Chapter 10

1. Chapter 10 Design Patterns

2. Motivation for Design Patterns • Most software systems contain certain common aspects that are frequently reinvented for each system • Solutions to these common problems may vary in quality from system to system • Design patterns seeks to communicate these classic solutions in an easy to understand manner

3. What are Design Patterns? • Design Patterns communicate solutions to common programming problems • The seminal book on design patterns, Design Patterns, Elements of Reusable Object-Oriented Software by Gamma et al, identifies three categories of design patterns • Creational • Structural • Behavioral

4. Creational Patterns • Abstract factory • Facilitates the creation of related objects without concrete class reference • Builder • If a class supports a number of different data representations, the builder encapsulates the details of the necessary conversions • Factory method • Defers determination of specific class to create to a subclass

5. More Creational Patterns • Prototype • Embodies information comprising prototypical instances • Singleton • Ensures that the class has at most one instance and that other classes have global access to it

6. Structural Patterns • Adapter • Translates between interacting objects for which the method signatures are incompatible • Bridge • Creates a more flexible mechanism for multiple implementations of an abstract class than inheritance • Composite • Facilitates a generic hierarchical grouping of objects where the class and number of objects may change • Decorator • Allows dynamic addition of new behavior to objects by acting as an outer wrapper containing the added features

7. More Structural Patterns • Facade • Provides a front end for a series of classes so that users of these classes only need to know of and use the standard set of methods implemented by the facade • Flyweight • Describes a strategy for using an otherwise prohibitive number of very small-scale objects with minimal overhead • Proxy • A means by which the instantiation of certain classes may be delayed until the proxied object is needed

8. Behavioral Patterns • Chain of responsibility • A strategy that sets up a series of objects that may respond to a message depending on the context of the request • Command • Allows methods to be encapsulated in objects and invoked by a standard means as determined by the interface of the common class • Interpreter • If a set of problems are conducive to being represented as a simple grammar, a class may be defined for each rule

9. More Behavioral Patterns • Iterator • Separates the class elements necessary for access and traversal from the aggregating class and encapsulates them in the iterator • Mediator • Helps to avoid highly coupled class definitions by moving explicit object references to an intermediary object • Memento • A means to store the otherwise encapsulated internal state of an object for the purpose of creating a check-point or undoing transactions

10. More Behavioral Patterns • Observer • Allows multiple dependent instances of a class to be notified of relevant changes to the observed class • State • A class hierarchy is defined around a set of states for an object, allowing the behavior of an object to vary depending on a state by substituting the state-specific subclass in its composition • Strategy • Selects a set of related algorithms and encapsulates them in such a way that they may be interchangeable for solving certain problems

11. More Behavioral Patterns • Template method • Structures a method in such a way that elements of the algorithm are deferred to subclasses • Visitor • Allows the introduction of new operations on the objects that comprise the visited object