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 systems).

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 to your current class also.

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

Why 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: A subclass is a class that 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 that 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.


The syntax of Java Inheritance

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.


Types of inheritance in java

1) Single Inheritance

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

File: TestInheritance.java

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();  
}}

2) Multilevel Inheritance

When there is a chain of inheritance, it is known as multilevel inheritance. As you can see in the example given below, BabyDog class inherits the Dog class which again inherits the Animal class, so there is a multilevel inheritance.

File: TestInheritance2.java

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();  
}}

3) Hierarchical Inheritance

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

File: TestInheritance3.java

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  
}}