Kotlin Map mapNotNull()
Syntax & Examples

Syntax of Map.mapNotNull()

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

fun <K, V, R : Any> Map<out K, V>.mapNotNull( transform: (Entry<K, V>) -> R? ): List<R>

This mapNotNull() extension function of Map returns a list containing only the non-null results of applying the given transform function to each entry in the original map.

✐ Examples

1 Filter keys by even values and return non-null results

In this example,

We create a map named map1 with key-value pairs.
We use the mapNotNull function on map1, applying a transform function that returns the key if its corresponding value is even, otherwise null.
The non-null results are collected into a list.
We print the resulting list to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map1 = mapOf("key1" to 1, "key2" to 2, "key3" to 3)
    val result = map1.mapNotNull { (key, value) -> if (value % 2 == 0) key else null }
    println(result)
}

Output

[key2]

2 Filter and return non-null values from the map

In this example,

We create a map named map2 with key-value pairs, including null values.
We use the mapNotNull function on map2, applying a transform function that returns the value if it is not null.
The non-null values are collected into a list.
We print the resulting list to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map2 = mapOf("apple" to null, "banana" to 6, "cherry" to 7)
    val result = map2.mapNotNull { (_, value) -> value }
    println(result)
}

Output

[6, 7]

3 Return non-null values from the map

In this example,

We create a map named map3 with key-value pairs, including a null value.
We use the mapNotNull function on map3, applying a transform function that returns the value if it is not null.
The non-null values are collected into a list.
We print the resulting list to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map3 = mapOf(1 to "one", 2 to "two", 3 to null)
    val result = map3.mapNotNull { (_, value) -> value }
    println(result)
}

Output

[one, two]

Summary

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