Arrays and indexed access

A fixed row of marks can use an array, where indexed positions refer to elements. Kotlin distinguishes generic Array from specialized IntArray, which avoids boxing each integer on the JVM. Array size is fixed, although its elements can be replaced.

Zero is the first index, and size is one past the last valid position. An invalid subscript throws instead of returning null. getOrNull provides a nullable lookup. Assigning an array to another variable shares its reference; use copyOf for a separate array value.

Connect this to read-only and mutable lists and classes, objects and shared references.

Save the complete source as Main.kt and use the Kotlin/JVM environment. No separate input file is needed.

Kotlin/JVM · complete program

fun main() {
  val scores = intArrayOf(70, 80)
  val alias = scores
  val copy = scores.copyOf()
  alias[0] = 75
  println(scores[0])
  println(copy[0])
  println(scores.getOrNull(5))
}

Expected output

75
70
null

Follow the execution

Alias and scores refer to the same array, so the update is visible as 75. Copy retains 70 because copyOf created separate element storage. The absent indexed lookup returns null.

Try this in the Kotlin compiler by pasting the complete program.

Your task: Does val prevent alias[0] from changing?

Kotlin/JVM · complete program

fun main() {
  val scores = intArrayOf(70, 80)
  val alias = scores
  val copy = scores.copyOf()
  alias[0] = 75
  println(scores[0])
  println(copy[0])
  println(scores.getOrNull(5))
}

Worked answer

No. Val prevents reassigning the array reference, not replacement of its mutable elements.

An array copy is shallow if its elements are object references. Copying their slots does not clone the objects. For growable ordered collections, use an appropriate list instead of pretending append can change an array’s fixed size.

No. Val prevents reassigning the array reference, not replacement of its mutable elements. An array copy is shallow if its elements are object references.

Further reading: official Kotlin documentation for this topic.