first, last

first, last

These functions return the first and the last element of the collection correspondingly. You can also use them with a predicate; in this case, they return the first or the last element that matches the given predicate.

If a collection is empty or doesn't contain elements matching the predicate, the functions throw NoSuchElementException.

fun main() {

    val numbers = listOf(1, -2, 3, -4, 5, -6)            // 1
    
    val first = numbers.first()                          // 2
    val last = numbers.last()                            // 3
    
    val firstEven = numbers.first { it % 2 == 0 }        // 4
    val lastOdd = numbers.last { it % 2 != 0 }           // 5

    println("Numbers: $numbers")
    println("First $first, last $last, first even $firstEven, last odd $lastOdd")
}

1. Defines a collection of numbers.
2. Picks the first element.
3. Picks the last element.
4. Picks the first even element.
5. Picks the last odd element.

firstOrNull, lastOrNull

These functions work almost the same way with one difference: they return null if there are no matching elements.

fun main() {

   val words = listOf("foo", "bar", "baz", "faz")         // 1
   val empty = emptyList<String>()                        // 2
   
   val first = empty.firstOrNull()                        // 3
   val last = empty.lastOrNull()                          // 4
   
   val firstF = words.firstOrNull { it.startsWith('f') }  // 5
   val firstZ = words.firstOrNull { it.startsWith('z') }  // 6
   val lastF = words.lastOrNull { it.endsWith('f') }      // 7
   val lastZ = words.lastOrNull { it.endsWith('z') }      // 8

   println("Empty list: first is $first, last is $last")
   println("Word list: first item starting with 'f' is $firstF, first item starting with 'z' is $firstZ")
   println("Word list: last item ending with 'f' is $lastF, last item ending with 'z' is $lastZ")
}

1. Defines a collection of words.
2. Defines an empty collection.
3. Picks the first element from empty collection. It supposed to be null.
4. Picks the last element from empty collection. It supposed to be null as well.
5. Picks the first word starting with 'f'.
6. Picks the first word starting with 'z'.
7. Picks the last word ending with 'f'.
8. Picks the last word ending with 'z'.