val handler: Handler = { req, res -> res.body("Hello") }

Is it bad practice? To declare lambdas? No, it's perfectly fine.

kingsley: that works with the java version, but if i convert the interface to kotlin:

@FunctionalInterface
@Throws(Exception::class)
fun handle(request: Request, response: Response)

i need to add a constructor in order to do that

Yes. Kotlin interfaces are not SAM capable. You'd need use a functional type with typealias instead to achieve the same

typealias Handler = (Request, Response) -> Unit

thanks, that helps

You're welcome

maybe the best approach here would be to keep the functional interfaces as java, since i intend to use this with both java and kotlin? or is there a better approach?

Yes. You can do that. Using Kotlin's functional type results in

FunctionN

interfaces in Java which might not be as pretty as a specialized interface like your

Handler