OOPS

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.

Class
A class describes all the attributes of objects, as well as the methods that implement the behavior of member objects. It’s a comprehensive data type which represents a blue print of objects. It’s a template of object.

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.

Relationship between Classes and Objects
Class is a definition, while object is an 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 specific car object with fixed max speed and urgent brakes.

Characteristics of Object Oriented System’s

Abstraction
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 achieve any color in world. It’s a model of real world or concept.

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

Communication using Messages
When application wants to achieve certain task it can only be done using combination of objects. A single object can not do the entire task. Example if we want to make order processing form. We will use Customer object, Order object, Product object and Payment object to achieve 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 (Inheritance and Aggregation)
In object oriented world objects have relation and hierarchies in between them. There are basically three kind of relationship in Object Oriented world:

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

Aggregation
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.

Inheritance
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 specialized class like CAR, which will add new attributes and use some existing qualities of the parent class. Its demonstrate more of a parent-child relationship. This kind of hierarchy is called inheritance.