Using Kotlin Generics in Nested Interfaces

Question

I'm trying to write an interface for sending generic events to a remote server, along with a listener which returns a response that includes the list of events that was sent/attempted to be sent.

interface Sender<T> {
  interface Response {
    val success: Boolean
    val events: List<T> // unresolved reference
  }

  ...
}

If I change the T to a *, it compiles, but the type doesn't match what I need in the listener.

How can I "inherit" the generic type T in a sub-interface like this?

In a nested interface? You can't.

Louis Wasserman
– Louis Wasserman

2025-06-10 23:48:05 +00:00
Commented Jun 10 at 23:48 — Louis Wasserman
– Louis Wasserman, Commented Jun 10 at 23:48

Louis Wasserman · Accepted Answer · 2025-06-10 23:50:24Z

1

You can't. The only thing you can make work is

interface Sender<T> {
  interface Response<T> {
    val success: Boolean
    val events: List<T> // unresolved reference
  }

  ...
  fun send(response: Response<T>) // or
  fun send(): Response<T>
}

You can't get around repeating the <T>, not with an interface, because you must be able to refer to Sender.Response even when you are not in a Sender object, or you have several Sender objects with different Ts, or whatever.

answered Jun 10 at 23:50

Louis Wasserman

200k26 gold badges367 silver badges437 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Using Kotlin Generics in Nested Interfaces

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related