What is the smartest way to copy a Map in Kotlin?

Question

I'd like to get a new instance of some Map with the same content but Map doesn't have a built-in copy method. I can do something like this:

val newInst = someMap.map { it.toPair() }.toMap()

But it looks rather ugly. Is there any more smarter way to do this?

Answer 1

Just use the HashMap constructor:

val original = hashMapOf(1 to "x")
val copy = HashMap(original)

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Update for Kotlin 1.1:

Since Kotlin 1.1, the extension functions Map.toMap and Map.toMutableMap create copies.

Answer 2

Use putAll method:

val map = mapOf("1" to 1, "2" to 2)
val copy = hashMapOf<String, Int>()
copy.putAll(map)

Or:

val map = mapOf("1" to 1, "2" to 2)
val copy = map + mapOf<String, Int>() // preset

Your way also looks idiomatic to me.

Answer 3

The proposed way of doing this is:

map.toList().toMap()

However, the java's method is 2 to 3 times faster:

(map as LinkedHashMap).clone()

Anyway, if it bothers you that there is no unified way of cloning Kotlin's collections (and there is in Java!), vote here: https://youtrack.jetbrains.com/issue/KT-11221