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?
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.
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.
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.
