Design Principles
The Open/Closed Principle Software entities (classes, modules, etc) should be open for extension, but closed for modification The Liskov Substitution Principle Liskov’s notion of “subtype” is based on the notion of substitutability; that is, if S is a subtype of T, then objects of type T in a program may be replaced with objects of type S without altering any of the desirable properties of that program (e.g., correctness). ie Derived classes must be usable through the base class interface without the need for the user to know the difference ...