Inheritance in Java

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.

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: 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.

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.

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.

Java Inheritance Example

    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);  
    }  
    }  

Test it Now

 Programmer salary is:40000.0
 Bonus of programmer is:10000

Types of inheritance in java

• Single
• Multilevel
• Hierarchical
• Multiple
• Hybrid

    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...	

       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...

          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...

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.

    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?  
    }  
    }  



 Compile Time Error

Aggregation in Java

If a class have an entity reference, it is known as Aggregation. Aggregation represents HAS-A relationship

Consider a situation, Employee object contains many informations such as id, name, emailId etc. It contains one more object named address, which contains its own informations such as city, state, country, zipcode etc. as given below.

    class Employee{  
    int id;  
    String name;  
    Address address;//Address is a class  
    ...  
    }  

In such case, Employee has an entity reference address, so relationship is Employee HAS-A address.

Why use Aggregation?

For Code Reusability.

In this example, we have created the reference of Operation class in the Circle class.

    class Operation{  
     int square(int n){  
      return n*n;  
     }  
    }  
      
    class Circle{  
     Operation op;//aggregation  
     double pi=3.14;  
        
     double area(int radius){  
       op=new Operation();  
       int rsquare=op.square(radius);//code reusability (i.e. delegates the method call).  
       return pi*rsquare;  
     }  
      
         
        
     public static void main(String args[]){  
       Circle c=new Circle();  
       double result=c.area(5);  
       System.out.println(result);  
     }  
    }  

Output:78.5
     

When use Aggregation?

Code reuse is also best achieved by aggregation when there is no is-a relationship. Inheritance should be used only if the relationship is-a is maintained throughout the lifetime of the objects involved; otherwise, aggregation is the best choice.

Understanding meaningful example of Aggregation

In this example, Employee has an object of Address, address object contains its own informations such as city, state, country etc. In such case relationship is Employee HAS-A address.

Address.java

    public class Address {  
    String city,state,country;  
      
    public Address(String city, String state, String country) {  
        this.city = city;  
        this.state = state;  
        this.country = country;  
    }  
      
    }  

Emp.java

    public class Emp {  
    int id;  
    String name;  
    Address address;  
      
    public Emp(int id, String name,Address address) {  
        this.id = id;  
        this.name = name;  
        this.address=address;  
    }  
      
    void display(){  
    System.out.println(id+" "+name);  
    System.out.println(address.city+" "+address.state+" "+address.country);  
    }  
      
    public static void main(String[] args) {  
    Address address1=new Address("gzb","UP","india");  
    Address address2=new Address("gno","UP","india");  
      
    Emp e=new Emp(111,"varun",address1);  
    Emp e2=new Emp(112,"arun",address2);  
          
    e.display();  
    e2.display();  
          
    }  
    }  



Output:111 varun
       gzb UP india
       112 arun
       gno UP india