This second thought seems to be well supported, since there is a built-in method that facilitates it...

# getting-started

[Chris Cordero]

This second thought seems to be well supported, since there is a built-in method that facilitates it: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/to-map.html

[rbares]

MutableMap implements the read-only Map interface, so the usual mechanic for this is along the lines of:

private val _myMap = mutableMapOf<Int, Int>()
val myMap : Map<Int, Int> = _myMap

so no need to copy everything unless you want to provide a snapshot of the current values instead

[Robert Jaros]

But remember this way all clients will still be able to cast foo.myMap as MutableMap<Int,Int> and modify its content.

[Robert Jaros]

You could use delegation to protect from casting without copying data:

class Foo {
    private val _myMap = mutableMapOf<Int, Int>()
    private class MapWrapper(map: Map<Int,Int>): Map<Int, Int> by map
    val myMap: Map<Int,Int> = MapWrapper(_myMap)
}

[Chris Cordero]

Neat stuff. Thanks everyone.