I have ```private val actions = mutableMapOf lt String SomeI

Question

I have ```private val actions = mutableMapOf lt String SomeInterface gt fun lt T SomeInterface gt getOrNull key String T = actions key as T val tryToGetAFoo = getOrNull lt Bar gt something I have a Foo in the something key so this should be null Bar and Foo are both implementation of SomeInterface ``` what is this error that I m getting `class Bar cannot be cast to class Foo Bar and Foo are in unnamed module of loader app `

Chris Lee · Answer

Would be expected A `Foo` is not a `Bar` even though they implement the same interface The `as T` combined with `getOrNull lt Bar gt ` casts everything to a `Bar`

dave08 · Answer

But in the docs it says that the `as ` operator returns null if it doesn t succeed in casting

Joffrey · Answer

And what does the compiler warning says on that line

Joffrey · Answer

This cast is unchecked meaning that at runtime there is no cast at all so it won t behave as you expect

dave08 · Answer

`class Bar cannot be cast to class Foo Bar and Foo are in unnamed module of loader app `

Chris Lee · Answer

In Kotlin playground val tryToGetAFoo = getOrNull lt Bar gt something

Chris Lee · Answer

Sorry <https pl kotl in 8sSlmqCXK>

dave08 · Answer

Right that s exactly the error

Joffrey · Answer

< dave08> the thing you wrote here is not the compiler warning that s the runtime error you get The warning should be something like `Unchecked cast SomeInterface to T` This means that even though you re writing an explicit cast here with `as ` it will not check the type of the value at runtime so the cast will never fail and the value will always be returned as is That is why the function happily returns a `Foo` even when called with ` lt Bar gt ` The cast that fails is probably not the one you wrote with `as ` but the one the compiler does automatically afterwards during the variable assignment To fix it you can use an inline function with `reified T` so the cast is not unchecked ```private inline fun lt reified T SomeInterface gt getOrNull key String T = actions key as T```

Chris Lee · Answer

Yep It works with reification ```inline fun lt reified T SomeInterface gt getOrNull key String T = actions key as T```

dave08 · Answer

Yeah but if you noticed in the question `actions` is private in the class so an inline function won t work here

Joffrey · Answer

Then make it ` PublishedApi internal`

Chris Lee · Answer

or have a helper function to bridge the visibility gap

dave08 · Answer

gt or have a helper function how I can t cast to a KClass

Chris Lee · Answer

dave08 · Answer

gt Then make it ` PublishedApi internal` It s used in the same module

dave08 · Answer

Oh that s good

Joffrey · Answer

If you don t want to make `actions` visible to the module you can either provide an untyped helper as suggested by Chris or make a typed helper that uses a `KClass` and then a reified helper for convenience ```inline fun lt reified T SomeInterface gt getOrNull key String T = getOrNull key T class fun lt T SomeInterface gt getOrNull key String type KClass lt T gt T = type safeCast actions key ```

dave08 · Answer

Does that `type safeCast` require the reflection libs

Joffrey · Answer

Ah maybe I just tried on the playground but it s in `kotlin reflect full` so there is a chance yes

Joffrey · Answer

Maybe Chris s option is better for your case but I m not sure it s much better than just exposing `actions` as a read only map to be frank smile

dave08 · Answer

True in general but here I m abstracting away the fact that the key is a value class internally

Chris Lee · Answer

exposing the map as readonly also allows changing `getOrNull` to be an extension function for a more natural feel ```val actions = mutableMapOf lt String SomeInterface gt interface SomeInterface class Foo SomeInterface class Bar SomeInterface inline fun lt reified T SomeInterface gt Map lt String SomeInterface gt getOrNull key String T = actions key as T public fun getAction key String SomeInterface return actions key I have a Foo in the something key so this should be null Bar and Foo are both implementation of SomeInterface You can edit run and share this code <http play kotlinlang org|play kotlinlang org> fun main actions foo = Foo actions bar = Bar val tryToGetAFoo = actions getOrNull lt Bar gt foo ```

Chris Lee · Answer

gt True in general but here I m abstracting away the fact that the key is a value class internally Is it really abstracted away though Having to pass in a key to getOrNull exposes that there s a key value relationship

dave08 · Answer

Yeah but outside of this class I need to allow access w o wrapping the key everywhere I need to access it s contents If I d be passing around the wrapped key which might happen in the future then I d probably change that

Joffrey · Answer

What s the point of wrapping the key if not to make consumers type safe

Joffrey · Answer

I would have expected the opposite internally you can use raw values but for your public API you want better types for your consumers safety

dave08 · Answer

I m refactoring so I need to do things step by step I don t want to break too much and have to rewrite all the unit tests at once

Joffrey · Answer

ah ok it sounded like an actual design decision not a temporary state for the refac Then I understand slightly smiling face