Hiding instance variables of a class

Question

I'm wondering why Java has this strange behavior regarding a superclass and a subclass having instance variables with the same name.

Let's say we have the following class definitions:

class Parent {
    int var = 1;
}

class Child extends Parent {
    int var = 2;
}

By doing this, we are supposed to have hidden the superclass's variable var. And if we do not explicitly specify a way to access Parent's var via a super call, then we should never be able to access var from an instance of a child.

But when we have a cast, this hiding mechanism breaks:

Child child = new Child();
Parent parent = (Parent)child;
System.out.println(parent.var); // prints out 1, instead of 2

Doesn't this completely circumvent the whole point of field hiding? If this is the case, then doesn't that render the the idea completely useless?

EDIT: I am referring specifically to this article in the Java Tutorials. It mentions

Within the subclass, the field in the superclass cannot be referenced by its simple name. Instead, the field must be accessed through super...

From what I read there, it seems to imply that the developers of Java had some kind of technique in mind in doing this. Though I agree that it is a rather obscure concept and would probably bad practice in general.

Answer 1

In Java, data members are not polymorphic. This means that Parent.var and Child.var are two distinct variables that happen to have the same name. You're not in any sense "overriding" var in the derived class; as you have discovered yourself, both variables can be accessed independently of one another.

The best way forward really depends on what you're trying to achieve:

1. If Parent.var should not be visible to Child, make it private.
2. If Parent.var and Child.var are two logically distinct variables, give them different names to avoid confusion.
3. If Parent.var and Child.var are logically the same variable, then use one data member for them.

Answer 2

The "point" of field hiding is merely to specify the behaviour of code which does give a variable the same name as one in its superclass.

It's not meant to be used as a technique to genuinely hide information. That's done by making the variables private to start with... I would strongly recommend using private variables in virtually all cases. Fields are an implementation detail which should be hidden from all other code.