How to Use flatMap on List in Kotlin: Syntax and Examples

In Kotlin, you can use the flatMap function on a list to transform each element into a list and then flatten all those lists into a single list. It combines mapping and flattening in one step, making it easy to work with nested collections.

Syntax

The flatMap function takes a lambda that returns a list for each element. It applies this lambda to every element, then merges all resulting lists into one.

• list.flatMap { element -> listOfTransformedElements }
• The lambda returns a list for each element.
• The final result is a single flattened list.

kotlin

val result = list.flatMap { element -> listOf(element, element * 2) }

Example

This example shows how to use flatMap to double each number and keep the original, then flatten the results into one list.

kotlin

fun main() {
    val numbers = listOf(1, 2, 3)
    val doubledAndOriginal = numbers.flatMap { number -> listOf(number, number * 2) }
    println(doubledAndOriginal)
}

Output

[1, 2, 2, 4, 3, 6]

Common Pitfalls

A common mistake is using map instead of flatMap when you want a single flattened list. map returns a list of lists, not a flat list.

Also, ensure the lambda returns a list, not a single element.

kotlin

fun main() {
    val numbers = listOf(1, 2, 3)
    // Wrong: map returns List<List<Int>>
    val mapped = numbers.map { number -> listOf(number, number * 2) }
    println(mapped) // Output: [[1, 2], [2, 4], [3, 6]]

    // Correct: flatMap returns List<Int>
    val flatMapped = numbers.flatMap { number -> listOf(number, number * 2) }
    println(flatMapped) // Output: [1, 2, 2, 4, 3, 6]
}

Output

[[1, 2], [2, 4], [3, 6]]
[1, 2, 2, 4, 3, 6]

Quick Reference

• flatMap: transforms and flattens a list of lists into a single list.
• Use when each element maps to multiple elements.
• Returns a flat list, unlike map which returns nested lists.

Key Takeaways

Use flatMap to transform and flatten nested lists in one step.

The lambda inside flatMap must return a list for each element.

Avoid using map when you want a single flat list; it returns nested lists.

The result of flatMap is a single list containing all elements from the inner lists.

FlatMap is useful for working with nested or grouped data in Kotlin collections.