A quick questions on generics. Is it possible to e...

# announcements

[Martin Brehovsky]

A quick questions on generics. Is it possible to enforce non-nullable type from a nullable type? Something like this:

class Foo<T: Any?> {
  
    abstract fun returnValue(): T
    abstract fun returnValueAsNonNullable(): T!! // this doesn't work 

}

The problem is I need to define the T type as either nullable or non-nullable, but I'm trying to bridge semantics of reactive streams, which are always non-nullable and represent null by empty stream state, and futures which are possibly nullable. However I need to differentiate between a nullable and non-nullable future, therefore decide whether to allow the reactive stream to return empty value (as null) or fail.

[Matthias Geisler]

Why not?:

class Foo<T: Any> {
    abstract fun returnValue(): T?
    abstract fun returnValueAsNonNullable(): T
}

[Martin Brehovsky]

because that would make returnValue() always nullable, which is not what I want. I want the this function to be either nullable or non-nullable based on the developer desire. However need to return always non-nullable from returnValueAsNonNullable

[Sourabh Rawat]

Btw T: Any? is same as T

[Martin Brehovsky]

I wanted to be explicit there.

[Martin Brehovsky]

but pretty much need to define a possibly nullable type T as non-nullable version of itself

[udalov]

T!! will be possible since Kotlin 1.6

[Martin Brehovsky]

This is a great news, but is there any pattern I could follow in the meantime?

[Fleshgrinder]

You can solve it with an extension function but that's useless for your exact use case because it cannot be abstract. I fear there's no way ... well ... unless you use two type parameters. Foo<A : Any, B : A?>