Sealed Classes

Sealed Classes

Marking a class or trait sealed restricts all its direct subtypes to being defined in the same source file, letting the compiler check that a match expression covers every possible case exhaustively. This is especially powerful combined with case classes to model a fixed, closed set of variants (like a simple enum-style hierarchy). The compiler warns if a match on a sealed hierarchy misses a case.

Note: sealed combined with case classes gives you compiler-checked exhaustiveness warnings on match expressions -- a very common, powerful Scala pattern.

Example: Sealed Classes

sealed trait Shape
case class Circle(radius: Double) extends Shape
case class Square(side: Double) extends Shape

object Main extends App {
  def area(shape: Shape): Double = shape match {
    case Circle(r) => Math.PI * r * r
    case Square(s) => s * s
  }

  println(area(Circle(2)))
  println(area(Square(3)))
}