Why it is not possible to override mutable variable in scala?

Question

Why it is not possible to override mutable variable in scala ?

class Abs(var name: String){
}

class AbsImpl(override var name: String) extends Abs(name){
}
         


        

Answer 1

If you could override a var with a var, then the overriding member could have a narrower type. (That's how overriding is defined.)

You could then assign a value of a wider type and then read it expecting the narrower type, and fail.

Illustration of the setter involved:

scala> class A ; class B extends A
defined class A
defined class B

scala> abstract class C { var x: A } ; class D extends C { var x: B = _ }
:13: error: class D needs to be abstract, since variable x in class C of type A is not defined
(Note that an abstract var requires a setter in addition to the getter)
       abstract class C { var x: A } ; class D extends C { var x: B = _ }
                                             ^

scala> abstract class C { var x: A }
defined class C

scala> class D extends C { var x: B = _ ; def x_=(a: A) = ??? }
defined class D