How to Add Entry to Map in Kotlin: Simple Guide

In Kotlin, you can add an entry to a mutable map using the put(key, value) function or the indexing operator [key] = value. Immutable maps cannot be changed after creation, so use mutableMapOf() to create a map you can add entries to.

Syntax

To add an entry to a mutable map in Kotlin, use either:

• map.put(key, value) - adds or updates the entry with the given key.
• map[key] = value - a shorter, idiomatic way to add or update an entry.

Note that the map must be mutable (created with mutableMapOf()) to add entries.

kotlin

val map = mutableMapOf<String, Int>()
map.put("apple", 3)       // Using put()
map["banana"] = 5        // Using indexing operator

Example

This example shows how to create a mutable map, add entries using both put() and the indexing operator, and print the map contents.

kotlin

fun main() {
    val fruitCounts = mutableMapOf<String, Int>()
    fruitCounts.put("apple", 3)       // Add entry with put()
    fruitCounts["banana"] = 5        // Add entry with indexing
    println(fruitCounts)                // Print the map
}

Output

{apple=3, banana=5}

Common Pitfalls

Trying to add entries to an immutable map causes errors. Immutable maps created with mapOf() cannot be changed after creation.

Wrong:

Using mapOf() and then trying to add entries will fail.

kotlin

fun main() {
    val map = mapOf("apple" to 3)
    // map["banana"] = 5  // Error: Val cannot be reassigned - map is immutable
}

Quick Reference

Summary of how to add entries to maps in Kotlin:

Action	Code Example	Notes
Create mutable map	val map = mutableMapOf()	Map can be changed
Add or update entry	map.put(key, value)	Adds or updates key with value
Add or update entry (idiomatic)	map[key] = value	Shorter syntax for put()
Immutable map	val map = mapOf(key to value)	Cannot add or change entries

Key Takeaways

Use mutableMapOf() to create a map you can add entries to.

Add entries with map.put(key, value) or map[key] = value syntax.

Immutable maps created with mapOf() cannot be modified after creation.

The indexing operator map[key] = value is the most common way to add or update entries.

Trying to add entries to an immutable map causes compile-time errors.