Putting element into HashMap with Map interface

Question

I'm trying Kotlin and I've encountered a small problem that I can't resolve. When I have the following construction I can put elements into the map:

val map = HashMap<String, String>()
map["asd"] = "s"
map.put("34", "354")

However when I create a map with the Map interface I can only read them, what I'm doing wrong ?

val map: Map<String, String> = HashMap<String, String>();
map.put("24", "34") //error
map["23"] = "23" //error

Or maybe I'm confusing something about interfaces in Kotlin ?

Solution

In the first example map gets the type of HashMap, in the second example you cast it to the Interface Map.

Map is a readonly map, there is no put/set, see here

In order to be able to edit the map, you should use MutableMap

Comments

[Big Kahuna]

Coming from Java, it's important to remember that Map is not coming from java.util.Map but from Map defined in Collections.kt. As a Kotlin newbie, I've been caught out by this as well. As mentioned above Map is readonly; to change the map use MutableMap. There's also helper methods to create different maps in Maps.kt i.e. hashMapOf etc