How to initialize an array in Kotlin?

Small tutorial with code examples and video about arrays in Kotlin
18 August 2017   7330

What is array in Kotlin?

According to the official website, arrays in Kotlin are represented by the Array class, that has get and set functions (that turn into []by operator overloading conventions), and size property, along with a few other useful member functions:

class Array<T> private constructor() {
    val size: Int
    operator fun get(index: Int): T
    operator fun set(index: Int, value: T): Unit

    operator fun iterator(): Iterator<T>
    // ...

To create an array, we can use a library function arrayOf() and pass the item values to it, so that arrayOf(1, 2, 3) creates an array [1, 2, 3]. Alternatively, the arrayOfNulls() library function can be used to create an array of a given size filled with null elements.

Another option is to use a factory function that takes the array size and the function that can return the initial value of each array element given its index:

// Creates an Array<String> with values ["0", "1", "4", "9", "16"]
val asc = Array(5, { i -> (i * i).toString() })

As we said above, the [] operation stands for calls to member functions get() and set().

Note: unlike Java, arrays in Kotlin are invariant. This means that Kotlin does not let us assign an Array<String> to an Array<Any>, which prevents a possible runtime failure.

How to initialize an array in Kotlin?

Kotlin also has specialized classes to represent arrays of primitive types without boxing overhead: ByteArrayShortArrayIntArray and so on. These classes have no inheritance relation to the Array class, but they have the same set of methods and properties. Each of them also has a corresponding factory function:

val x: IntArray = intArrayOf(1, 2, 3)
x[0] = x[1] + x[2]

The video above will get you familiar with arrays in Kotlin.

JetBrains to Roll Out Kotlin/Native 0.8

Version 0.8 of Kotlin/Native brings new features and bug fixes
13 July 2018   138

The project team of Kotlin / Native introduced the release of version 0.8. Corrections and additions have concerned the security of multithreaded programming, the expansion of library functionality and support for iOS devices. As always, alot of bugs are fixed.


Implemented the concept of object freezing

object Config {
  var width: Int = 10
  private set
  init {
    val file = fopen("config.txt", "r")
    if (file != null) {
              try {
              val buffer = ByteArray(2 * 1024)
                 while (true) {
                 val nextLine = fgets(buffer.refTo(0), buffer.size, file)?.toKString()
                 if (nextLine == null || nextLine.isEmpty()) break
                 val records = nextLine.split('=')
                 if (records.size != 2) continue
                 val key = records[0].trim()
                 val value = records[1].trim()
                 when (key) {
                         "width" -> width = value.toInt()
             } finally {

This code considers the configuration file and will later receive the Config.width property. 

The Config file itself is divided for any number of threads or vorkers. After publication, the object is frozen and any attempt to change it will result in an exception (InvalidMutabilityException).

The second peculiarity of parallelism in the Kotlin / Native project is indivisible values and references. Let's add an image:

object Config {
private val icon_ = AtomicReference()
val icon: Image
get() = icon_.get()!!
fun updateIcon(file: String) {
val imageData = Image(file)
icon_.compareAndSwap(icon_.get(), imageData.freeze())

All information about the icon in the AtomicReference is frozen. Any changes are indivisible and similar information is visible to each client.

Kotlin Libraries

Starting v 0.8, the standard library (kotlin. *) is compatible with other platforms. Standard number generator and the function of mixing the collections are designed to eliminate the use of sharpened for a specific platform API. As an experiment, the project supports other libraries, for example, kotlinx.coroutines, kotlinx.serialization, and also the Ktor HTTP client.

Changes for iOS devices

Fixed issues with the publication in the App Store and the initialization of the platform. Support for the 32-bit ARM architecture of iOS processors, which allows using Kotlin / Native on older devices from Apple added.