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What is Object Oriented Programming?
It is a problem solving technique to develop software systems. It is a technique to think real world in terms of objects. Object maps the software model to real world concept. These objects have responsibilities and provide services to application or other objects.
What is a Class?
A class describes all the attributes of objects, as well as the methods that implement the behavior of member objects. It is a comprehensive data type which represents a blue print of objects. It is a template of object.
What is an Object?
It is a basic unit of a system. An object is an entity that has attributes,behavior, and identity. Objects are members of a class. Attributes and behavior of an object are defined by the class definition.
What is the relation between Classes and Objects?
They look very much same but are not same. Class is a definition, while object is a instance of the class created. Class is a blue print while objects are actual objects existing in real world. Example we have class CAR which has attributes and methods like Speed,Brakes,Type of Car etc. Class CAR is just a prototype,now we can create real time objects which can be used to provide functionality. Example we can create a Maruti car object with 100 km speed and urgent brakes.
What are different properties provided by Object-oriented systems?
Following are characteristics of Object Oriented Systems :


It allows complex real world to be represented in simplified manner. Example color is abstracted to RGB. By just making the combination of these three colors we can acheive any color in world. It is a model of real world or concept.


It is a process of hiding all the internal details of an object from the outside world.

Communication using messages

When application wants to acheive certain task it can only be done using combination of objects. A single object cannot do all the task. Example if we want to make order processing form. we will use Customer object,Order object,Product object and Payment object to acheive this functionality. In short these objects should communicate with each other. This is achieved when objects send messages to each other.

Object lifetime

All objects have life time. Objects are created and initialized necessary functionalities are done and later the object is destroyed. Every object have there own state and identity which differ from instance to instance.

Class hierarchies
In object oriented world objects have relation and hierarchies in between them. There are basically three kind of relationship in Object Oriented World


This is the simplest relationship between objects. Example every customer has sales. So Customer object and sales object have an association relation between them.


This is also called as composition model. Example in order to make a "Accounts" class it has use other objects example "Voucher", "Journal" and "Cash" objects. so accounts class is aggregation of these three objects.


Hierarchy is used to define more specialized classes based on a preexisting generalized class. Example we have VEHICLE class and we can inherit this class make more specialize class like CAR, which will add new attributes and use some existing qualities of the parent class. Its shows more of a parent-child relationship. This kind of hierarchy is called inheritance.


When inheritance is used to extend a generalized class to a more specialized class, it includes behavior of the top class. The inheriting class often implement a behavior that can be somewhat different than the generalized class, but the name of the behavior can be same. It is important that a given instance of an object use the correct behavior, and the property of polymorphism allows this to happen automatically
What are abstract classes?
Following are features of a abstract class :

You can not create a object of abstract class
Abstract class is designed to act as a base class (to be inherited by other classes). Abstract class is a design concept in program development and provides a base upon which other classes are built.
Abstract classes are similar to interfaces. After declaring an abstract class,it cannot be instantiated on its own,it must be inherited.
In VB.NET abstract classes are created using "MustInherit" keyword. In C# we have "Abstract" keyword.
Abstract classes can have implementation or pure abstract methods which should be implemented in the child class.
What is a Interface?
Interface is a contract that defines the signature of the functionality. So if a class is implementing a interface it says to the outer world, that it provides specific behavior. Example if a class is implementing Idisposable interface that means it has a functionality to release unmanaged resources. Now external objects using this class knowthat it has contract by which it can dispose unused unmanaged objects.

Single Class can implement multiple interfaces.
If a class implements a interface then it has to provide implementation to all its methods.
What is the difference between abstract classes and interfaces?
Following are the differences between abstract and interfaces :

Abstract classes can have concrete methods while interfaces have no methods implemented.
Interfaces do not come in inheriting chain, while abstract classes come in inheritance.
What is a delegate?
Delegate is a class that can hold a reference to a method or a function. Delegate class has a signature and it can only reference those methods whose signature is complaint with the class. Delegates are type-safe functions pointers or callbacks.
What are events?
As compared to delegates events works with source and listener methodology. So listeners who are interested in receiving some events they subscribe to the source.Once this subscription is done the source raises events to its entire listener when needed. One source can have multiple listeners.

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