null vermeiden

Das Problem mit null

Was ist Option?

object Main {
  def main(args: Array[String]): Unit = {
    val present: Option[Int] = Some(42)
    val absent: Option[Int] = None
    println(present)
    println(absent)
  }
}

Some erstellen

object Main {
  def main(args: Array[String]): Unit = {
    val name: Option[String] = Some("Ada")
    println(name)
  }
}

Fehlen mit None darstellen

object Main {
  def lookup(found: Boolean): Option[Int] =
    if (found) Some(100) else None
  def main(args: Array[String]): Unit = {
    println(lookup(true))
    println(lookup(false))
  }
}

Funktionen, die fehlschlagen können

object Main {
  def findUser(id: Int): Option[String] =
    if (id == 1) Some("Grace") else None
  def main(args: Array[String]): Unit = {
    println(findUser(1))
    println(findUser(2))
  }
}

Mit isDefined und isEmpty prüfen

object Main {
  def main(args: Array[String]): Unit = {
    val a: Option[Int] = Some(5)
    val b: Option[Int] = None
    println(a.isDefined)
    println(b.isEmpty)
  }
}

Pattern Matching für ein Option

object Main {
  def greet(name: Option[String]): String = name match {
    case Some(n) => s"Hello, $n"
    case None    => "Hello, stranger"
  }
  def main(args: Array[String]): Unit = {
    println(greet(Some("Bo")))
    println(greet(None))
  }
}

Option aus Nullable-Werten

object Main {
  def main(args: Array[String]): Unit = {
    val maybe = Option("data")
    val nothing = Option(null)
    println(maybe)
    println(nothing)
  }
}

Rückgabewerte der Standardbibliothek vom Typ Option

object Main {
  def main(args: Array[String]): Unit = {
    val xs = List(1, 2, 3)
    val empty = List.empty[Int]
    println(xs.headOption)
    println(empty.headOption)
  }
}

Warum Option statt null?

Alles zusammenführen

object Main {
  val prices = Map("apple" -> 3, "pear" -> 5)
  def priceOf(item: String): Option[Int] = prices.get(item)
  def main(args: Array[String]): Unit = {
    priceOf("apple") match {
      case Some(p) => println(s"costs $p")
      case None    => println("not for sale")
    }
    println(priceOf("banana"))
  }
}

Schnelltest

Zusammenfassung