Chapter 10 Design Patterns

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

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

Creational Patterns Abstract factory Builder Factory method 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

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

Structural Patterns Adapter Bridge Composite Decorator 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

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

Behavioral Patterns Chain of responsibility Command Interpreter 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

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

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

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