Tagged Ids using Phantom types

Ids.scala

trait Entity
trait User extends Entity
trait Product extends Entity

case class Id[T<:Entity](id:String)


def buy(pId:Id[Product],uId:Id[User])="Bought product %s for user %s".format(pId.id,uId.id)

val pId=new Id[Product]("1")
val uId=new Id[User]("2")

/*
scala> buy(pId,uId)
res2: String = Bought product 1 for user 2

scala> buy(uId,pId)
<console>:16: error: type mismatch;
 found   : Id[User]
 required: Id[Product]
              buy(uId,pId)
                  ^
*/

JavaIds.java

/*
  In Java due to popular request
*/

public interface Entity{
}

public interface User extends Entity{
}

public interface Product extends Entity{
}

public class Id<T extends Entity>{
  private String id;
  public Id(String id){
      this.id=id;
  }

  public String getId(){
      return id;
  }
}


public class Buyer {

    public static String buy(Id<Product> pId, Id<User> uId){
        return "Bought product "+pId.getId()+" for user "+uId.getId();
    }

    public static void main(String[] args){
        Id<Product> pId=new Id<Product>("1");
        Id<User> uId=new Id<User>("2");
        
        System.out.println(buy(pId,uId));
        /*
        Doesn't compile
        System.out.println(buy(uId,pId));
        */
    }
}