Kotlin Tutorials

Kotlin Map mapKeys()
Syntax & Examples

Syntax of Map.mapKeys()

The syntax of Map.mapKeys() extension function is:

fun <K, V, R> Map<out K, V>.mapKeys( transform: (Entry<K, V>) -> R ): Map<R, V>

This mapKeys() extension function of Map returns a new Map with entries having the keys obtained by applying the transform function to each entry in this Map and the values of this map.


✐ Examples

1 Map keys to their lengths

In this example,

  • We create a map named map1 with key-value pairs.
  • We use the mapKeys() function to transform the keys of map1 by their lengths.
  • The resulting map transformedMap has keys representing the lengths of the original keys in map1.
  • We print the transformed map to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map1 = mapOf("apple" to 1, "banana" to 2, "cherry" to 3)
    val transformedMap = map1.mapKeys { entry -> entry.key.length }
    println(transformedMap)
}

Output

{5=1, 6=3}

2 Map keys to uppercase

In this example,

  • We create a map named map2 with key-value pairs.
  • We use the mapKeys() function to transform the keys of map2 to uppercase.
  • The resulting map transformedMap has keys that are the uppercase versions of the original keys in map2.
  • We print the transformed map to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map2 = mapOf("apple" to 1, "banana" to 2, "cherry" to 3)
    val transformedMap = map2.mapKeys { entry -> entry.key.toUpperCase() }
    println(transformedMap)
}

Output

{APPLE=1, BANANA=2, CHERRY=3}

3 Map keys to their values

In this example,

  • We create a map named map3 with key-value pairs.
  • We use the mapKeys() function to transform the keys of map3 to their corresponding values.
  • The resulting map transformedMap has keys that are the values of the original map map3.
  • We print the transformed map to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map3 = mapOf("apple" to 1, "banana" to 2, "cherry" to 3)
    val transformedMap = map3.mapKeys { entry -> entry.value }
    println(transformedMap)
}

Output

{1=1, 2=2, 3=3}

Summary

In this Kotlin tutorial, we learned about mapKeys() extension function of Map: the syntax and few working examples with output and detailed explanation for each example.