Writing a method to return a java Class constrained to a certain type in Scala

For example, I have this:

abstract class Vehicle

class Car extends Vehicle

And I want to write a method that returns a java Class, but constrained to only Classes of type Vehicle.

object CarDealer {
  def getVehicularClass: Class[Vehicle] = classOf[Car]
}

I am not able to开发者_Python百科 get the following different approaches to work:

1. The way I would normally deal with this

   object CarDealer {
     def getVehicularClass[A <: Vehicle]: Class[A] = classOf[Car]
   }
   
   [error]  found   : java.lang.Class[Car](classOf[Car])
   [error]  required: Class[A]
   [error]   def getVehicularClass[A <: Vehicle]: Class[A] = classOf[Car]
   

2. Using an abstract type

   type V <: Vehicle
   

3. Using an implicit manifest (seems like it only constrains method arguments and not return values)

Can anyone help?

Thanks!

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Vehicle is the supertype. You don't know that some subtype A of Vehicle is going to include Car, so you can't write it generically that way. What if someone asked for a Bus? The CarDealer can't help, but the type signature promises that it can make it work for any A <: Vehicle that you ask for.

If you don't care about preserving the real type of the vehicle, just write

object CarDealer {
  def getVehicularClass: Class[Vehicle] = classOf[Car]
}

Alternatively, if you want some interface obeyed, you specify in the interface what you want:

trait VehicleClass[A <: Vehicle] {
  def getVehicularClass: Class[A]
}

and then you have your object state which subclass of vehicle it is promising to return:

object CarDealer extends VehicleClass[Car] {
  def getVehicularClass = classOf[Car]
}

and now all promises are specified and lived up to.

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The type Class[T] is invariant in type T. Thus even if A extends B, classOf[A] is not a subtype of classOf[B]. Therefore there's no exact solution to your problem...

---

This constrain is adequately expressed with existential types:

object CarDealer {
  def getVehicularClass: Class[_ <: Vehicle] = classOf[Car]
}

Of course, there might be other requirements that make an existential type inappropriate, but your problem is not particularly well stated.