密封类

特质（trait）和类（class）可以用sealed标记为密封的，这意味着其所有子类都必须与之定义在相同文件中

sealed abstract class Furniture
case class Couch() extends Furniture
case class Chair() extends Furniture
def findPlaceToSit(piece: Furniture): String = piece match {
  case a: Couch => "Lie on the couch"
  case b: Chair => "Sit on the chair"
}