1. Design Patterns

2. 1 Paradigm 4 Concepts 9 Principles 23 Patterns

3. 1 Paradigm Object-Orientation is good! Not every-body agrees…

4. 4 Concepts • The foundations of Object-Orientation • Abstraction • Encapsulation • Inheritance • Polymorphism

5. 9 Principles • Just knowing the four concepts does not make you a good software designer… • Provide no guidance to design decisions • Concepts are made operational through a set of of principles for good Object-Oriented design

6. 9 Principles • Isolate the aspects of your application that vary • Program to an interface, not an implementation • Favor composition over inheritance • Strive for loosely coupled designs between interacting objects • Classes should be open for extension, but closed for modification • Depend upon abstractions, not upon concrete classes • Principle of Least Knowledge – only talk to your closest friends • The Hollywood principle – don’t call us, we’ll call you • A class should only have one reason to change

7. 9 Principles • Where do the principles come from…? • Accumulated wisdom, learned from solving real-life design problems • Some obvious… • Some not-so-obvious… • Some overlapping…

8. 9 Principles • Principles can be used operationally when designing; as guidelines and sanity check • What problems are we solving when designing software…? • Examined by the ”Gang of Four” in the 90’es • Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides

10. 23 Patterns • Authors examined a large base of high-quality code, to identify how common software design problems were solved • No theoretical work; pattens are discovered, not invented • 23 common strategies for solving design problems were identified, and dubbed design patterns

11. 23 Patterns • Design patterns fall in three categories • Creational: How are objects created • Structural: How are objects combined • Behavioral: How do objects interact

12. 23 Patterns • Creational patterns • Singleton • Prototype • Builder • Factory Method • Abstract Factory

13. 23 Patterns • Structural patterns • Adapter • Bridge • Composite • Decorator • Facade • Flyweight • Proxy

14. 23 Patterns • Behavioral patterns • Chain of responsibility • Command • Interpreter • Iterator • Mediator • Memento • Observer • State • Strategy • Template Method • Visitor

15. Design Patterns • Are distilled knowledge • Learned from practical experience • Solve a problem in a given context • Provides a common vocabulary • Is not the solution to all SW design issues… • …but almost 