How to Sort Map by Key in Kotlin: Simple Guide

In Kotlin, you can sort a map by its keys using the toSortedMap() function, which returns a new map sorted by keys. Alternatively, you can use entries.sortedBy { it.key } to get a sorted list of pairs by key.

Syntax

The main way to sort a map by key in Kotlin is using toSortedMap(). This function returns a new SortedMap with keys in ascending order.

Another way is to use entries.sortedBy { it.key } which returns a list of map entries sorted by key.

kotlin

val sortedMap = originalMap.toSortedMap()

val sortedList = originalMap.entries.sortedBy { it.key }

Example

This example shows how to sort a map by its keys using toSortedMap(). It prints the original map and the sorted map.

kotlin

fun main() {
    val originalMap = mapOf(3 to "Three", 1 to "One", 2 to "Two")
    println("Original map: $originalMap")

    val sortedMap = originalMap.toSortedMap()
    println("Sorted map by key: $sortedMap")
}

Output

Original map: {3=Three, 1=One, 2=Two}
Sorted map by key: {1=One, 2=Two, 3=Three}

Common Pitfalls

A common mistake is trying to sort the map in place, but Kotlin maps are immutable by default, so sorting returns a new map.

Also, using sortedBy on a map directly returns a list, not a map, which may confuse beginners.

kotlin

fun main() {
    val map = mapOf(2 to "B", 1 to "A")

    // Wrong: This does not sort the map in place
    // map.sortedBy { it.key } // returns List, not Map

    // Right: Use toSortedMap() to get a sorted map
    val sortedMap = map.toSortedMap()
    println(sortedMap)
}

Output

{1=A, 2=B}

Quick Reference

• toSortedMap(): Returns a new map sorted by keys.
• entries.sortedBy { it.key }: Returns a list of entries sorted by key.
• Maps are immutable by default; sorting returns a new collection.

Key Takeaways

Use toSortedMap() to get a new map sorted by keys in ascending order.

sortedBy on map entries returns a list, not a map.

Kotlin maps are immutable; sorting creates a new collection.

toSortedMap() is the simplest way to sort a map by key.

Remember to print or use the returned sorted map, as original map stays unchanged.