Accessing private instances in child class

Question

I've seen answers for questions related to mine on Stack Overflow, but I am still left with some ambiguity. A parent class method has access to its own private instance variables. If a child class inherits the class, what happens when the getA() method is called on an instance of the Child class? Does it return the a from the Parent class or the a from the Child class?

class Parent {
    private int a = 10;  
    public int getA() {
       return a;
    }
 }

class Child extends Parent {
    private int a = 22;              
 }

public class Test { 
   public static void main(String []args) throws Exception {        
       Child c = new Child(); 
       System.out.println(c.getA());
   }    
}

score 3 · Accepted Answer · answered Apr 01 '15 at 06:27

As the Method getA() is inherited, if you call this Method, you'll always invoke the Parent's Method.

The current Object will be treated as a Parent and not as a Child, and the Parent's a will be returned.

Even though you have your own variable a, this variable wont override the Parent's a. They are different from each other, have different addresses and different values.

If you want getA() to return Child's a, you need to override the Method to return your new variable.

class Child extends Parent {
    private int a = 22;

    @Override
    public int getA(){
        return a;
    }
 }

You could also "go crazy" and do stuff like the following:

class Child extends Parent {
    private int a = 22;       

    @Override
    public int getA(){
        int superA = super.getA();

        return a+superA;    
    }
 }

That way you could return the sum of Parent's and Child's a.

(Just an example)

score 2 · Answer 2 · edited May 23 '17 at 12:14

2

This is a duplicate of this SO post.

The variable a in subclass Child hides the a in the parent class Parent.

edited May 23 '17 at 12:14

Community

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answered Apr 01 '15 at 06:17

Tim Biegeleisen

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I know that it hides, that's not what I want actually – Kishore Kumar Korada Apr 01 '15 at 06:19
@KishoreKumarKorada then what do you want? – user253751 Apr 01 '15 at 06:22
2

I know that fields can't be inherited but methods does. So when I have getA() in child class doesn't it pick child's a instance variable? – Kishore Kumar Korada Apr 01 '15 at 06:23
@KishoreKumarKorada note that whether private fields are inherited depends on how you define "inherited", and there isn't an official definition. In some ways they are, in some ways they aren't. (I removed the part from my answer that said they weren't) – user253751 Apr 01 '15 at 06:29

user253751 · Answer 3 · 2015-04-01T06:28:12.783

1

Fields cannot be overridden.

In your code, an instance of child (like the one referred to by c) has two different fields, which are both called a.

You cannot access Parent's private variables inside Child, full stop. That's the entire point of private. There is nothing* you can write inside Child to make the parent's a field equal 22.

* Technically, you could with reflection. Reflection doesn't count though, since its purpose is essentially to allow you to break things, and do things that are otherwise impossible.

edited Apr 01 '15 at 06:28

answered Apr 01 '15 at 06:18

user253751

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As I understood the question, it's not about changing `Parent`'s `a` to be 22, but why `getA()` will always return the `Parent`'s `a`, even though he has his own new `a` variable – Loki Apr 01 '15 at 06:20
The take-home message to remember here is that methods **can** be overridden by their subclasses, but variables **cannot** be overridden. – Tim Biegeleisen Apr 01 '15 at 06:21
@Loki The original question was mistaken on that point, I believe. – Tim Biegeleisen Apr 01 '15 at 06:21
@Loki Yes, That's what exactly I want to know – Kishore Kumar Korada Apr 01 '15 at 06:25

score 1 · Answer 4 · answered Apr 01 '15 at 06:19

Private variables are local to the class and in your code you are inheriting the properties of the parent class so you can access getA() and it will return the parent's attribute. And you cannot access child's variable unless you have public getter method for child attribute.

score 1 · Answer 5 · answered Apr 01 '15 at 06:27

Stick to basics "private member variable will only be accessed within the class by its own member functions and fields cannot be inherited" so basically when you are accessing the private variable A in parent class , the method is supposed to access its own private member rather then the private field of any child class.

Accessing private instances in child class

5 Answers5