items + 4 // creates new list
    println(bigger) // [1, 2, 3, 4]
}

Underlying Implementation

Under the hood, listOf returns a Java ArrayList wrapped in an unmodifiable view. Read-only is an interface contract, not a runtime guarantee against reflection.

fun main() {
    val xs = listOf(1, 2, 3)
    println(xs::class.simpleName) // ArrayList
}

Pairs and to

The infix function to creates a Pair<A, B> — handy for map literals and tuples.

fun main() {
    val pair = "key" to 42
    println(pair) // (key, 42)
    val m = mapOf("a" to 1, "b" to 2)
    for ((k, v) in m) println("$k -> $v")
}

Iterating Read-Only Collections

Read-only collections support all the standard iteration patterns: for, forEach, map, filter.

fun main() {
    val nums = setOf(1, 2, 3, 4)
    val doubled = nums.map { it * 2 }
    println(doubled) // [2, 4, 6, 8]
}

Concatenating Lists

The + operator produces a new combined list (read-only behavior preserved).

fun main() {
    val a = listOf(1, 2)
    val b = listOf(3, 4)
    val combined = a + b
    println(combined) // [1, 2, 3, 4]
}

Quick Check

What happens when you call add(4) on a list created with listOf(1, 2, 3)?

Answer it in the interactive lesson

Recap

listOf, setOf, and mapOf create read-only collections. They cannot be modified through their interfaces. Use + to produce new collections, and emptyList()/emptySet()/emptyMap() for typed empties.

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Frequently Asked Questions

Is the “listOf, setOf, mapOf: Read-Only Collections” lesson free?

Yes — any 3 lessons of the Kotlin Academy learning path, including “listOf, setOf, mapOf: Read-Only Collections”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Kotlin Academy course includes 4 lessons in total.

What will I learn in “listOf, setOf, mapOf: Read-Only Collections”?

Build immutable collections and understand their access patterns. You practise Kotlin Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Kotlin Academy?

No prior experience is required. Kotlin Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.

How long does the “listOf, setOf, mapOf: Read-Only Collections” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Kotlin Academy lesson?

Yes. Every Kotlin Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. listOf, setOf, mapOf: Read-Only Collections
  2. mutableListOf, mutableMapOf: Mutable Builders
  3. Accessing Elements: get, first, last, getOrNull
  4. Iterating and Transforming Collections with Standard Library
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