How to access the private variables of a class in its subclass?

Question

This is a question I was asked in an interview: I have class A with private members and Class B extends A. I know private members of a class cannot be accessed, but the qu

Answer 1

To access private variables of parent class in subclass you can use protected or add getters and setters to private variables in parent class..

Answer 2

Reflection? Omitting imports, this should work:

public class A {

    private int ii = 23;

}

public class B extends A {

    private void readPrivateSuperClassField() throws Exception {
        Class<?> clazz = getClass().getSuperclass();
        Field field = clazz.getDeclaredField("ii");
        field.setAccessible(true);
        System.out.println(field.getInt(this));
    }

    public static void main(String[] args) throws Exception {
        new B().readPrivateSuperClassField();
    }

}

It'll not work if you do something like that before the of invocation readPrivateSuperClassField();:

System.setSecurityManager(new SecurityManager() {
        @Override
        public void checkMemberAccess(Class<?> clazz, int which) {
            if (clazz.equals(A.class)) {
                throw new SecurityException();
            } else {
                super.checkMemberAccess(clazz, which);    
            }
        }
    });

And there are other conditions under which the Reflection approach won't work. See the API docs for SecurityManager and AccessibleObject for more info. Thanks to CPerkins for pointing that out.

I hope they were just testing your knowledge, not looking for a real application of this stuff ;-) Although I think an ugly hack like this above can be legit in certain edge cases.

Answer 3

The architecture is broken. Private members are private because you do not want them accessed outside the class and friends.

You can use friend hacks, accessors, promote the member, or #define private public (heh). But these are all short term solutions - you will probably have to revisit the broken architecture at some stage.

Answer 4

By using public accessors (getters & setters) of A's privates members ...