Map creation (mapOf, mutableMapOf) in Kotlin

Introduction

Maps help you store pairs of related information, like a name and a phone number. Using mapOf and mutableMapOf lets you create these pairs easily.

When you want to look up a value by a key, like finding a phone number by a person's name.

When you need to store settings or options with names and values.

When you want to group data where each item has a unique label.

When you want to change or add pairs after creating the map, use a mutable map.

When you only need to read data without changing it, use an immutable map.

Syntax

Kotlin

val mapName = mapOf(key1 to value1, key2 to value2)
val mutableMapName = mutableMapOf(key1 to value1, key2 to value2)

mapOf creates a map you cannot change later (immutable).

mutableMapOf creates a map you can add or remove pairs from (mutable).

Examples

This creates an immutable map of color names to their hex codes.

Kotlin

val colors = mapOf("red" to "#FF0000", "green" to "#00FF00")

This creates a mutable map with user information you can update later.

Kotlin

val userInfo = mutableMapOf("name" to "Alice", "age" to 30)

Updates the age in the mutable map to 31.

Kotlin

userInfo["age"] = 31

Sample Program

This program shows how to create an immutable map of country capitals and a mutable map of game scores. It prints the capital of France, then shows the scores before and after updating.

Kotlin

fun main() {
    val capitals = mapOf("France" to "Paris", "Japan" to "Tokyo")
    println("Capital of France: ${capitals["France"]}")

    val scores = mutableMapOf("Alice" to 10, "Bob" to 8)
    println("Initial scores: $scores")

    scores["Alice"] = 15
    scores["Charlie"] = 12
    println("Updated scores: $scores")
}

Important Notes

Immutable maps cannot be changed after creation; trying to add or remove items will cause an error.

Mutable maps let you add, update, or remove pairs anytime.

Use the to keyword to create pairs inside the map.

Summary

mapOf creates a fixed map you cannot change.

mutableMapOf creates a map you can change later.

Maps store pairs of keys and values for easy lookup.