val l1 = listOf(1) + listOf(2)
val l2 = listOf(1, 2)
println(l1::class) // prints java.util.ArrayList
println(l2::class) // prints java.util.Arrays$ArrayList
is an implementation detail. In the unlikely event you care about the actual type of list, you can always instantiate the type you want directly. I explained more about the two different classes in my reply to your other thread.