Principles of Class Design (SOLID)

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        SRP: Single Responsibility Principle
               – An object should have only a single responsibility & all the responsibility should be entirely encapsulated by the class.
               – There should never be more than one reason for a class to change
            OCP: Open/Closed Principle
                – Software entities should be open for extension, but closed for modification
            LSP: Liskov Substitution Principle
                – Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program
           ISP: Interface Segregation Principle 
              – many client specific interfaces are better than one general purpose interface
              – Once an interface has gotten too 'LARGE' split it into smaller and more specific interfaces. So that, any clients of the interface will only know about the methods that pertain to them. No client should be forced to depend on methods it does not use
           DIP: Dependency Inversion Principle
              – Depend upon Abstractions. Do not depend upon concretions. 
              – Dependency Injection (DI) is one method of following this principle.

   Class Relationship:
•          Dependency
       Weaker form of relationship which indicates that, one class depends on another because, it uses it at some point of time.
Dependency exists, if a class is a parameter variable or local variable of a method of another class.

•          Association
Loose form of relationship (Student can enroll in multiple Courses, and A Course can have multiple Students)

•          Aggregation
Whole part relationship. Part can exist without Whole. (Engine can exist even if Car is destroyed, the same Engine could be used in a different Car)

•          Composition
Stronger form of whole part relationship. Part cannot exist without Whole. (OrderDetail cannot exist if Order is deleted. If Order is deleted, OrderDetail also gets deleted)

•          Generalization / Inheritance
       


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