4.5. What is a Class? A class is simply a representation of a type of object. It is the blueprint/ plan/ template that describe the details of an object. A class is the blueprint from which the individual objects are created. Class is composed of three things: a name, attributes, and operations. Collapse | Copy Code public class Student { } According to the sample given below we can say that the Student object, named objectStudent, has been created out of the Student class. Collapse | Copy Code Student objectStudent = new Student(); In real world, youll often find many individual objects all of the same kind. As an example, there may be thousands of other bicycles in existence, all of the same make and model. Each bicycle has built from the same blueprint. In object-oriented terms, we say that the bicycle is an instance of the class of objects known as bicycles. In the software world, though you may not have realized it, you have already used classes. For example, the TextBox control, you always used, is made out of the TextBox class, which defines its appearance and capabilities. Each time you drag a TextBox control, you are actually creating a new instance of the TextBox class. How to identify and design a Class? This is an art; each designer uses different techniques to identify classes. However according to Object Oriented Design Principles, there are five principles that you must follow when design a class, SRP - The Single Responsibility Principle - A class should have one, and only one, reason to change. OCP - The Open Closed Principle - You should be able to extend a classes behavior, without modifying it. LSP - The Liskov Substitution Principle- Derived classes must be substitutable for their base classes. DIP - The Dependency Inversion Principle- Depend on abstractions, not on concretions. ISP - The Interface Segregation Principle- Make fine grained interfaces that are client specific. For more information on design principles, please refer to Object Mentor. Additionally to identify a class correctly, you need to identify the full list of leaf level functions/ operations of the system (granular level use cases of the system). Then you can proceed to group each function to form classes (classes will group same types of functions/ operations). However a well defined class must be a meaningful grouping of a set of functions and should support the re-usability while increasing expandability/ maintainability of the overall system. In software world the concept of dividing and conquering is always recommended, if you start analyzing a full system at the start, you will find it harder to manage. So the better approach is to identify the module of the system first and then dig deep in to each module separately to seek out classes. A software system may consist of many classes. But in any case, when you have many, it needs to be managed. Think of a big organization, with its work force exceeding several thousand employees (let’s take one employee as a one class). In order to manage such a work force, you need to have proper management policies in place. Same technique can be applies to manage classes of your software system as well. In order to manage the classes of a software system, and to reduce the complexity, the system designers use several techniques, which can be grouped under four main concepts named Encapsulation, Abstraction, Inheritance, and Polymorphism. These concepts are the four main gods of OOP world and in software term, they are called four main Object Oriented Programming (OOP) Concepts.
Posted on: Sun, 23 Mar 2014 09:40:28 +0000
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