Restricting parent class members to its immediate child class only

Question

Suppose I have four classes in java with given hierarchy.

class A {}
class B extends A {} 
class C extends B {} 
class D extends C {} 

As per my understanding all the accessible fields of class A will be available to all its child classes through inheritance. Now what if I want few fields of class A to be available to class B only.

Is there any way in java so that I can restrict certain fields of parent class to its immediate child class only?

Answer 1

Not that I would do anything like this in production, but here you go:

class A {
    private int a = 1;

    public int getA() {
        if (getClass().equals(A.class) || 
            getClass().getSuperclass().equals(A.class)) {
            return a;
        } else {
            throw new UnsupportedOperationException("hahaha");
        }
    }        

    public static void main(String[] args) {
        A a = new A();
        B b = new B();
        C c = new C();
        System.out.println(a.getA());
        System.out.println(b.getA());
        System.out.println(c.getA());
    }
}

class B extends A {}

class C extends B {}

So basically you can use this trick to allow only classes you want (using either the blacklist or whitelist approach) - to call getA().

OUTPUT:

1
1
Exception in thread "main" java.lang.IllegalStateException: hahaha
    ...