Mutable List in Kotlin

Introduction

Mutable List is a fundamental concept every Kotlin developer should understand. Collections let you store and transform groups of values. Kotlin separates read-only and mutable views so you can express intent clearly in your APIs.

MutableListOf() creates a list that supports add, remove and update operations. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• mutableListOf() creates a list that supports add, remove and update operations.
• Use immutable list when data should not change after creation.
• Mutable lists are useful for dynamic data.

Syntax

val list = mutableListOf(1, 2, 3)

Mutable List in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    val numbers = mutableListOf(1, 2, 3)
    numbers.add(4)
    numbers.removeAt(0)
    println(numbers)
}

Sample Output

[2, 3, 4]

When to use

Use collections when the number of items is dynamic or when you need map/set semantics instead of a plain list.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. val numbers = mutableListOf(1, 2, 3) assigns or updates a value used later in the program.

3. The println(numbers) statement writes a line to the console — this produces part of the sample output below.

4. MutableListOf() creates a list that supports add, remove and update operations.

5. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Prefer immutable listOf, setOf, mapOf for data that should not change after creation.
• Use map, filter, and fold instead of manual loops when transforming collections.
• Pick the smallest collection type that fits — don’t use a List when a Set is semantically correct.

Common Mistakes

• Modifying a list while iterating it — use filter or iterator remove carefully.
• Using mutableListOf when an immutable list would suffice, exposing accidental mutation.
• Calling get on a Map without checking key existence — prefer getOrDefault or getValue.

Key Points

• mutableListOf() creates a list that supports add, remove and update operations.
• Use immutable list when data should not change after creation.
• Mutable lists are useful for dynamic data.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.