Comparison of Inheritance in C++ and Java

2.5

The purpose of inheritance is same in C++ and Java. Inheritance is used in both languages for reusing code and/or creating is-a relationship. There are following differences in the way both languages provide support for inheritance.

1) In Java, all classes inherit from the Object class directly or indirectly. Therefore, there is always a single inheritance tree of classes in Java, and Object class is root of the tree. In Java, if we create a class that doesn’t inherit from any class then it automatically inherits from Object class . In C++, there is forest of classes; when we create a class that doesn’t inherit from anything, we create a new tree in forest.

Following Java example shows that Test class automatically inherits from Object class.

class Test { 
    // members of test
}
class Main {
  public static void main(String[] args) {
    Test t = new Test();    
    System.out.println("t is instanceof Object: " + (t instanceof Object));
  }
}

Output:

t is instanceof Object: true



2) In Java, members of the grandparent class are not directly accessible. See this G-Fact for more details.



3) The meaning of protected member access specifier is somewhat different in Java. In Java, protected members of a class “A” are accessible in other class “B” of same package, even if B doesn’t inherit from A (they both have to be in the same package). For example, in the following program, protected members of A are accessible in B.

// filename B.java
class A {
    protected int x = 10, y = 20;
}
 
class B {
    public static void main(String args[]) {
        A a = new A();
        System.out.println(a.x + " " + a.y);
    }
}



4) Java uses extends keyword for inheritence. Unlike C++, Java doesn’t provide an inheritance specifier like public, protected or private. Therefore, we cannot change the protection level of members of base class in Java, if some data member is public or protected in base class then it remains public or protected in derived class. Like C++, private members of base class are not accessible in derived class.
Unlike C++, in Java, we don’t have to remember those rules of inheritance which are combination of base class access specifier and inheritance specifier.



5) In Java, methods are virtual by default. In C++, we explicitly use virtual keyword. See this G-Fact for more details.



6) Java uses a separte keyword interface for interfaces, and abstract keyword for abstract classes and abstract functions.

Following is a Java abstract class example.

// An abstract class example
abstract class myAbstractClass {
   
   // An abstract method
   abstract void myAbstractFun();
  
   // A normal method
   void fun() { 
      System.out.println("Inside My fun");
   } 
}

public class myClass extends myAbstractClass {
   public void myAbstractFun() {
      System.out.println("Inside My fun");
   }
}

Following is a Java interface example

// An interface example
public interface myInterface {
   // myAbstractFun() is public and abstract, even if we don't use these keywords
   void myAbstractFun();  // is same as public abstract void myAbstractFun()
}

// Note the implements keyword also.
public class myClass implements myInterface {
   public void myAbstractFun() {
      System.out.println("Inside My fun");
   }
}



7) Unlike C++, Java doesn’t support multiple inheritance. A class cannot inherit from more than one class. A class can implement multiple interfaces though.



8 ) In C++, default constructor of parent class is automatically called, but if we want to call parametrized constructor of a parent class, we must use Initializer list. Like C++, default constructor of the parent class is automatically called in Java, but if we want to call parametrized constructor then we must use super to call the parent constructor. See following Java example.

package main;
 
class Base {
    private int b;
    Base(int x) {
        b = x;
        System.out.println("Base constructor called");
    }                
}
 
class Derived extends Base {
    private int d;
    Derived(int x, int y) {
        // Calling parent class parameterized constructor
        // Call to parent constructor must be the first line in a Derived class
        super(x);
        d = y;
        System.out.println("Derived constructor called");
    }                    
}
 
class Main{
    public static void main(String[] args) {
      Derived obj = new Derived(1, 2);
    }
}

Output:

Base constructor called
Derived constructor called

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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