Kotlin List single()
Syntax & Examples

Syntax of List.single()

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

fun <T> List<T>.single(): T

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

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

This extension function returns the single element matching the given predicate, or throws exception if there is no or more than one matching element.

✐ Examples

1 Example

In this example,

We create a list of integers named list containing the element 42.
We use the single() function to get the single element from the list.
The result, which is 42, is stored in result.
We print result using println().

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf(42)
    val result = list.single()
    println(result)
}

Output

2 Example

In this example,

We create a list of characters named list containing elements 'a', 'b', 'c'.
We use the single function with a predicate that checks if an element is equal to 'b'.
The result, which is 'b', is stored in result.
We print result using println().

Kotlin Program

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

Output

3 Example

In this example,

We create a list of strings named list containing the element "apple".
We use the single function with a predicate that checks if the length of the element is greater than 5.
Since there is no element in the list that satisfies the predicate, calling single() with this predicate throws an exception.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf("apple")
    val result = list.single { it.length > 5 }
    println(result)
}

Output

Exception in thread "main" java.util.NoSuchElementException: Collection contains no element matching the predicate.

Summary

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