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

Currently in Kotlin stdlib there are no implementations of List (Map) that would not also implement MutableList (MutableMap). However due to Kotlin's delegation feature the implementations become one liners:

class ImmutableList(private val inner:List) : List by inner
class ImmutableMap(private val inner: Map) : Map by inner

You can also enhance the creation of the immutable counterparts with extension methods:

fun  Map.toImmutableMap(): Map {
    if (this is ImmutableMap) {
        return this
    } else {
        return ImmutableMap(this)
    }
}

fun  List.toImmutableList(): List {
    if (this is ImmutableList) {
        return this
    } else {
        return ImmutableList(this)
    }
}

The above prevents a caller from modifying the List (Map) by casting to a different class. However there are still reasons to create a copy of the original container to prevent subtle issues like ConcurrentModificationException:

class ImmutableList private constructor(private val inner: List) : List by inner {
    companion object {
        fun  create(inner: List) = if (inner is ImmutableList) {
                inner
            } else {
                ImmutableList(inner.toList())
            }
    }
}

class ImmutableMap private constructor(private val inner: Map) : Map by inner {
    companion object {
        fun  create(inner: Map) = if (inner is ImmutableMap) {
            inner
        } else {
            ImmutableMap(hashMapOf(*inner.toList().toTypedArray()))
        }
    }
}

fun  Map.toImmutableMap(): Map = ImmutableMap.create(this)
fun  List.toImmutableList(): List = ImmutableList.create(this)

While the above is not hard to implement there are already implementations of immutable lists and maps in both Guava and Eclipse-Collections.