Kotlin Vertx Type Mismatch found Future<Unit> expected Handler<AsyncResult<Void>>

Question

I thought that in Kotlin, Unit was equivalent to Void. With Vert.x Service Discovery, it is not possible to pass a Future<Unit> to unpublish(String id, Handler<AsyncResult<Void>> resultHandler) (gives a type mismatch) yet it will accept Future<Void> without any problem. Why is this and is there a solution or will I just have to live with using Void?

Answer 1

Unit is not equivalent to Void, it is equivalent to void in kotlin.

In java, void is a keyword, but Void is a class. so the code below can't be compiled:

fun foo():Void{/**need return a Void instance exactly**/}

fun bar():Void{ return Unit; }
//                     ^--- type mismatch error

java applies the same rule, for example:

Void canNotBeCompiled(){
  // must return a Void instance exactly.
}

Void foo(){
  return Void.TYPE;
}

Void nil(){
  return null;
}

Finally the Unit documentation also says:

The type with only one value: the Unit object. This type corresponds to the void type in Java.