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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.

Practice Print a mutable list of three names

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

Starter code

Main.kt
fun main() {
    // Create a mutable list of three names and print it
}

Sample solution

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

Expected output

[Ava, Liam, Nora]

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which one creates a mutable list of Int?

  • listOf(1, 2, 3)
  • mutableListOf(1, 2, 3) Correct answer
  • list(1, 2, 3)
  • arrayOf(1, 2, 3)

Explanation: mutableListOf returns a mutable list; add and remove work.

2 Fill in the blank: mutable list

Question 
val numbers = ____(1, 2, 3)
Answer 
val numbers = mutableListOf(1, 2, 3)

Explanation: mutableListOf(1, 2, 3) gives a list you can add to.

3 Match list expression to behavior

Term Meaning
listOf(...) Read-only list
mutableListOf(...) Mutable list
size Element count
[i] Index access

All lessons in Beginner

1 Kotlin basics

  1. What is Kotlin?
  2. Your first program
  3. The main function
  4. Comments
  5. String templates
  6. Summary - Kotlin basics

2 Variables

  1. val and var
  2. Type inference
  3. const
  4. lateinit
  5. by lazy
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. Boolean and Char
  4. String
  5. Any and Unit
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Elvis operator
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String templates
  3. String methods
  4. substring
  5. split and replace
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. if expression
  4. when
  5. when expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Creating arrays
  2. Array access
  3. Array size
  4. Array loop
  5. Array methods
  6. Summary - Arrays

11 Lists

  1. Creating lists
  2. List operations
  3. List access
  4. List iteration
  5. List methods
  6. Summary - Lists

12 Maps and Sets

  1. Creating sets
  2. Set operations
  3. Creating maps
  4. Map operations
  5. Map iteration
  6. Summary - Maps and Sets

13 Functions

  1. Defining functions
  2. Return values
  3. Expression body
  4. Default parameters
  5. Named arguments
  6. Summary - Functions

14 Null safety

  1. Nullable types
  2. ?, !! and ?:
  3. let
  4. Null check
  5. lateinit
  6. Summary - Null safety

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. Regex
  2. matches and find
  3. replace
  4. StringBuilder
  5. trim and pad
  6. Summary - Advanced strings

17 Enums

  1. enum class
  2. Enum values
  3. Enum properties
  4. when with enum
  5. Enum and range
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. try expression
  5. Custom exceptions
  6. Summary - Exceptions

19 Class basics

  1. class
  2. Constructor
  3. Properties
  4. Methods
  5. this
  6. Summary - Class basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together