Monday, 1 May 2017

Inheritance in Java

Inheritance in java is a mechanism in which one object acquires all the properties and behaviors of parent object.

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 parent class, and you can add new methods and fields also.

Inheritance represents the IS-A relationship, also known as parent-child relationship.

Why use inheritance in java


  • For Method Overriding (so runtime polymorphism can be achieved).
  • For Code Reusability.

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.

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

Java Inheritance Example

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As displayed in the above figure, Programmer is the subclass and Employee is the superclass. Relationship between two classes is Programmer IS-A Employee.It means that Programmer is a type of Employee.

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);  
}  
}  

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 in java


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. We will learn about interfaces later.

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When a class extends multiple classes i.e. known as multiple inheritance. For Example: 

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Single Inheritance Example


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();  
}}  

Output:

barking...
eating...

Multilevel Inheritance Example


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();  
}}  

Output:

weeping...
barking...
eating...

Hierarchical Inheritance Example


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  
}}  

Output:

meowing...
eating...