Kotlin List singleOrNull()
Syntax & Examples

Syntax of List.singleOrNull()

There are 2 variations for the syntax of List.singleOrNull() extension function. They are:

fun <T> List<T>.singleOrNull(): T?

This extension function returns single element, or null if the list is empty or has more than one element.

fun <T> Iterable<T>.singleOrNull( predicate: (T) -> Boolean ): T?

This extension function returns the single element matching the given predicate, or null if element was not found or more than one element was found.

✐ Examples

1 Example

In this example,

We create a list named list containing an integer [8].
We call the singleOrNull() function on list to get the single element, if exists, or null if the list is empty or has more than one element.
Since the list has only one element, the resulting value is 8, and is stored in singleElement.
Finally, we print the singleElement to standard output using the println function.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf(8);
    val singleElement = list.singleOrNull();
    println(singleElement);
}

Output

2 Example

In this example,

We create a list named list containing the characters ['a', 'b', 'c'].
We call the singleOrNull function with a predicate on list to get the single element matching the given predicate, or null if no such element is found or more than one element is found.
The predicate checks if the element is equal to 'b'.
The result, which is the single element 'b' or null, is stored in singleElement.
Finally, we print the singleElement to standard output using the println function.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf('a', 'b', 'c');
    val singleElement = list.singleOrNull { it == 'b' };
    println(singleElement);
}

Output

Summary

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