why?

Can you please make your example compilable?

so should work?

Probably. But I really have no idea, because your example makes no sense at all.

in Java we used to use interfaces to communicate between fragments/activities

This seems to have nothing to do with interfaces vs lambdas. Your

listener

is just defined in the wrong scope.

I want to launch some data to a Fragment when get executed at a certain point to another activity

You need to make a compilable example if you want us to be able to help you.

OK, you mean a git repository?

No, just edit your first post. The code snippet you posted doesn't compile.

ah i get

This works:

fun main() {
    val myFoo = Foo()
    myFoo.call = { println("hello") }
    
    myFoo.call?.invoke()
}

class Foo(var call: (() -> Unit)? = null)