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scalaobjectinheritancefinal

What's the point of declaring an object as "final"?

I just noticed that it's possible to declare objects as final in Scala:

final object O

What's the point of doing that? One cannot inherit from objects anyway:

object A
object B extends A // not found: type A
Solution

  • Not that anyone does this, but:

    $ scala -Yoverride-objects
Welcome to Scala version 2.11.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_11).
Type in expressions to have them evaluated.
Type :help for more information.

scala> trait A { object O } ; trait B extends A { override object O }
defined trait A
defined trait B

scala> trait A { final object O } ; trait B extends A { override object O }
<console>:8: error: overriding object O in trait A;
 object O cannot override final member
       trait A { final object O } ; trait B extends A { override object O }
                                                                        ^

    Possibly sometimes people want to do it. (For instance.)