Kotlin Map toMap()
Syntax & Examples

Syntax of toMap()

There are 2 variations for the syntax of Map.toMap() extension function. They are:

fun <K, V> Map<out K, V>.toMap(): Map<K, V>

This extension function returns a new read-only map containing all key-value pairs from the original map.

fun <K, V, M : MutableMap<in K, in V>> Map<out K, V>.toMap( destination: M ): M

This extension function populates and returns the destination mutable map with key-value pairs from the given map.

✐ Examples

1 Convert to read-only map

In this example,

We create a map named map1 containing integer keys and character values.
We then apply the toMap() function on map1.
As a result, a new read-only map containing all key-value pairs from map1 is returned.
We print the result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map1 = mapOf(1 to 'a', 2 to 'b', 3 to 'c')
    val result = map1.toMap()
    println(result)
}

Output

{1=a, 2=b, 3=c}

2 Populate mutable map

In this example,

We create a map named map2 containing character keys and integer values.
We create an empty mutable map named mutableMap.
We then apply the toMap() function on map2, populating the mutableMap with key-value pairs from map2.
As a result, mutableMap is returned with the key-value pairs from map2.
We print the result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map2 = mapOf('a' to 1, 'b' to 2, 'c' to 3)
    val mutableMap = mutableMapOf<Char, Int>()
    val result = map2.toMap(mutableMap)
    println(result)
}

Output

{a=1, b=2, c=3}

3 Populate mutable map with different types

In this example,

We create a map named map3 containing integer keys and character values.
We create an empty mutable map named mutableMap with different key and value types.
We then apply the toMap() function on map3, populating the mutableMap with key-value pairs from map3.
As a result, mutableMap is returned with the key-value pairs from map3.
We print the result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map3 = mapOf(1 to 'a', 2 to 'b', 3 to 'c')
    val mutableMap = mutableMapOf<Int, Char>()
    val result = map3.toMap(mutableMap)
    println(result)
}

Output

{1=a, 2=b, 3=c}

Summary

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