Kotlin List mapNotNull()
Syntax & Examples

Syntax of List.mapNotNull()

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

fun <T, R : Any> Iterable<T>.mapNotNull( transform: (T) -> R? ): List<R>

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

✐ Examples

1 Example

In this example,

We create a list named list1 containing the integers 1, 2, 3.
We then apply the mapNotNull function to list1, which takes a lambda with one parameter: it.
Within the lambda, we check if the element is even. If it is, we return the element multiplied by 2; otherwise, we return null.
The mapNotNull function returns a new list containing only the non-null results.
The resulting list, containing doubled values of even numbers, is stored in result.
Finally, we print the value of result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf(1, 2, 3, 4);
    val result = list1.mapNotNull { if (it % 2 == 0) it * 2 else null }
    print(result);
}

Output

[4, 8]

2 Example

In this example,

We create a list named list1 containing the characters 'a', 'b', 'c'.
We then apply the mapNotNull function to list1, which takes a lambda with one parameter: it.
Within the lambda, we check if the element is equal to 'b'. If it is, we return the element; otherwise, we return null.
The mapNotNull function returns a new list containing only the non-null results.
The resulting list, containing only the element 'b', is stored in result.
Finally, we print the value of result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf(&quot;a&quot;, &quot;b&quot;, &quot;c&quot;);
    val result = list1.mapNotNull { if (it == &quot;b&quot;) it else null }
    print(result);
}

Output

[b]

3 Example

In this example,

We create a list named list1 containing the strings 'apple', 'banana', 'cherry'.
We then apply the mapNotNull function to list1, which takes a lambda with one parameter: it.
Within the lambda, we check if the length of the string is greater than 5. If it is, we return the element; otherwise, we return null.
The mapNotNull function returns a new list containing only the non-null results.
The resulting list, containing strings with a length greater than 5, is stored in result.
Finally, we print the value of result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf("apple", "banana", "cherry");
    val result = list1.mapNotNull { if (it.length > 5) it else null }
    print(result);
}

Output

[banana, cherry]

Summary

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