scala covariant

gistfile1.scala

class Fruit {
  def selfIntro() = "Fruit"
}

class Apple extends Fruit{
  override def selfIntro() = "Apple"
}

class Package[T] (val contents:Seq[T]) {
  def this() = this(List[T]())
  def putIn[T](xs:T*) = {new Package(for(x <- xs) yield x) }
  def getAll() = contents
}


def cut(p:Package[Fruit]) {p.getAll foreach (x => Console.println(x.selfIntro + " cutted"))}

val fruitPackage = new Package[Fruit].putIn(new Fruit, new Fruit, new Fruit)
val applePackage = new Package[Apple].putIn(new Apple, new Apple, new Apple)

-----------------------------协变-----------------------
class Package[+T] (val contents:Seq[T]) {
  def this() = this(List[T]())
  def putIn[U >: T](xs:U*) = {new Package(for(x <- xs) yield x) }
  def getAll() = contents
}

---------------------------逆变完整代码-------------------------

class Fruit {
  def selfIntro() = "Fruit"
}

class Apple extends Fruit{
  override def selfIntro() = "Apple"
}

class RedApple extends Apple{
  override def selfIntro() = "RedApple"
}

class Cutter[-T] {
  def cut[U <: T](x:U)  = x match {
    case (f:Fruit) => Console.println("cut a " ++ f.selfIntro)
    case _ => Console.println("don't know what to do")
  }
}

class Chef[T] {
  def cutIt(c:Cutter[T], p:T) = c.cut(p)
}
///测试结果，切红苹果的刀是不能切苹果的，而且水果的刀是可以的，这正是我们需要的
scala> new Chef[Apple].cutIt(new Cutter[RedApple], new Apple)
<console>:16: error: type mismatch;
 found   : Cutter[RedApple]
 required: Cutter[Apple]
              new Chef[Apple].cutIt(new Cutter[RedApple], new Apple)
                                    ^

scala> new Chef[Apple].cutIt(new Cutter[Apple], new Apple)
cut a Apple

scala> new Chef[Apple].cutIt(new Cutter[Fruit], new Apple)
cut a Apple