How are Kotlin Array's toList and asList different?

Question

The Kotlin Array class offers asList(), toList(), and toMutableList() methods. The first two methods both return a List and are described in the Kotlin reference as follows:

• asList() returns a List that wraps the original Array.
• toList() returns a List containing all elements [of the original Array].

These methods appear interchangeable. How do these two methods differ in practice?

Answer 1

TL;DR

The list created with asList keeps a reference to the original Array.
The list created with toList/toMutableList is backed by a copy of the original Array.

Explanation

asList

The asList function creates a list that reuses the same Array instance, which implies that changes to the original array also have impact on the List:

val arr = arrayOf(1, 2, 3) val l1 = arr.asList()  arr[0] = 4 println(l1) // [4, 2, 3] 

toList

This isn't the case for toList/toMutableList since the array is copied:

val arr = arrayOf(1, 2, 3) val l2 = arr.toList()  arr[0] = 4 println(l2) // [1, 2, 3] 

The Kotlin source code can be found here.

Answer 2

Basically asList() still maintains a reference to the original Array. That means mutations to that list will also mutate the underlying Array.

toList() is simply copying the values of the Array into a new List, but there is no lingering link afterwards.

For most use-cases, they probably are interchangeable. asList() will likely have slightly better performance (since it isn't performing a copy) and toList() will be a "safe" copy against unexpected mutations.