Question

What is the reason for vals not (?) being automatically final in singleton objects? E.g.

object NonFinal {
   val a = 0
   val b = 1

   def test(i: Int) = (i: @annotation.switch) match {
      case `a` => true
      case `b` => false
   }
}

results in:

<console>:12: error: could not emit switch for @switch annotated match
          def test(i: Int) = (i: @annotation.switch) match {
                                                     ^

Whereas

object Final {
   final val a = 0
   final val b = 1

   def test(i: Int) = (i: @annotation.switch) match {
      case `a` => true
      case `b` => false
   }
}

Compiles without warnings, so presumably generates the faster pattern matching table.

Having to add final seems pure annoying noise to me. Isn’t an object final per se, and thus also its members?

Answer 1

This is addressed explicitly in the specification, and they are automatically final:

Members of final classes or objects are implicitly also final, so
the final modifier is generally redundant for them, too. Note, however, that
constant value definitions (§4.1) do require an explicit final modifier, even if
they are defined in a final class or object.

Your final-less example compiles without errors (or warnings) with 2.10-M7, so I’d assume that there’s a problem with the @switch checking in earlier versions, and that the members are in fact final.

---

Update: Actually this is more curious than I expected—if we compile the following with either 2.9.2 or 2.10-M7:

object NonFinal {
  val a = 0
}

object Final {
  final val a = 0
}

javap does show a difference:

public final class NonFinal$ implements scala.ScalaObject {
  public static final NonFinal$ MODULE$;
  public static {};
  public int a();
}

public final class Final$ implements scala.ScalaObject {
  public static final Final$ MODULE$;
  public static {};
  public final int a();
}

You see the same thing even if the right-hand side of the value definitions isn’t a constant expression.

So I’ll leave my answer, but it’s not conclusive.