Understanding the concept of inheritance in Java

Question

I am just refreshing the oops features of the java. So, I have a little confusion regarding inheritance concept. For that I have a following sample code :

class Super{
    int index = 5;
    public void printVal(){
        System.out.println("Super");
    }
}
class Sub extends Super{
    int index = 2;
    public void printVal(){
        System.out.println("Sub");
    }
}
public class Runner {
    public static void main(String args[]){
        Super sup = new Sub();
        System.out.println(sup.index+",");
        sup.printVal();
    }
}

Now above code is giving me output as : 5,Sub.

Here, we are overriding printVal() method, so that is understandable that it is accessing child class method only.

But I could not understand why it's accessing the value of x from Super class...

Thanks in advance....

Answer 1

This is called instance variable hiding - link. Basically you have two separate variables and since the type of the reference is Super it will use the index variable from Super.

Answer 2

Objects have types, and variables have types. Because you put:

Super sup = new Sub();

Now you have a variable sup of type Super which refers to an object of type Sub.

When you call a method on an object, the method that runs is chosen based on the type of the object, which is why it prints "Sub" instead of "Super".

When you access a field in an object, the field is chosen based on the type of the variable, which is why you get 5.