List Basics

A List is an ordered row of items that you can look at, but not change, once you've made it with listOf.

In this page:

Creating a List
Accessing Elements by Index
Common List Properties and Functions
Lists Are Read-Only, Not Necessarily Immutable

Creating a List

listOf(...) builds a read-only List containing the given elements in the given order, which is the most common way to work with a fixed collection of values.

Example: Creating a List

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

Accessing Elements by Index

Elements are retrieved using square brackets with a zero-based index, such as fruits[0] for the first element.

Example: Accessing Elements by Index

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fun main() {
    val fruits = listOf("apple", "banana", "cherry")
    println("First: ${fruits[0]}")
    println("Last: ${fruits[fruits.size - 1]}")
}

Common List Properties and Functions

List provides convenient read-only members like .size, .first(), .last(), .isEmpty(), and .contains(item) for inspecting the collection without mutating it.

Example: Common List Properties and Functions

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fun main() {
    val fruits = listOf("apple", "banana", "cherry")
    println("Size: ${fruits.size}")
    println("First: ${fruits.first()}")
    println("Contains banana: ${fruits.contains("banana")}")
}

Lists Are Read-Only, Not Necessarily Immutable

A List reference cannot itself add or remove elements, but if it points to the same object as a MutableList reference elsewhere, changes made through that other reference are still visible.

Note: This is why List is called read-only rather than immutable in Kotlin's terminology.

Example: Lists Are Read-Only, Not Necessarily Immutable

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fun main() {
    val mutable = mutableListOf("a", "b")
    val readOnlyView: List<String> = mutable
    mutable.add("c")
    println(readOnlyView)
}

Common Mistakes

Trying to call .add() on a List created with listOf(), forgetting it is read-only and not necessarily mutable.
Assuming a read-only List can never change; if its underlying value is a MutableList, changes made through that reference are still visible through the read-only view.
Using index-based access without checking bounds, causing an IndexOutOfBoundsException on an invalid index.

Chapter Summary

listOf(...) creates a read-only List -- you cannot add, remove, or replace elements through it.
Elements are accessed by zero-based index using list[index] or list.get(index).
List provides many read-only helper functions like .size, .first(), .last(), and .contains().
A read-only List type does not guarantee true immutability if another reference to the same underlying collection is mutable.

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List Basics

Creating a List

Accessing Elements by Index

Common List Properties and Functions

Lists Are Read-Only, Not Necessarily Immutable

Chapter Quiz — Complete all 7 topics to unlock

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