Co&ContraVariance in Scala (and not Java)

covariant-result.txt

> run-main Covariant
[info] Running Covariant 
55.0
[success] Total time: 0 s, completed Feb 2, 2013 6:13:40 PM
> 

Covariant.scala

object Covariant extends App {

  trait Vehicle {
    def name:String
    def velocity:Float
  }
  case class Car(name:String, velocity:Float) extends Vehicle

  object VehiculeStats {
    def meanVelocity(vs:List[Vehicle]):Float =
      vs match {
        case Nil => 0
        case vs => vs.map{_.velocity}.sum / vs.size
      }
  }

  val christine = Car("Christine", 55f)
  val cars = List(christine)

  val m = VehiculeStats.meanVelocity(cars)
  println(m)
}

Error.txt

[info] Compiling 1 Scala source and 1 Java source to /home/noootsab/src/test/target/scala-2.10/classes...
[error] /home/noootsab/src/test/Test.java:41: meanVelocity(java.util.List<Test.Vehicle>) in Test.VehiculeStats cannot be applied to (java.util.List<Test.Car>)
[error]     Float mv = VehiculeStats.meanVelocity(cars);
[error]                             ^
[error] 1 error
[error] (compile:compile) javac returned nonzero exit code
[error] Total time: 1 s, completed Feb 2, 2013 5:25:28 PM
> 

OptionCov.scala

object OptionCov extends App {
  import Covariant._

  val christine = Car("Christine", 55f)
  val someCar:Option[Car] = Some(christine)

  val someVehicle:Option[Vehicle] = someCar

  println(someVehicle) //prints Some(Christine)
}

Test.java

import java.util.*;

public class Test {

  static public abstract class Vehicle {
    public String name;

    public Float velocity;

    public String toString() {
      return this.name;
    }
  }

  static public class Car extends Vehicle {
    public Car(String  name, Float velocity) {
      this.name = name;
      this.velocity = velocity;
    }
  }

  static class VehiculeStats {
    public static Float meanVelocity(List<Vehicle> vs) {
      if (vs.isEmpty()) {
        return 0f;
      } else {
        Float acc = 0f;
        for (Vehicle v : vs) {
          acc += v.velocity;
        }
        return acc / vs.size();
      }
    }
  }

  public static void main(String[] args) {
    Car christine = new Car("Christine", 55f);

    List<Car> cars = Arrays.asList(christine);

    Float mv = VehiculeStats.meanVelocity(cars);
    System.out.println("Mean Velocity : " + mv);
  }

}