Danger: Shadowed member values

It is possible (although not recommended) to declare a public property in a super class and have a private value or variable in a subclass:

1. scala> class X (val i:Int)
2. defined class X
4. scala> class Y(i:Int) extends X(19) {
5.      | println(i)
6.      | }
7. defined class Y
9. scala> val y = new Y(100)
10. 100
11. y: Y = Y@3e55a58f
13. scala> y.i
14. res7: Int = 19

This begs the question: how can the subclass access the superclass instance of i? Let me re-iterate. Just because it is possible this is a bad idea. This pattern seems like it is begging to create bugs. But who knows there is probably some reason out there where this is required.

The way to access the X.i value from Y is to declare which value you want using the syntax this:X :

1. scala> class Y(i:Int) extends X(19) {
2.      | println("Y.i"+i)              
3.      | println("X.i"+(this:X).i)
4.      | }
5. defined class Y
7. scala> new Y(39)
8. Y.i39
9. X.i19
10. res8: Y = Y@451dfada

Repeat: Watch out! Because I learned this danger while writing this topic:

1. scala> class Y(i:Int) extends X(19) {
2.      | println("1. "+i)              
3.      | println("2. "+this.i)         
4.      | println("3. "+(this:Y).i)     
5.      | println("4. "+(this:X).i)     
6.      | }
7. defined class Y
9. scala> new Y(44)
10. 1. 44
11. 2. 44
12. 3. 19  // this is created via (this:Y).i !
13. 4. 19
14. res0: Y = Y@338bd37a

It seems that the syntax (this:X).i accesses the public accessor for i. The public access is a method call I think so it always obtains the value for the property. Shocking!