Let’s discuss the most useful cases for the extension methods. First use-case: extension methods allow to apply themselves only selectively. For example, the following method only applies to a list of Person:

fun Iterable<Person>.averageAge(): Double = map { it.age }.filterNotNull().average()

No longer it is necessary to create classes which only perform summarization functionality over a collection of objects - you can now attach that functionality directly to a Iterable or a Collection.

Second use-case is even more interesting. Let’s create a Session object as follows:

object Session {
    operator fun get(key: String): Any? = VaadinSession.getCurrent().getAttribute(key)
    operator fun set(key: String, value: Any?) = VaadinSession.getCurrent().setAttribute(key, value)
}

This will allow to store stuff into the Session as follows: Session["key"] = value. But this is not the interesting part. The interesting part comes here:

fun login(person: Person) {
    Session["user"] = person
}
fun logout() {
    Session["user"] = null
}
val Session.loggedInUser: Person? get()= Session["user"] as Person?

Yes, yes, login/logout, standard stuff, what’s your point? The main point is that we extended the Session with the loggedInUser property. And by “we” I mean that anyone can do this - another class, another jar file, another module. Consider this: lots of jars, lots of modules contributing stuff to the central Session class. You can use that stuff, as long as those jars are on your classpath. This is not about pluggability, this is about modularity and about discoverability of services. Your IDE will auto-discover all services and will provide you a neat list in the form of auto-completion on the Session object. And in some applications, this approach may even replace simple injections - that is, injections with one implementation and with no interceptors. But - don’t we strive for all injections to be simple?

Interesting times we live in ;) And that’s all folks, thank you for reading these ramblings ;) If you wish to see these ideas in practice, please check out the Vaadin on Kotlin project on Github - it includes the production-ready db{} function, Grid integration for rendering of your DB tables in your page and more. Check out the sample project as well.