Extend generic type - PriorityQueue

Question

I can't understand why I need () and hence where MyTypeQueOrdering goes. Here is header of PriorityQueue, found on official github:

class PriorityQueue[A](implicit val ord: Ordering[A])

Here is my try (which works):

class MyType{

}

object MyTypeQueOrdering extends Ordering[MyType]{
    def compare (n1:MyType, n2:MyType) = -1
}

class MyTypeQue extends PriorityQueue[MyType]()(MyTypeQueOrdering){

}

... but I can't figure out why I need (). Does PriorityQueue[MyType]() return something?

Answer 1

Try making MyTypeQueOrdering an implicit object:

object Implicits {
  //implicit objects can't be top-level ones
  implicit object MyTypeQueOrdering extends Ordering[MyType] {
    def compare(n1: MyType, n2: MyType) = -1
  }
}

This way you can omit both parentheses:

import Implicits._

class MyTypeQue extends PriorityQueue[MyType] { ... }

The reason you need the empty parentheses in your example is because PriorityQueue[MyType](MyTypeQueOrdering) would assume you're trying to pass the ordering as a constructor parameter. So that's why you need to explicitly show no-arg instantiation and then passing the ordering