Creating lists

Learn to create lists with listOf and mutableListOf.

Creating lists: listOf and mutableListOf

Kotlin has two kinds of lists. listOf returns a read-only list: you can read its elements but you cannot add or remove. mutableListOf gives you a mutable list.

val names = listOf("Ava", "Liam")
val nums = mutableListOf(1, 2, 3)
nums.add(4)

The type is usually inferred: listOf("a") becomes a List<String>. The list length comes from size; an element is reached with [i] or get(i).

Read-only vs mutable lists

Calling add on a list built with listOf is a compile error, because the returned type is List<T> and has no add method. mutableListOf returns MutableList<T>.

val a = listOf(1, 2)   // List<Int>
val b = mutableListOf(1, 2)  // MutableList<Int>
b.add(3)   // ok
a.add(3)   // compile error

This distinction guarantees at compile time whether a list can be changed later, which makes the code safer.

Exercise Print a mutable list of three names

Create a MutableList<String> of three names and print it with println.

Show example solution

fun main() {
    val names = mutableListOf("Ava", "Liam", "Nora")
    println(names)
}

Test yourself

Which one creates a mutable list of Int?
- listOf(1, 2, 3)
- mutableListOf(1, 2, 3)
- list(1, 2, 3)
- arrayOf(1, 2, 3)
Show answer

mutableListOf(1, 2, 3) — mutableListOf returns a mutable list; add and remove work.

Fill in the blank: mutable list

val numbers = ____(1, 2, 3)

Show answer

val numbers = mutableListOf(1, 2, 3)

Match list expression to behavior

listOf(...)

Read-only list
mutableListOf(...)

Mutable list
size

Element count
[i]

Index access

Next lesson: List operations

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