Parent and Child Classes Having Same Data Member in Java

In C++ we have all class member methods as non-virtual. In order to make so, we have to use a keyword as a prefix known as virtual. Whereas in Java, we have all class member methods as virtual by default. In order to make them non-virtual, we use the keyword final.

Reference variables in Java are basically variables holding the address of the object in hexadecimal type which later is converted to the binary system that basically is the address of the object to be stored on the heap memory.

Reference variables that differ from primitive types as their size can not be calculated. In Java, the reference variable of the Parent class is capable to hold its object reference as well as its child object reference. Let's see about non-method members with the help of an example.

Example:

// Java Program to Demonstrate that Non-method Members
// are Accessed according to Reference Type
// (Unlike methods that are accessed according
// to the referred object)

// Class 1
// Super class
class Parent {
    int value = 1000;

    // Constructor of super class
    Parent()
    {

        // Print statement
        System.out.println("Parent Constructor");
    }
}

// Class 2
// Sub class
class Child extends Parent {

    int value = 10;

    // Constructor of sub class
    Child()
    {

        // Print statement
        System.out.println("Child Constructor");
    }
}

// Class 3
// Main class
class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating an object of sub class inside main()
        // method
        Child obj = new Child();

        // Printing the reference of child type
        System.out.println("Reference of Child Type :"
                           + obj.value);

        // Note that doing "Parent par = new Child()"
        // would produce same result
        Parent par = obj;

        // Par holding obj will access the value
        // variable of parent class

        // Printing the reference of parent type
        System.out.println("Reference of Parent Type : "
                           + par.value);
    }
}

Parent Constructor
Child Constructor
Reference of Child Type :10
Reference of Parent Type : 1000

Output Explanation: If a parent reference variable is holding the reference of the child class and we have the "value" variable in both the parent and child class, it will refer to the parent class "value" variable, whether it is holding child class object reference. The reference holding the child class object reference will not be able to access the members (functions or variables) of the child class. This is because the parent reference variable can only access fields that are in the parent class. Thus the type of reference variable decides which version of "value" will be called and not the type of object being instantiated. It is because the compiler uses a special run-time polymorphism mechanism only for methods. (There the type of object being instantiated decides which version of the method to be called).

Note: It is made possible to access child data members using parent pointer with typecasting.