Retrieve single elements

Kotlin collections provide a set of functions for retrieving single elements from collections. Functions described on this page apply to both lists and sets.

As the definition of list says, a list is an ordered collection. Hence, every element of a list has its position that you can use for referring. In addition to functions described on this page, lists offer a wider set of ways to retrieve and search for elements by indices. For more details, see List-specific operations.

In turn, set is not an ordered collection by definition. However, the Kotlin Set stores elements in certain orders. These can be the order of insertion (in LinkedHashSet), natural sorting order (in SortedSet), or another order. The order of a set of elements can also be unknown. In such cases, the elements are still ordered somehow, so the functions that rely on the element positions still return their results. However, such results are unpredictable to the caller unless they know the specific implementation of Set used.

Retrieve by position

For retrieving an element at a specific position, there is the function elementAt(). Call it with the integer number as an argument, and you'll receive the collection element at the given position. The first element has the position 0, and the last one is (size - 1).

elementAt() is useful for collections that do not provide indexed access, or are not statically known to provide one. In case of List, it's more idiomatic to use indexed access operator (get() or []).

fun main() {
//sampleStart
    val numbers = linkedSetOf("one", "two", "three", "four", "five")
    println(numbers.elementAt(3))    

    val numbersSortedSet = sortedSetOf("one", "two", "three", "four")
    println(numbersSortedSet.elementAt(0)) // elements are stored in the ascending order
//sampleEnd
}

There are also useful aliases for retrieving the first and the last element of the collection: first() and last().

fun main() {
//sampleStart
    val numbers = listOf("one", "two", "three", "four", "five")
    println(numbers.first())    
    println(numbers.last())    
//sampleEnd
}

To avoid exceptions when retrieving element with non-existing positions, use safe variations of elementAt():

elementAtOrNull() returns null when the specified position is out of the collection bounds.
elementAtOrElse() additionally takes a lambda function that maps an Int argument to an instance of the collection element type. When called with an out-of-bounds position, the elementAtOrElse() returns the result of the lambda on the given value.

fun main() {
//sampleStart
    val numbers = listOf("one", "two", "three", "four", "five")
    println(numbers.elementAtOrNull(5))
    println(numbers.elementAtOrElse(5) { index -> "The value for index $index is undefined"})
//sampleEnd
}

Retrieve by condition

Functions first() and last() also let you search a collection for elements matching a given predicate. When you call first() with a predicate that tests a collection element, you'll receive the first element on which the predicate yields true. In turn, last() with a predicate returns the last element matching it.

fun main() {
//sampleStart
    val numbers = listOf("one", "two", "three", "four", "five", "six")
    println(numbers.first { it.length > 3 })
    println(numbers.last { it.startsWith("f") })
//sampleEnd
}

If no elements match the predicate, both functions throw exceptions. To avoid them, use firstOrNull() and lastOrNull() instead: they return null if no matching elements are found.

fun main() {
//sampleStart
    val numbers = listOf("one", "two", "three", "four", "five", "six")
    println(numbers.firstOrNull { it.length > 6 })
//sampleEnd
}

Use the aliases if their names suit your situation better:

find() instead of firstOrNull()
findLast() instead of lastOrNull()

fun main() {
//sampleStart
    val numbers = listOf(1, 2, 3, 4)
    println(numbers.find { it % 2 == 0 })
    println(numbers.findLast { it % 2 == 0 })
//sampleEnd
}

Retrieve with selector

If you need to map the collection before retrieving the element, there is a function firstNotNullOf(). It combines 2 actions:

Maps the collection with the selector function
Returns the first non-null value in the result

firstNotNullOf() throws the NoSuchElementException if the resulting collection doesn't have a non-nullable element. Use the counterpart firstNotNullOfOrNull() to return null in this case.

fun main() {
//sampleStart
    val list = listOf<Any>(0, "true", false)
    // Converts each element to string and returns the first one that has required length
    val longEnough = list.firstNotNullOf { item -> item.toString().takeIf { it.length >= 4 } }
    println(longEnough)
//sampleEnd
}

Random element

If you need to retrieve an arbitrary element of a collection, call the random() function. You can call it without arguments or with a Random object as a source of the randomness.

fun main() {
//sampleStart
    val numbers = listOf(1, 2, 3, 4)
    println(numbers.random())
//sampleEnd
}

On empty collections, random() throws an exception. To receive null instead, use randomOrNull()

Check element existence

To check the presence of an element in a collection, use the contains() function. It returns true if there is a collection element that equals() the function argument. You can call contains() in the operator form with the in keyword.

To check the presence of multiple instances together at once, call containsAll() with a collection of these instances as an argument.

fun main() {
//sampleStart
    val numbers = listOf("one", "two", "three", "four", "five", "six")
    println(numbers.contains("four"))
    println("zero" in numbers)
    
    println(numbers.containsAll(listOf("four", "two")))
    println(numbers.containsAll(listOf("one", "zero")))
//sampleEnd
}

Additionally, you can check if the collection contains any elements by calling isEmpty() or isNotEmpty().

fun main() {
//sampleStart
    val numbers = listOf("one", "two", "three", "four", "five", "six")
    println(numbers.isEmpty())
    println(numbers.isNotEmpty())
    
    val empty = emptyList<String>()
    println(empty.isEmpty())
    println(empty.isNotEmpty())
//sampleEnd
}

Last modified: 26 十一月 2024