What's the point of declaring an object as “fi

问题:

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

回答1:

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.)

回答2:

It makes no difference; object definitions are always final. The language specification explicitly mentions this in 5.4 Modifiers:

final is redundant for object definitions.