Sort map by keys in Kotlin

This article explores different ways to sort a map in Kotlin according to the natural ordering of its keys.

1. Using TreeMap

A TreeMap is sorted according to the natural ordering of its keys. The idea is to pass your map to the TreeMap constructor to get a new tree map containing the same mappings but ordered according to its keys’ natural ordering.

import java.util.TreeMap

fun main() {

var map: MutableMap<String, String> = HashMap()

map["UNITED STATES"] = "WASHINGTON, D.C."

map["UNITED KINGDOM"] = "LONDON"

map["ITALY"] = "ROME"

map["SPAIN"] = "MADRID"

val sortedMap: MutableMap<String, String> = TreeMap(map)

println(sortedMap)

}

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Output:

{ITALY=ROME, SPAIN=MADRID, UNITED KINGDOM=LONDON, UNITED STATES=WASHINGTON, D.C.}

2. Using LinkedHashMap

Alternatively, you can collect the sorted mappings in a LinkedHashMap, which remembers the iteration order of keys. This can be done with either sorted(), sortedBy() or sortedWith() function.

1. Using sorted() function

fun main() {

val map: MutableMap<String, String> = HashMap()

map["UNITED STATES"] = "WASHINGTON, D.C."

map["UNITED KINGDOM"] = "LONDON"

map["ITALY"] = "ROME"

map["SPAIN"] = "MADRID"

val sortedMap: MutableMap<String, String> = LinkedHashMap()

map.keys.sorted().forEach { sortedMap[it] = map[it]!! }

println(sortedMap)

}

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Output:

{ITALY=ROME, SPAIN=MADRID, UNITED KINGDOM=LONDON, UNITED STATES=WASHINGTON, D.C.}

2. Using sortedBy() function

fun main() {

val map: MutableMap<String, String> = HashMap()

map["UNITED STATES"] = "WASHINGTON, D.C."

map["UNITED KINGDOM"] = "LONDON"

map["ITALY"] = "ROME"

map["SPAIN"] = "MADRID"

val sortedMap: MutableMap<String, String> = LinkedHashMap()

map.entries.sortedBy { it.key }.forEach { sortedMap[it.key] = it.value }

println(sortedMap)

}

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Output:

{ITALY=ROME, SPAIN=MADRID, UNITED KINGDOM=LONDON, UNITED STATES=WASHINGTON, D.C.}

3. Using sortedWith() function

fun main() {

val map: MutableMap<String, String> = HashMap()

map["UNITED STATES"] = "WASHINGTON, D.C."

map["UNITED KINGDOM"] = "LONDON"

map["ITALY"] = "ROME"

map["SPAIN"] = "MADRID"

val sortedMap: MutableMap<String, String> = LinkedHashMap()

map.entries

.sortedWith(java.util.Map.Entry.comparingByKey())

.forEach { sortedMap[it.key] = it.value }

println(sortedMap)

}

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Output:

{ITALY=ROME, SPAIN=MADRID, UNITED KINGDOM=LONDON, UNITED STATES=WASHINGTON, D.C.}

That’s all about sorting map by keys in Kotlin.

Sort map by keys in Kotlin

1. Using TreeMap

2. Using LinkedHashMap

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