How Does Default Virtual Behavior Differ in C++ and Java?

Let us discuss how the default virtual behavior of methods is opposite in C++ and Java. It is very important to remember that in the C++ language class member methods are non-virtual by default. They can be made virtual by using virtual keywords. For example, Base::show() is non-virtual in following program and program prints “Base::show() called”. 

Example:

CPP

// C++ Program to Illustrate How
// Default Virtual Behave
// Different in C++ and Java

// Importing required libraries
// Input output stream
#include <iostream>

using namespace std;

// Class 1
// Superclass
class Base {

    // Granting public access via
    // public access modifier
public:
    // In c++, non-virtual by default
    // Method of superclass
    void show()
    {

        // Print statement
        cout << "Base::show() called";
    }
};

// Class 2
// Subclass
class Derived : public Base {

    // Granting public access via public access modifier
public:
    // Method of subclass
    void show()
    {

        // Print statement
        cout << "Derived::show() called";
    }
};

// Main driver method
int main()
{
    // Creating object of subclass
    Derived d;

    // Creating object of subclass
    // with different reference
    Base& b = d;

    // Calling show() method over
    // Superclass object
    b.show();

    getchar();

    return 0;
}

Output:

Base::show() called

Output Explanation: Adding virtual before definition of Base::show() makes program print “Derived::show() called”. In Java, methods are virtual by default and can be made non-virtual by using the final keyword. For example, in the following java program, show() is by default virtual and the program prints “Derived::show() called“.

Let us see what happens in the case we use the same concept in a java programming language via the example proposed below.

Example:

Java

// Java Program to Illustrate
// How Default Virtual Behave
// Different in C++ and Java

// Importing required classes
import java.util.*;

// Class 1
// Helper class
class Base {

    // Method of sub class
    // In java, virtual by default
    public void show()
    {

        // Print statement
        System.out.println("Base::show() called");
    }
}

// Class 2
// Helper class extending Class 1
class Derived extends Base {

    // Method
    public void show()
    {

        // Print statement
        System.out.println("Derived::show() called");
    }
}

// Class 3
// Main class
public class GFG {

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

        // Creating object of superclass with
        // reference to subclass object
        Base b = new Derived();
        ;

        // Calling show() method over Superclass object
        b.show();
    }
}

Output

Derived::show() called

Note: Unlike C++ non-virtual behavior, if we add final before the definition of the show() in Base, then the above program fails in the compilation.