Kotlin Tutorials

Kotlin Set mapNotNullTo()
Syntax & Examples

Set.mapNotNullTo() extension function

The mapNotNullTo() extension function in Kotlin applies the given transform function to each element in the original set, and appends only the non-null results to the given destination.

Syntax of Set.mapNotNullTo()

The syntax of Set.mapNotNullTo() extension function is:

fun <T, R : Any, C : MutableCollection<in R>> Set<T>.mapNotNullTo(destination: C, transform: (T) -> R?): C

This mapNotNullTo() extension function of Set applies the given transform function to each element in the original collection and appends only the non-null results to the given destination.

Parameters

ParameterOptional/RequiredDescription
destinationrequiredThe collection to which the non-null results will be appended.
transformrequiredA function that takes an element and returns the transformed result, which may be null.

Return Type

Set.mapNotNullTo() returns value of type C.


✐ Examples

1 Transforming a set of integers by doubling each element, filtering out nulls, and appending to a list

Using mapNotNullTo() to transform a set of integers by doubling each element, filtering out null results, and appending them to a list.

For example,

  1. Create a set of integers.
  2. Create an empty mutable list to serve as the destination.
  3. Use mapNotNullTo() with a transform function that doubles each element and filters out nulls.
  4. Print the resulting list.

Kotlin Program

fun main() {
    val numbers = setOf(1, 2, 3, 4, 5)
    val destination = mutableListOf<Int>()
    numbers.mapNotNullTo(destination) { if (it % 2 == 0) it * 2 else null }
    println(destination)
}

Output

[4, 8]

2 Transforming a set of strings by getting their lengths, filtering out nulls, and appending to a list

Using mapNotNullTo() to transform a set of strings by getting the length of each string, filtering out null results, and appending them to a list.

For example,

  1. Create a set of strings.
  2. Create an empty mutable list to serve as the destination.
  3. Use mapNotNullTo() with a transform function that returns the length of each string, and filters out nulls.
  4. Print the resulting list.

Kotlin Program

fun main() {
    val strings = setOf("one", "two", "three")
    val destination = mutableListOf<Int>()
    strings.mapNotNullTo(destination) { if (it.length > 3) it.length else null }
    println(destination)
}

Output

[5]

3 Transforming a set of custom objects by extracting a specific property, filtering out nulls, and appending to a list

Using mapNotNullTo() to transform a set of custom objects by extracting a specific property, filtering out null results, and appending them to a list.

For example,

  1. Create a data class.
  2. Create a set of custom objects.
  3. Create an empty mutable list to serve as the destination.
  4. Use mapNotNullTo() with a transform function that extracts a specific property from each object, and filters out nulls.
  5. Print the resulting list.

Kotlin Program

data class Person(val name: String, val age: Int)

fun main() {
    val people = setOf(Person("Alice", 30), Person("Bob", 25), Person("Charlie", 35))
    val destination = mutableListOf<String>()
    people.mapNotNullTo(destination) { if (it.age > 30) it.name else null }
    println(destination)
}

Output

[Charlie]

Summary

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