Classier Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 195

8.4.8.3]. Because className is a field, Derived.className hides Base.className rather than
overriding it [JLS 8.3]. It is legal, though inadvisable, for one field to hide another when the hiding
field has an access modifier that provides less access than the hidden field. In fact, it is legal for a
hiding field to have a type that is completely unrelated to that of the field it hides: The Derived
class would be legal even if Derived.className were of type GregorianCalendar .
The compilation error in our program occurs when class PrivateMatter TRies to access
Derived.className . Although Base has a public field className , this field is not inherited into
Derived because it is hidden by Derived.className . Within the class Derived , the field name
className refers to the private field Derived.className . Because this field is declared private , it
is not accessible to the class PrivateMatter . Therefore, the compiler generates an error message
something like this:
PrivateMatter.java:11: className has private access in Derived
System.out.println(new Derived().className);
^
Note that it is possible to access the public field Base.className in a Derived instance even
though it is hidden, by casting the Derived instance to Base . The following version of
PrivateMatter prints Base :
public class PrivateMatter {
public static void main(String[] args) {
System.out.println(( (Base) new Derived()).className);
}
}
This demonstrates a big difference between overriding and hiding. Once a method is overridden in a
subclass, you can't invoke it on an instance of the subclass (except from within the subclass, by
using the super keyword). You can, however, access a hidden field by casting the subclass instance
to a superclass in which the field is not hidden.