Kotlin Tutorials

Kotlin Map flatMap()
Syntax & Examples

Syntax of flatMap()

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

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

This flatMap() extension function of Map returns a single list of all elements yielded from results of transform function being invoked on each entry of original map.


✐ Examples

1 Flatten a map of lists of strings

In this example,

  • We create a map where keys are integers and values are lists of strings.
  • We use the flatMap function on the map, applying a lambda that accesses the value of each entry and flattens it into a single list.
  • The resulting list contains all elements from the lists in the map.
  • We print the flattened list.

Kotlin Program

fun main(args: Array<String>) {
    val map = mapOf(1 to listOf("one", "uno"), 2 to listOf("two", "dos"))
    val flatMapped = map.flatMap { it.value }
    println(flatMapped)
}

Output

[one, uno, two, dos]

2 Flatten a map of lists of integers with transformation

In this example,

  • We create a map where keys are strings and values are lists of integers.
  • We use the flatMap function on the map, applying a lambda that maps each integer in the lists to its double value.
  • The resulting list contains all elements from the transformed lists in the map.
  • We print the flattened and transformed list.

Kotlin Program

fun main(args: Array<String>) {
    val map = mapOf("A" to listOf(1, 2), "B" to listOf(3, 4))
    val flatMapped = map.flatMap { it.value.map { it * 2 } }
    println(flatMapped)
}

Output

[2, 4, 6, 8]

3 Flatten a list of objects with lists as properties

In this example,

  • We define a data class Person with a name and a list of hobbies.
  • We create a list of Person objects, each with a name and a list of hobbies.
  • We use the flatMap function on the list of Person objects, applying a lambda that accesses the hobbies list of each person and flattens them into a single list.
  • The resulting list contains all hobbies from all people in the list.
  • We print the flattened list of hobbies.

Kotlin Program

fun main(args: Array<String>) {
    data class Person(val name: String, val hobbies: List<String>)
    val people = listOf(Person("Alice", listOf("Reading", "Running")), Person("Bob", listOf("Swimming", "Singing")))
    val flatMapped = people.flatMap { it.hobbies }
    println(flatMapped)
}

Output

[Reading, Running, Swimming, Singing]

Summary

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