callbacks.kt

class Manager {

  var service: Service? = null

  init {
    service = Service(::print)
    //service = Service({ message -> println(message) })
    //service = Service { println(it) }
  }

  private fun print(message: String) {
    println(message)
  }
}

class Service(callMeMaybe: (String) -> Unit) {
  init {
    callMeMaybe.invoke("hello")
  }
}