Kotlin: Is Read-Only List immutable?

A List is a container, which is an object that holds other objects. Containers are also called collections.

A List is read-only – you can read its contents but not write to list. However if the underlying implementation is a MutableList and you retain a mutable reference to that implementation, you can still modify it via that mutable reference.

fun main() {
  val a = mutableListOf(1)
  val b: List<Int> = a
  print(b) // [1]

  a += 2
  print(b) // [1,2]
}

As we can see, even thought the b is read only, the value can still be modified since it is referenced to the a which is MutableList.

Therefore, Read-Only List is still modifiable.