Is it possible to create a static extension function to a Ja

Question

Is it possible to create a static extension function to a Java class It doesn t have a Companion to attach it to but the non static version is genericized

Shawn · Answer

Extension functions are all static and yes you can write extension functions for Java classes

locke · Answer

Then what would the signature be `fun lt Class gt lt function gt lt arguments gt ` yields a request for a type argument from the compiler and ` lt Class gt Companion lt function gt lt arguments gt ` yields an unresolved reference

Shawn · Answer

I think I might ve misunderstood what you meant but let s tackle the syntax first

Shawn · Answer

`fun JavaClass function args ` should work just fine where do the type arguments come into play

Shawn · Answer

Also technically you can define a generic extension function like this ``` fun lt T gt T function args ```

locke · Answer

The fact that `JavaClass` is genericized and so the compiler requests a type argument i e it s scoping that function to only `JavaClass lt T gt ` I am trying to get it to work with just `JavaClass function ` without needing to give it a type argument

Shawn · Answer

Ah I see what you mean now

Shawn · Answer

Should just be able to use star projection here if the type isn t relevant

Shawn · Answer

``` fun JavaClass lt gt function ```

locke · Answer

I am currently trying using that is losing access to static operators For context I m trying to add a composite function to `Observable` in RxJava It s using both `fromIterable` and `interval` that are static on `Observable` I guess since they re static I can just use `Observable fromIterable` instead of `this fromIterable` but that certainly feels like a code smell

Shawn · Answer

why is that a code smell Calling static methods by way of an instance is the code smell

locke · Answer

In my brain `this` should refer to the `Companion` in this context and thus those methods should be available in scope without needing to define it I e just calling `fromIterable` which is implicitly `this fromIterable`

locke · Answer

But I guess that s not how it works

locke · Answer

Though interestingly this method doesn t actually work it seems `Observable function` doesn t expose this extension

Shawn · Answer

expose in what sense

locke · Answer

You can t call it in client code `Observable function` doesn t actually reference the `Observable lt gt function` declaration I m sure if I had an instance of `Observable` I would have access but that s not the goal

Shawn · Answer

Are you trying to call extension methods from Java

karelpeeters · Answer

< Shawn> I may be misunderstanding but I think you missed that he wants static extension functions

Shawn · Answer

Ahh as though they were defined as a static method within the class body

locke · Answer

< Shawn> no I m calling them from Kotlin The goal is to be able to write an extension function such that I can in Kotlin declare `Observable myFunction lt arguments gt `

locke · Answer

Yes exactly

Shawn · Answer

that was my bad

locke · Answer

All good technical problems are hard to communicate slightly smiling face

locke · Answer

Is this even possible I am not sure it is

Shawn · Answer

Unfortunately I don t think it is confused

karelpeeters · Answer

No same for Kotlin classes that don t define a companion themselves

locke · Answer

That s unfortunate Oh well thanks guys

gildor · Answer

Please vote for this feature and share your use cases <https youtrack jetbrains com issue KT 11968>