What are the differences between final class and sealed class in Scala?

Question

There are two types of modifiers in Scala: final and sealed

What are the differences between them? When should you use one over the other?

Answer 1

sealed classes (or traits) can still be inherited in the same source file (where final classes can't be inherited at all).

Use sealed when you want to restrict the number of subclasses of a base class (see "Algebraic Data Type").

As one of the very practical benefits of such a restriction the compiler can now warn you about non-exaustive pattern matches:

sealed trait Duo
case class One(i:Int) extends Duo
case class Two(i:Int, j:Int) extends Duo

def test(d:Duo) {
  match {
    case One(x) => println(x) // warning since you are not matching Two
  }
}