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Design Patterns
Vince Huston
new software
design blog


Design Pattern: identify a popular design strategy and document the domain-independent leverage.
Adapter: use an extra level of indirection to make 2 incompatible objects compatible.


GoF Design Patterns

GoF Structure Similarities

Left-Right symbol = wrapper/wrappee or delegation or "has a" relationship
Adapter ... wrap a legacy object that provides an incompatible interface with an object that supports the desired interface
Facade ... wrap a complicated subsystem with an object that provides a simple interface
Proxy ... wrap an object with a surrogate object that provides additional functionality
Up-Down symbol = inheritance hierarchy (promote interface to a base class and bury implementation alternatives in derived classes)
Strategy ... define algorithm interface in a base class and implementations in derived classes
Factory Method ... define "createInstance" placeholder in the base class, each derived class calls the "new" operator and returns an instance of itself
Visitor ... define "accept" method in first inheritance hierarchy, define "visit" methods in second hierarchy ... a.k.a. "double dispatch"
Category: a wrapper wraps an inheritance hierarchy
Builder ... the "reader" delegates to its configured "builder" ... each builder corresponds to a different representation or target
State ... the FiniteStateMachine delegates to the "current" state object, and that state object can set the "next" state object
Bridge ... the wrapper models "abstraction" and the wrappee models many possible "implementations" ... the wrapper can use inheritance to support abstraction specialization
Observer ... the "model" broadcasts to many possible "views", and each "view" can dialog with the "model"
Category: recursive composition
Composite ... derived Composites contain one or more base Components, each of which could be a derived Composite
Decorator ... a Decorator contains a single base Component, which could be a derived ConcreteComponent or another derived Decorator
Chain of Responsibility ... define "linked list" functionality in the base class and implement "domain" functionality in derived classes
Interpreter ... map a domain to a language, the language to a recursive grammar, and the grammar to the Composite pattern
Cloud symbol = promote X to "full object status"
Command ... encapsulate an object, the method to be invoked, and the parameters to be passed behind the method signature "execute"
Iterator ... encapsulate the traversal of collection classes behind the interface ""
Mediator ... decouple peer objects by encapsulating their "many to many" linkages in an intermediary object
Memento ... encapsulate the state of an existing object in a new object to implement a "restore" capability
Prototype ... encapsulate use of the "new" operator behind the method signature "clone" ... clients will delegate to a Prototype object when new instances are required
Category: miscellaneous
Abstract Factory ... model "platform" (e.g. windowing system, operating system, database) with an inheritance hierarchy, and model each "product" (e.g. widgets, services, data structures) with its own hierarchy ... platform derived classes create and return instances of product derived classes
Template Method ... define the "outline" of an algorithm in a base class ... common implementation is staged in the base class, peculiar implementation is represented by "place holders" in the base class and then implemented in derived classes
Flyweight ... when dozens of instances of a class are desired and performance boggs down, externalize object state that is peculiar for each instance, and require the client to pass that state when methods are invoked
Singleton ... engineer a class to encapsulate a single instance of itself, and "lock out" clients from creating their own instances



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