case class inheritance

Scala 禁止case class inheritance

case class Person(name: String, age: Int)
case class FootballPlayer(name: String, age: Int, number: Int) extends Person(name, age)

在编译时会报出以下错误：

Error:(5, 12) case class FootballPlayer has case ancestor Person, but case-to-case inheritance is prohibited. To overcome this limitation, use extractors to pattern match on non-leaf nodes.
case class FootballPlayer(name: String, age: Int, number: Int) extends Person(name, age)
^

原因有挺多，下边的两篇文章讲了下理由，但我还是没明白其中的必要性。

http://tech.schmitztech.com/scala/caseclassinheritence.html

http://stackoverflow.com/questions/11158929/what-is-so-wrong-with-case-class-inheritance

---

我们可以使得两个case class继承同一个trait来实际于case class继承的行为，但是这样就得自己写extractor了。比如

  sealed trait Person{
    def age: Int
    def name: String
  }
  case class FootballPlayer(name: String, age: Int, number: Int) extends Person
  val torres = FootballPlayer("Fernando Torres", 31, 19)
  val Person(name, age) = torres // can't resolve symbal Person

会编译出错, 因为Person是个trait，它并不支持extractor的语法。

需要给Person加个Companion object，像这样

  object Person{
    def unapply(p: Person) = Some((p.name, p.age))
  }

这时就能用extractor了

  val torres = FootballPlayer("Fernando Torres", 31, 19)
  val Person(name, age) = torres

---

编译器会为case class生成equals方法，但普通类就不会了。

　　sealed trait Person{
    def age: Int
    def name: String
  }

  case class FootballPlayer(name: String, age: Int, number: Int) extends Person
  class Doctor(val name: String, val age: Int) extends Person
  val torresA = FootballPlayer("Fernando Torres", 31, 19)
  val torresB = FootballPlayer("Fernando Torres", 31, 19)
  println(torresA == torresB)//true

  val docA = new Doctor("C", 30)
  val docB = new Doctor("C", 30)
  println(docA == docB)//false

这时，当两个FootballPlayer的构造参数相同，它们就相等。但是对于Doctor类来说不是这样了。

当给FootballPlayer这个case class的父类Person定义了equals方法之后，就不是这样了。

 sealed trait Person{self =>
    def age: Int
    def name: String
    override def equals(that: Any):Boolean = {
      that match{
        case p: Person => p.age == self.age && p.name == self.name
        case _ => false
      }
    }
    override def hashCode: Int = {
      var hash = 1
      hash = hash * 31 + age
      hash = hash * 31 + {if(name !=null) name.hashCode else 0}
      hash
    }
  }
  case class FootballPlayer(name: String, age: Int, number: Int) extends Person
  class Doctor(val name: String, val age: Int) extends Person
  val torresA = FootballPlayer("Fernando Torres", 31, 19)
  val torresB = FootballPlayer("Fernando Torres", 31, 19)
  torresA.equals(torresB)
  println(torresA == torresB)//true

  val docA = new Doctor("C", 30)
  val docB = new Doctor("C", 30)
  println(docA == docB) //true

  val footballPlayerC = FootballPlayer("C", 30, 30)
  println(footballPlayerC == docA) //true
  println(footballPlayerC.hashCode())//1958
  println(docA.hashCode())//1958

貌似这时候case class就不会生成equals方法了, 转而使用父类Person的equals方法。并且要记得同时在Person中覆盖hashCode方法，不然就破坏了equals对hashCode的要求。