Versiegelte Traits

Was ist ein Trait?

Ein einfaches Trait

trait Shape
case class Circle(r: Double) extends Shape
case class Square(side: Double) extends Shape
object Main {
  def main(args: Array[String]): Unit = {
    val s: Shape = Circle(2.0)
    println(s)
  }
}

Was bedeutet sealed?

sealed trait Color
case object Red extends Color
case object Green extends Color
case object Blue extends Color
object Main {
  def main(args: Array[String]): Unit = {
    val c: Color = Green
    println(c)
  }
}

Case-Objekte als Varianten

sealed trait Direction
case object North extends Direction
case object South extends Direction
object Main {
  def main(args: Array[String]): Unit = {
    val d: Direction = North
    println(d)
  }
}

Objekte und Klassen kombinieren

sealed trait Event
case object Started extends Event
case class Failed(reason: String) extends Event
object Main {
  def main(args: Array[String]): Unit = {
    val e: Event = Failed("timeout")
    println(e)
  }
}

Pattern Matching für ein sealed Trait

sealed trait Color
case object Red extends Color
case object Green extends Color
case object Blue extends Color
object Main {
  def hex(c: Color): String = c match {
    case Red   => "#FF0000"
    case Green => "#00FF00"
    case Blue  => "#0000FF"
  }
  def main(args: Array[String]): Unit = {
    println(hex(Green))
  }
}

Compiler-Warnungen

Traits können Member deklarieren

sealed trait Animal {
  def sound: String
}
case object Dog extends Animal { def sound = "Woof" }
case object Cat extends Animal { def sound = "Meow" }
object Main {
  def main(args: Array[String]): Unit = {
    println(Dog.sound)
    println(Cat.sound)
  }
}

Warum sealed?

Die Hierarchie verwenden

sealed trait Payment
case class Cash(amount: Int) extends Payment
case class Card(number: String) extends Payment
object Main {
  def summary(p: Payment): String = p match {
    case Cash(a) => s"cash $a"
    case Card(n) => s"card ending ${n.takeRight(4)}"
  }
  def main(args: Array[String]): Unit = {
    println(summary(Cash(50)))
    println(summary(Card("123456789012")))
  }
}

Alles zusammenführen

sealed trait Result
case object Pending extends Result
case class Success(value: Int) extends Result
case class Error(msg: String) extends Result
object Main {
  def show(r: Result): String = r match {
    case Pending    => "waiting"
    case Success(v) => s"ok: $v"
    case Error(m)   => s"fail: $m"
  }
  def main(args: Array[String]): Unit = {
    List(Pending, Success(7), Error("boom")).foreach(r => println(show(r)))
  }
}

Schnelltest

Zusammenfassung