How to turn a Mutable Collection into an Immutable one

Question

I was writing a small piece of code in which I internally handle my data in a mutable map, which in turn has mutable lists.

I wanted to expose my data to the API use

Answer 1

A classic solution is to copy your data, so that even if modified, the change would not affect the private class property:

class School {
    private val roster = mutableMapOf>()

    fun db(): Map> = roster.mapValuestTo {it.value.toList}
}