Introduction to Design Patterns

1. Introduction to Design Patterns 1

2. A pattern is based on the principles of object-oriented programming: abstraction, inheritance, polymorphism and association • A pattern is a recurringsolutionto a standard problem, in a context • A pattern is independent of the application domain • A pattern is a three-part rule, which expresses a relation between a certain context, a problem, and a solution Pattern??

3. Using patterns offers a few key advantages: • Promote reuse • Facilitate a proven solution • Provide a common vocabulary • “Design patterns help a designer get a design right faster” Design Pattern Advantages

4. Creational patterns • Focus on the best way to create instances of objects to promote flexibility • Structural patterns • Focus on the composition of classes and objects into larger structures • Behavioral patterns • Focus on the interaction between classes or objects • Distribute responsibility Types of Patterns

5. Some patterns are used together, for example the composite pattern is sometimes used with the iterator or visitor • Some patterns are also defined as alternatives for others, e.g. the prototype pattern can be used as an alternative to the abstract factory pattern • Some patterns, e.g. the composite and decorator pattern, result in similar designs Relationships between Patterns

6. Abstract Factory • Factory for building related objects • Builder • Separates an object construction from its representation • Factory Method • Creates an instance of several derived classes • Prototype • Factory for cloning new instances from a prototype • Singleton • A class of which a single object can exist Creational Design Patterns

7. Adapter • Match interfaces of different classes • Bridge • Separates an object’s interface from its implementation • Composite • A tree structure of simple and composite objects • Decorator • Add responsibilities of objects dynamically • Facade • A single class that represents the entire subsystem Structural Patterns

8. Flyweight • A fine-grained objects shared efficiently • Proxy • An object representing another object Structural Patterns (cont.)

9. Chain of Responsibility • Passes a request between a chain of objects • Command • Encapsulates a command request as an object • Iterator • Elements of a collection are accessed sequentially • Interpreter • Language elements are included • Mediator • Defines simplified communication between classes Behavioral Patterns

10. Memento • Capture and restores an object’s internal state • Observer • A way of notifying change to a number of classes • State • Alter an object’s behavior when its state changes • Strategy • Encapsulates an algorithm inside a class • Template Method • Defer the exact steps of an algorithm to a subclass Behavioral Patterns (cont.)

11. Visitor • Defines a new operation to a class without change Behavioral Patterns (cont.)

12. Design patterns enable large-scale reuse of software architectures and also help document systems • Patterns help improve developer communication • Pattern names form a common vocabulary • Patterns help ease the transition to Object Oriented technology Benefits of Design Patterns

13. Patterns do not lead to direct code reuse • Patterns are deceptively simple • Teams may suffer from pattern overload • Patterns are validated by experience and discussion rather than by automated testing • Integrating patterns is a human-intensive activity Drawbacks of Design Patterns

14. Obtain an overview of the pattern • Obtain an understanding of the classes and objects and relationships between them • Choose application-specific names for the components of the patterns • Define the classes • Choose application-specific names for the operations defined in the pattern • Implement the necessary operations and relationships Suggestions for Effective Use

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16. References