How to add elements to CopyInWriteCollection using Kotlin style? - kotlin

Question

Assume we have a custom collection
class CopyOnWriteCollection<T> {
// returns copy of collection with new element
fun add(element: T): CopyOnWriteCollection<T> {
...
}
}
if i need to add several elements i would do something like this:
val newCollection = oldCollection
.add(1)
.add(2)
.add(3)
And newCollection contains elements from oldCollection and also contains 1,2,3.
Perfect!
But how can i add elements from another collection using forEach of map?
val collection = CopyOnWriteCollection()
(1..3).forEach { collection.add(it) } // this approach works only with mutable collections

Answer 1

You can use an imperative loop, or you can use the fold()function:
fun main() {
var collection = CopyOnWriteCollection<Int>()
var collection2 = collection
for (i in 1..3) {
collection = collection.add(i)
}
println(collection)
collection2 = (1..3).fold(collection2) { coll, i -> coll.add(i) }
println(collection2)
}
class CopyOnWriteCollection<T> {
private val list = mutableListOf<T>()
// returns copy of collection with new element
fun add(element: T): CopyOnWriteCollection<T> {
val copy = CopyOnWriteCollection<T>()
copy.list.addAll(this.list)
copy.list.add(element)
return copy;
}
override fun toString() = list.toString()
}

Answer 2

If the CopyOnWriteCollection class is under your control, I'd approach this by adding an addAll() method to it:
/** Returns copy of collection with new element. */
fun addAll(elements: Collection<T>): CopyOnWriteCollection<T> {
// ...
}
You could then call that with e.g.
val newCollection = oldCollection.addAll(listOf(1, 2, 3))
(Or you could take a vararg instead of a collection.)
That's likely to take a lot less time and memory.
(Also, if you really need to write your own collection class, I'd strongly recommend implementing Collection (or one of its subinterfaces if appropriate), or extending a class which does.  That will give access to the huge range of extension methods and other goodies in the stdlib.)