Abstract class vs Interface

Abstract class

  • An abstract class is a class that is declared with 'abstract' keyword 
  • It may or may not include abstract methods
  • An abstract class cannot be instantiated. 
  • An abstract class can implement interface without even providing the implementation of interface methods.

  Syntax

        abstract class Payment 
        {
  //constants and methods
        }

  Example
  abstract class Payment
        {
  abstract void doPayment();
  }
  class CreditCardPayment extends Payment
        {
                @Override
  void doPayment()
                                    {
                   Sysout("Payment with CreditCard");
  }
  }

Interface

  • Interface is the way to achieve abstraction in Java.
  • 'Interface' keyword is used to create Interface.
  • Just like Abstract class Interface also cannot be instantiated.
  • All the fields declared in interface are public, static and final by default.
  • All the methods declared in interface are public and abstract by default. 
  • You can also create concrete method using 'default' keyword (introduced in Java8).
  • You can also create class inside Interface.

  Syntax
interface Payment
{
   //constants
   //abstract methods
   //concrete methods (introduced in Java8) using 'default' 
}

  Example

        interface Payment 
        {
             //constants
            String type= "PAYMENT";
            //abstract method
            void doPayment();

            //concrete method
           default void printMessage() {                                         System.out.println();
           }
         }
         
         public class CreditCard implements Payment
         {
            @Override
            public void doPayment() {
System.out.println("value is : "+ type);
            }
            public static void main(String[] args){
final CreditCard creditCard = new CreditCard();
creditCard.doPayment();
creditCard.printMessage();
            }
        }

             *printMessage() is default method, creating a concrete method in interface is possible with                  Java 8


Which should we use, abstract class or interface?

<< If you want to use abstract class >>

  • Basically abstract class should be used with closely related classes, for example 
    • "Animal" is abstract class Then its subclasses should be Lion, Tiger, Fox etc.
    • "Shape" is abstract class Then its subclasses should be circle, triangle, rectangle etc
  • When you expect that subclass has no other class to extend
  • When you want to declare non-static or non-final fields.

<< If you want to use Interface >>

  • You expect that unrelated classes would implement your interface. 
  • You want to specify the behaviour of a particular data type, but not concerned about who implements its behaviour.
  • You want to take advantage of multiple inheritance of type.
  • Examples - Cloneable, Comparable, Comparator


========================Thank You============================


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