How do I safely get a value from a Map?

Use map.getOrDefault("key", defaultValue) or map["key"] ?: defaultValue. Avoid map["key"]!! which throws NullPointerException if the key does not exist.

What is the to infix for maps?

"name" to "Juned" creates a Pair used by mapOf. It reads naturally: mapOf("a" to 1, "b" to 2).

Beginner 8 min readKotlin 2.0

Kotlin Map — Key-Value Pairs with mapOf and mutableMapOf

A Map stores key-value pairs. Each key is unique. Kotlin has read-only (mapOf) and mutable (mutableMapOf) maps.

What You Will Learn

Create maps with mapOf
Access values by key
Add and update entries with mutableMapOf
Iterate over map entries
Check key and value membership

Read-Only Map with mapOf

Create key-value pairs using the to infix function.

mapOf

kotlin
fun main() {
    val capitals = mapOf(
        "India" to "New Delhi",
        "France" to "Paris",
        "Japan" to "Tokyo"
    )
    println(capitals["India"])
    println(capitals["Germany"])  // key not found
    println(capitals.size)
    println("France" in capitals)
}

Output

New Delhi null 3 true

"India" to "New Delhi" is the Pair syntax. capitals["India"] retrieves the value. capitals["Germany"] returns null because the key does not exist.

Beginner Tip: Use getOrDefault("Germany", "Unknown") to provide a fallback when a key might not exist.

Mutable Map

mutableMapOf lets you add, update, and remove entries.

mutableMapOf

kotlin
fun main() {
    val scores = mutableMapOf("Alice" to 85, "Bob" to 90)
    scores["Carol"] = 78       // add new entry
    scores["Alice"] = 92       // update existing
    scores.remove("Bob")
    println(scores)
}

Output

{Alice=92, Carol=78}

scores["key"] = value adds or updates. .remove() deletes by key. Bob is removed.

Iterating a Map

Iterate over entries, keys, or values.

Iterating

kotlin
fun main() {
    val prices = mapOf("Coffee" to 2.5, "Tea" to 1.8, "Juice" to 3.0)
    for ((item, price) in prices) {
        println("$item: $$price")
    }
    println("Keys: ${prices.keys}")
    println("Values: ${prices.values}")
}

Output

Coffee: $2.5 Tea: $1.8 Juice: $3.0 Keys: [Coffee, Tea, Juice] Values: [2.5, 1.8, 3.0]

Destructuring (item, price) unpacks each Map.Entry. .keys returns the set of keys. .values returns a collection of values.

Practice Exercise

Exercisepredict output

What prints? val m = mapOf("a" to 1, "b" to 2) println(m["a"]) println(m["c"])

Quick Quiz

What does the to keyword do in Kotlin maps?

Kotlin Map — Key-Value Pairs with mapOf and mutableMapOf

What You Will Learn

Read-Only Map with mapOf

mapOf

Mutable Map

mutableMapOf

Iterating a Map

Iterating

Practice Exercise

Quick Quiz

Frequently Asked Questions

Related Tutorials