Arrays in Kotlin

Generic Arrays

Generic arrays in Kotlin are represented by Array<T>.

To create an empty array, use emptyArray<T>() factory function:

val empty = emptyArray<String>()

To create an array with given size and initial values, use the constructor:

var strings = Array<String>(size = 5, init = { index -> "Item #$index" })
print(Arrays.toString(a)) // prints "[Item #0, Item #1, Item #2, Item #3, Item #4]"
print(a.size) // prints 5

Arrays have get(index: Int): T and set(index: Int, value: T) functions:

strings.set(2, "ChangedItem")
print(strings.get(2)) // prints "ChangedItem"

// You can use subscription as well:
strings[2] = "ChangedItem"
print(strings[2]) // prints "ChangedItem"

Arrays of Primitives

These types do not inherit from Array<T> to avoid boxing, however, they have the same attributes and methods.

Kotlin type	Factory function	JVM type
BooleanArray	booleanArrayOf(true, false)	boolean[]
ByteArray	byteArrayOf(1, 2, 3)	byte[]
CharArray	charArrayOf('a', 'b', 'c')	char[]
DoubleArray	doubleArrayOf(1.2, 5.0)	double[]
FloatArray	floatArrayOf(1.2, 5.0)	float[]
IntArray	intArrayOf(1, 2, 3)	int[]
LongArray	longArrayOf(1, 2, 3)	long[]
ShortArray	shortArrayOf(1, 2, 3)	short[]

Extensions

average() is defined for Byte, Int, Long, Short, Double, Float and always returns Double:

val doubles = doubleArrayOf(1.5, 3.0)
print(doubles.average()) // prints 2.25

val ints = intArrayOf(1, 4)
println(ints.average()) // prints 2.5

component1(), component2(), ... component5() return an item of the array

getOrNull(index: Int) returns null if index is out of bounds, otherwise an item of the array

first(), last()

toHashSet() returns a HashSet<T> of all elements

sortedArray(), sortedArrayDescending() creates and returns a new array with sorted elements of current

sort(), sortDescending sort the array in-place

min(), max()

Iterate Array

You can print the array elements using the loop same as the Java enhanced loop, but you need to change keyword from : to in.

val asc = Array(5, { i -> (i * i).toString() })
for(s : String in asc){
    println(s);
}

You can also change data type in for loop.

val asc = Array(5, { i -> (i * i).toString() })
for(s in asc){
    println(s);
}

Create an array

val a = arrayOf(1, 2, 3) // creates an Array<Int> of size 3 containing [1, 2, 3].

Create an array using a closure

val a = Array(3) { i -> i * 2 } // creates an Array<Int> of size 3 containing [0, 2, 4]

Create an uninitialized array

val a = arrayOfNulls<Int>(3) // creates an Array<Int?> of [null, null, null]

The returned array will always have a nullable type. Arrays of non-nullable items can't be created uninitialized.