What is the purpose of empty class in Kotlin?

Question

I was going through Kotlin reference document and then I saw this.

The class declaration consists of the class name, the class header (specifying its type parameters, the primary constructor etc.) and the class body, surrounded by curly braces. Both the header and the body are optional; if the class has no body, curly braces can be omitted.

class Empty

Now I'm wondering what is the use of such class declaration without header and body

Answer 1

Empty classes can be useful to represent state along with other classes, especially when part of a sealed class. Eg.

sealed class MyState {
    class Empty : MyState()
    class Loading : MyState()
    data class Content(content: String) : MyState()
    data class Error(error: Throwable) : MyState()
}

In this way you can think of them like java enum entries with more flexibility.

Answer 2

An example of empty class usage from Spring Boot framework:

@SpringBootApplication
class FooApplication

fun main(args: Array<String>) {
    runApplication<FooApplication>(*args)
}

Answer 3

tldr: they want to demonstrate it's possible

even an empty class is of type Any and therefore has certain methods automatically. I think in most cases, this does not make sense, but in the documentation case it's used to show the simplest possible definition of a class.

The Java equivalent would be:

public final class Empty {
}