Playing around with generics in Kotlin, and though...

# announcements

[Rafa]

Playing around with generics in Kotlin, and thought this was interesting

class Example<T>(private val source: T){
    fun test(value: T?): T? = value
}

fun <T> create() = Example(null)

If we define the class to take in a non-nullable generic type, and still pass in null, Example(null) then the generic resolves to Example<Nothing?>. If we make the generic nullable T? then it resolves to Example<Nothing>.

I can't find anything online that confirms that null on a generic type returns "Nothing" and I guess it kind of make sense, but can anyone confirm as to how that happens?

[Shawn]

you may want to read these posts
https://proandroiddev.com/kotlins-nothing-type-946de7d464fb

https://blog.kotlin-academy.com/kotlins-nothing-its-usefulness-in-generics-5076a6a457f7

[Shawn]

to more directly answer your question, though, because `Nothing`and Nothing? are the “bottom” of the type system, the type of null as a literal parameter, absent any sort of annotation or projection that can tell the compiler that it is supposed to represent some other nullable type, has to resolve to Nothing?

[Pavlo Liapota]

Also note, that by default all generic types extend Any?, i.e. T : Any?, so T can be nullable.
You need to write

class Example<T : Any>

if you want that T can be non-nullable type only.
Then it would make sense to use T? in a function inside your class.

[Rafa]

Thanks for the answers and the resources!