Lists

A list is an ordered collection of values of the same type.

Real-world analogy: a list is a numbered row of lockers. Each locker holds one item, and you find an item by its locker number.

Read-only and mutable lists

Kotlin separates lists you can only read from lists you can change:

val fruits = listOf("apple", "banana", "cherry")     // read-only
val basket = mutableListOf("apple")                  // can change
val empty = mutableListOf<Int>()                     // empty, type given

Prefer read-only lists; use a mutable one only while you're building or changing the contents.

Reading elements

Positions (indexes) start at 0:

fun main() {
    val fruits = listOf("apple", "banana", "cherry")

    println(fruits[0])
    println(fruits[2])
    println(fruits.size)
    println(fruits.first())
    println(fruits.last())
    println(fruits.getOrNull(5))
    println(fruits.indexOf("banana"))
    println("cherry" in fruits)
}

apple
cherry
3
apple
cherry
null
1
true

getOrNull(5) returns null instead of crashing when the index is out of range.

Changing a mutable list

fun main() {
    val fruits = mutableListOf("apple", "banana")

    fruits.add("cherry")
    fruits.add(0, "apricot")
    fruits[1] = "avocado"
    println(fruits)

    fruits.remove("banana")
    fruits.removeAt(0)
    println(fruits)

    fruits += listOf("date", "fig")
    println(fruits)
}

[apricot, avocado, banana, cherry]
[avocado, cherry]
[avocado, cherry, date, fig]

Note that fruits is a val — the variable always refers to the same list — yet the list's contents can change. val stops reassignment, not modification.

Common mistakes

• Reading past the end: fruits[3] on a 3-element list throws IndexOutOfBoundsException. Use getOrNull when unsure.
• Trying to add to a listOf(...) list — read-only lists have no add function.

Practice

Create a mutable list of three cities, add a fourth, and print the list, its size, and the last city.

fun main() {
    val cities = mutableListOf("Lagos", "Lima", "Oslo")
    cities.add("Tokyo")
    println(cities)
    println(cities.size)
    println(cities.last())
}

[Lagos, Lima, Oslo, Tokyo]
4
Tokyo