# Passing Map with Non-Null keys to function that takes map with nullable keys? **URL:** \ **Category:** Support\ **Created:** [September 20, 2019, 9:26pm UTC](https://discuss.kotlinlang.org/t/passing-map-with-non-null-keys-to-function-that-takes-map-with-nullable-keys/14347 "2019-09-20T21:26:34Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![diathesis](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/diathesis/32/4410_2.png) [@diathesis](https://discuss.kotlinlang.org/u/diathesis)\ **Post date:** [September 20, 2019, 9:26pm UTC](https://discuss.kotlinlang.org/t/passing-map-with-non-null-keys-to-function-that-takes-map-with-nullable-keys/14347/1 "2019-09-20T21:26:34Z")
Say I have a function: ``` fun doSomething( mapping: Map ) ``` And I have a map: ``` val mapping = mapOf( "One": true, "Two": false ) ``` I can’t pass it in directly: ``` doSomething(mapping) ``` Because Kotlin says: ``` Required: Map Found: Map ``` I understand that a Map\ isn’t strictly the same thing as a Map\ – so what’s the simplest way to convert the map so that I can pass it in? I feel like there ought to be a simple way to do this, but I haven’t figured out what the simple way is. 😉 ---
**Author:** ![rrlynx](https://avatars.discourse-cdn.com/v4/letter/r/9de0a6/32.png) [@rrlynx](https://discuss.kotlinlang.org/u/rrlynx)\ **Post date:** [September 20, 2019, 11:26pm UTC](https://discuss.kotlinlang.org/t/passing-map-with-non-null-keys-to-function-that-takes-map-with-nullable-keys/14347/2 "2019-09-20T23:26:33Z")
You have to either specify the type of `mapping` variable or call `mapOf` with explicit type parameters. ---
**Author:** ![Wasabi375](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/wasabi375/32/4741_2.png) [@Wasabi375](https://discuss.kotlinlang.org/u/Wasabi375)\ **Post date:** [September 20, 2019, 11:33pm UTC](https://discuss.kotlinlang.org/t/passing-map-with-non-null-keys-to-function-that-takes-map-with-nullable-keys/14347/3 "2019-09-20T23:33:19Z")
The problem is that the generic type for the key is invariant. You can however still pass the map like this ```auto doSomething(mapping as Map) ``` This will however generate an unchecked cast warning, which you can suppress. ---
**Author:** ![diathesis](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/diathesis/32/4410_2.png) [@diathesis](https://discuss.kotlinlang.org/u/diathesis)\ **Post date:** [September 20, 2019, 11:45pm UTC](https://discuss.kotlinlang.org/t/passing-map-with-non-null-keys-to-function-that-takes-map-with-nullable-keys/14347/4 "2019-09-20T23:45:49Z")
> You have to either specify the type of `mapping` variable or call `mapOf` with explicit type parameters. Yeah, sorry, the example is a simplification of reality. I have a `Map` that I get from somewhere else, I know how to make my own `Map`, I’m just wondering what the cleanest way of converting the map I already have to the nullable-key version is. ---
**Author:** ![diathesis](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/diathesis/32/4410_2.png) [@diathesis](https://discuss.kotlinlang.org/u/diathesis)\ **Post date:** [September 20, 2019, 11:48pm UTC](https://discuss.kotlinlang.org/t/passing-map-with-non-null-keys-to-function-that-takes-map-with-nullable-keys/14347/5 "2019-09-20T23:48:24Z")
> You can however still pass the map like this Huh – I could swear I tried a cast (yay type erasure), but I just tried again and, yes, it works. Weird. Not sure why my attempt before failed now. 😉 ---
**Author:** ![gidds](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/gidds/32/10865_2.png) [@gidds](https://discuss.kotlinlang.org/u/gidds)\ **Post date:** [September 21, 2019, 11:47am UTC](https://discuss.kotlinlang.org/t/passing-map-with-non-null-keys-to-function-that-takes-map-with-nullable-keys/14347/6 "2019-09-21T11:47:52Z")
Note that such a cast is unsafe. The implementation of `doSomething()` could call `get(null)` on the map (which would be perfectly legal, as its parameter explicitly allows a null key). The cast could then cause a null-pointer exception. ---
**Author:** ![diathesis](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/diathesis/32/4410_2.png) [@diathesis](https://discuss.kotlinlang.org/u/diathesis)\ **Post date:** [September 22, 2019, 10:49pm UTC](https://discuss.kotlinlang.org/t/passing-map-with-non-null-keys-to-function-that-takes-map-with-nullable-keys/14347/7 "2019-09-22T22:49:11Z")
> The cast could then cause a null-pointer exception. Sorry, where’s the potential NPE? The biggest potential problem to me is that doSomething() could insert a null into the map which the caller could then access, not expecting a potential null, but that’s always a bit of a risk with generics given type erasure. ``` >>> fun doSomething( mapping: Map ) { ... println("mapping[null]: ${mapping[null]}") ... } >>> doSomething(mapOf(null to false)) mapping[null]: false >>> doSomething(mapOf("A" to true) as Map) mapping[null]: null ``` This seems … not terribad. ---
**Author:** ![Wasabi375](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/wasabi375/32/4741_2.png) [@Wasabi375](https://discuss.kotlinlang.org/u/Wasabi375)\ **Post date:** [September 23, 2019, 5:32am UTC](https://discuss.kotlinlang.org/t/passing-map-with-non-null-keys-to-function-that-takes-map-with-nullable-keys/14347/8 "2019-09-23T05:32:37Z")
The problem could be if the map is not backed by a java map but by a custom implementation in kotlin. In that case `mapping[null]` will fail with a NPE. ```auto class MyMap(val m: Map): Map by m { override operator fun get(key: K): V? = m[key] // kotlin will generate a null check for key, which would lead your code to fail } ``` That said, as long as you are sure that you are only dealing with java standard library maps you should be fine. ---
**Author:** ![diathesis](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/diathesis/32/4410_2.png) [@diathesis](https://discuss.kotlinlang.org/u/diathesis)\ **Post date:** [September 23, 2019, 2:22pm UTC](https://discuss.kotlinlang.org/t/passing-map-with-non-null-keys-to-function-that-takes-map-with-nullable-keys/14347/9 "2019-09-23T14:22:44Z")
Fair enough. In this case it’s definitely being created by `mapOf()`, which in its current implementation seems to be a `LinkedHashMap`, although there’s no guarantee that can’t change in the future, so there’s still some risk there in theory. ---
**Author:** ![temnur](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/temnur/32/7005_2.png) [@temnur](https://discuss.kotlinlang.org/u/temnur)\ **Post date:** [December 6, 2019, 2:32pm UTC](https://discuss.kotlinlang.org/t/passing-map-with-non-null-keys-to-function-that-takes-map-with-nullable-keys/14347/10 "2019-12-06T14:32:17Z")
For me it compiles, I had just seen it somewhere standard code: ```auto fun doSomething(mapping: Map): Unit {} val mapping = mapOf("One" to true, "Two" to false) doSomething(mapping) ``` ---
**Author:** ![Wasabi375](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/wasabi375/32/4741_2.png) [@Wasabi375](https://discuss.kotlinlang.org/u/Wasabi375)\ **Post date:** [December 6, 2019, 4:06pm UTC](https://discuss.kotlinlang.org/t/passing-map-with-non-null-keys-to-function-that-takes-map-with-nullable-keys/14347/11 "2019-12-06T16:06:40Z")
You are using `Map` instead of `Map`. But you might not always be in controll of the function.