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I have class that internally maintains a mutable list, and I want to provide an immutable view on this list. Currently I'm using the following:

/**The list that actually stores which element is at which position*/
private val list: MutableList<T> = ArrayList()
/**Immutable view of [list] to the outside.*/
val listView: List<T> get() = list.toList()
First question: Can this be done easier
Second question: How can I test that listView is actually immutable. I guess reflections are necessary?
                That's not an immutable view. It's a read-only view. Important distinction. The contents can be changing by whoever has access to the backing MutableList.
– Tenfour04
                Jun 23, 2020 at 2:29
If you only needed the compile-time type to be immutable, you could simply upcast your list:
val listView: List<T> get() = list
(Though if you checked and downcast that to MutableList, you could make changes — and those would affect the original list.)
However, if you want full immutability, that's tricky.  I don't think there are any general-purpose truly immutable lists in the Kotlin stdlib.
Although List and MutableList look like two different types, in Kotlin/JVM they both compile down to the same type in the bytecode, which is mutable.  And even in Kotlin, while Iterable.toList() does return a new list, the current implementation* actually gives a MutableList that's been upcast to List.  (Though mutating it wouldn't change the original list.)
Some third-party libraries provide truly immutable collections, though; see this question.
And to check whether a List is mutable, you could use a simple type check:
if (listView is MutableList)
No need to use reflection explicitly.  (A type check like that is considered to be implicit reflection.)
(* That can change, of course.  It's usually a mistake to read too much into the current code, if it's not backed up by the documentation.)
        
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