Inheritance

Inheritance in Java is a mechanism in which one object acquires all the properties and behaviors of a parent object. It is an important part of OOPs (Object Oriented programming system).

The idea behind inheritance in Java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of the parent class. Moreover, you can add new methods and fields in your current class also.

Note : Inheritance represents the IS-A relationship which is also known as a parent-child relationship.

Why do we use inheritance in java
For Method Overriding (so runtime polymorphism can be achieved). 

For Code Reusability.

Terms used in Inheritance

Class: A class is a group of objects which have common properties. It is a template or blueprint from which objects are created.

Sub Class/Child Class: Subclass is a class which inherits the other class. It is also called a derived class, extended class, or child class.

Super Class/Parent Class: Superclass is the class from where a subclass inherits the features. It is also called a base class or a parent class.

Reusability: As the name specifies, reusability is a mechanism which facilitates you to reuse the fields and methods of the existing class when you create a new class. You can use the same fields and methods already defined in the previous class.

 

Syntax:
class Subclass-name extends Superclass-name
    {
    //methods and fields
    }

The extends keyword indicates that you are making a new class that derives from an existing class. The meaning of "extends" is to increase the functionality.

 

In the terminology of Java, a class which is inherited is called a parent or superclass, and the new class is called child or subclass.

In the above figure, Programmer is the subclass and Employee is the superclass. 

The relationship between the two classes is Programmer IS-A Employee. It means that Programmer is a type of Employee.

 

Ex1:

class Employee

 {

 float salary=40000;

 }

  class Programmer extends Employee

  {

     int bonus=10000;

     public static void main(String args[])

    {

     Programmer p=new Programmer();

     System.out.println("Programmer salary is:"+p.salary);                               System.out.println("Bonus of Programmer is:"+p.bonus);

    }

  }

Output:

ubuntu@ubuntu:~/Desktop/rkjava$ javac Programmer.java
ubuntu@ubuntu:~/Desktop/rkjava$ java Programmer
Programmer salary is:40000.0
Bonus of Programmer is:10000

In the above example, Programmer object can access the field of own class as well as of Employee class
i.e. code reusability.
 
Types of inheritance

On the basis of class, there can be three types of inheritance in java: single, multilevel and hierarchical. In java programming, multiple and hybrid inheritance is supported through interface only.

Single Inheritance

When a class inherits another class, it is known as a single inheritance. In the example given below, Dog class inherits the Animal class, so there is the single inheritance.

Ex1:

class Animal

{

void eat()

{

System.out.println("eating...");

}

}

 

class Dog extends Animal

{

void bark()

{

System.out.println("barking...");

}

}

class TestInheritance

{

public static void main(String args[])

{

Dog d=new Dog(); 

d.bark();

d.eat();

}

}

Output: 

ubuntu@ubuntu:~/Desktop/rkjava$ javac TestInheritance.java
ubuntu@ubuntu:~/Desktop/rkjava$ java TestInheritance
barking...
eating...

Ex2:

class Faculty

{

String designation = "Asst.Professor";

String collegeName = "Gayatri Vidya Parishad"; 

void does()

{

System.out.println("Giving Lectures");

}

}

public class ComputerFaculty extends Faculty

{

String mainSubject = "ComputerScience"; 

public static void main(String args[])

{

ComputerFaculty obj = new ComputerFaculty(); 

System.out.println(obj.collegeName); 

System.out.println(obj.designation); 

System.out.println(obj.mainSubject);

obj.does();

}

}

Output:

ubuntu@ubuntu:~/Desktop/rkjava$ javac ComputerFaculty.java
ubuntu@ubuntu:~/Desktop/rkjava$ java ComputerFaculty
Gayatri Vidya Parishad
Asst.Professor
ComputerScience
Giving Lectures

Multilevel Inheritance

When there is a chain of inheritance, it is known as multilevel inheritance. It refers to a child and parent class relationship where a class extends the child class. 

For example class C extends class B and class B extends class A.

Ex:

class Animal

{

void eat()

{

System.out.println("eating...");

}

}

class Dog extends Animal

{

void bark()

{

System.out.println("barking...");

}

}

class BabyDog extends Dog

{

void weep()

{

System.out.println("weeping...");

}

}

class TestInheritance2

{

public static void main(String args[])

{

BabyDog d=new BabyDog(); 

d.weep();

d.bark();

d.eat();

}

}

Output: 

ubuntu@ubuntu:~/Desktop/rkjava$ javac TestInheritance2.java
ubuntu@ubuntu:~/Desktop/rkjava$ java TestInheritance2
weeping...
barking...
eating...

 

Hierarchical Inheritance

When two or more classes inherits a single class, it is known as hierarchical inheritance. In the example given below, Dog and Cat classes inherits the Animal class, so there is hierarchical inheritance.

Ex:

class Animal

{

void eat()

{

System.out.println("eating...");

}

}

class Dog extends Animal

{

void bark()

{

System.out.println("barking...");

}

}

class Cat extends Animal

{

void meow()

{

System.out.println("meowing...");

}

}

class TestInheritance3

{

public static void main(String args[])

{

Cat c=new Cat(); 

c.meow();

c.eat();

//c.bark(); //C.T.Error

}

}

Output: 

ubuntu@ubuntu:~/Desktop/rkjava$ javac TestInheritance3.java
ubuntu@ubuntu:~/Desktop/rkjava$ java TestInheritance3
meowing...
eating...

Why multiple inheritance is not supported in java?

To reduce the complexity and simplify the language, multiple inheritance is not supported in java.

Consider a scenario where A, B, and C are three classes. The C class inherits A and B classes. If A and B classes have the same method and you call it from child class object, there will be ambiguity to call the method of A or B class.

Since compile-time errors are better than runtime errors, Java renders compile-time error if you inherit 2 classes. So whether you have same method or different, there will be compile time error.

Ex: 

class A

{

void msg()

{

System.out.println("Hello");

}

}

class B

{

void msg()

{

System.out.println("Welcome");

}

}

class C extends A , B

{

//suppose if it were

public static void main(String args[])

{

C obj=new C();

obj.msg();//Now which msg() method would be invoked?

}

}

Output:

Compile Time Error