JLS 8 regarding keyword super and protected members

Question

In JLS 8 15.11.2-1 (page 505), I cannot understand what they mean by:

Note that super.x is not specified in terms of a cast, due to difficulties around access to protected members of the superclass.

Any help?

Answer 1

Consider this:

public class T2 {
    protected int x = 2;
}

/* in a different package */
public class T3 extends T2 {
    int x = 3;
    void test() {
        System.out.println(this.x); // prints 3
        System.out.println(super.x); // prints 2

        T2 this_as_t2 = (T2)this;
        System.out.println(this_as_t2.x); // Error: Can't access protected member x of class T2

        System.out.println(((T2)this).x); // Same error as above
    }
}

If super.x was equivalent to ((T2)this).x, then you wouldn't be able to use super.x to refer to the x field in T2.

So the specification does not say they're equivalent (because they aren't always). However, they're still equivalent in some situations - such as if both classes are in the same package, or if the field is public.