Diff_Interface&Abstract-class

What is Interface:
===================
-> The interface in java is a mechanism to achieve abstraction.
-> There can be only abstract methods in the java interface not method body.
-> It is used to achieve abstraction and multiple inheritance in Java.
-> Java Interface also represents IS-A relationship.

Why use Java interface?:
======================
1. It is used to achieve abstraction.
2. By interface, we can support the functionality of multiple inheritance.
3. It can be used to achieve loose coupling.

Abstraction:   (Security Purpose)
===========
-> Highlighting set of services what we are offering and hiding implementation details is nothing but Abstraction. Example: ATM.
-> Hiding internal details and showing functionality is known as abstraction.
-> For example: phone call, we don't know the internal processing.
-> In java, we use abstract class and interface to achieve abstraction.


Interface
=================
1. If we don't know anything about implementation just we have requirement specification
     then we should go for Interface.
2. Every methods inside Interface is always public and abstract.
3. We can't declare Interface methods with the following modifiers: private, protected,
     static, final, synchronized, native and strictfp.
4. Every variable present inside Interface must be public, static, final whether we are declaring
    or not.
5. We can't declare Interface variables with the following modifiers private, protected, transient
    and volatile.
6. For Interface variables compulsory we should perform initialization at the time of declaration,
    otherwise we will get compile time error.
7. Inside Interface we can't declare static and instance blocks.
8. Inside Interface we can't take constructor.
==========================================================================================

Abstract Class
=============
1. If we are talking about implementation but not completely (partial implementation)
     then we should go for Abstract class.
2. Every method present inside Abstract Class need not be public & abstract, we can
     take concrete methods also.
3. There are no restrictions on abstract class method modifiers.
4. Every variable present inside Abstract Class need not be public static and final.
5. There are no restrictions on Abstract class variable modifiers.
6. For Abstract Class variables which is not required to perform initialization at the time of declaration.
7. Inside Abstract Class we can declare static and instance blocks.
8. Inside Abstract Class we can take constructor.