listOf() returns MutableList

Question

In Kotlin, a list created using the listOf() function (which should be immutable) passes a type check against MutableList using the is operator.

Example:

fun main(args: Array<String>) {
    val list = listOf("I'm immutable")

    println(list is MutableList)
}

will print

true

Interestingly, an empty list created using listOf<String>() will fail the check and print false as it returns the singleton object EmptyList.

After some digging, it turns out that mutableListOf() creates a java.util.ArrayList whilst listOf() ends up creating a java.util.Arrays$ArrayList, but none of the classes involved implement MutableList, so why does a non-empty list still pass the type check against it? Hence, is there an alternative way of reliably checking if a list is mutable without having to check against it's actual implementation (is ArrayList etc.)?

Answer 1

The separation between List and MutableList is an illusion created by the Kotlin compiler. At runtime, Kotlin uses Java collection classes, which only have a single List interface containing both read and mutation methods. Compile-time references to List and MutableList are both compiled to java.util.List references. Therefore, it's not possible to detect whether a list is a MutableList at runtime.