Design Patterns

Here are my notes on using the vocabulary of design patterns to reuse designs.

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History: Where did this come from?

Though this be madness, yet there is method in 't. —William Shakespeare

The word “Design Patterns” was first used by architect Christoper Alexander in his article “Toward a Personal Workspace” and book with Sara Ishikawa and Murray Silverstein: “A Pattern Language: Towns, Buildings, Construction” Oxford University Press 1977 ISBN 0-19-501919-9.

Alexander believed that while most processes involved in designing physical structures were variable, there was always a single common invariant process underlying all other processes, which precisely defined the principles of the structure's design and construction. —AntiPatterns

The “Gang of Four” (Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides) applied Alexander's concepts to computer systems in their 1994 book $50 Design Patterns, Elements of Reusable Object-Oriented Software from Addison-Wesley, ISBN 0-201-63361-2. 395 pages. This “GoF” have their own newsgroup.

Specific Patterns

Early examples were in Smalltalk and C++, but have been applied to other languages:
Advanced C++ Programming Styles and Idioms by J. Coplien (Addison-Wesley, 1991 ISBN 0-201-54855-0).

Examples of Design Patterns in Microsoft's Visual Basic were provided by Griver, Arnheiter, Gellis (of Flash Creative) in their book “Visual Basic Developer's Guide to UML and Design Patterns”.

$33 Patterns in Java, Volume 1: A Catalog of Reusable Design Patterns Illustrated with UML by Mark Grand (John Wiley, 1998 ISBN 0-471-25839-3). 480 pages, but not a lot of detail among the typos.
$38 Volume 2 Mar. 1999 offers 50 specialized reusable patterns in 368 pages. I applaud the author for his discussion on White/Black Box and Regression Testing, which others argue are methodologies not design patterns.
$30 Volume 3 Java Enterprise Design Patterns Feb. 2001

Data Model Patterns: Conventions of Thought by David Hay (Dorset House, 1995 ISBN 0-932633-29-3).

CORBA Design Patterns by Thomas Mowbray (John Wiley, 1997 ISBN 0-471-15882-8).

$60 Object-Oriented Software Development in Java: Principles, Patterns, and Frameworks by Xiaoping Jia. Addison-Wesley, October 1999. 550 pages. A great introduction: thorough yet suscinct. Analysis Patterns: Reusable Object Models by Martin Fowler (Addison-Wesley, 1996 ISBN 0-201-89542-0).

Design Patterns for Object-Oriented Development by Wolfgang Pree (Addison-Wesley, 1995 ISBN 0-201-42294-8).

Pattern Oriented Software Architecture: A System of Patterns by F. Buschman, et. al. (John Wiley, 1996 ISBN 0-471-95869-7).

Pattern Languages of Program Design by J. Coplien and D. Schmidt (Addison-Wesley, 1995 ISBN 0-201-60734-4).
Pattern Languages of Program Design 2 by John Vlissides, J. Coplien and Normal Kerth (Addison-Wesley, 1996 ISBN 0-201-89527-7).
Pattern Languages of Program Design 3 by R. Martin (Addison-Wesley, 1997 ISBN 0-201-31011-2).
Pattern Hatching: Design Patterns Applied by John Vlissides (Addison-Wesley, 1998 ISBN 0-201-43293-5).


AntiPatterns

This book presents patterns to describe the roots of organizational disfunctions (such as the “Seven Deadly Sins”) that undermine or enhance system develoopment efforts:

AntiPatterns: Refactoring Software, Architectures, and Projects in Crisis by William Brown, Raphael C. Malveau, Hays W. McCormick III, William H. Brow, and Thomas J. Mowbray (John Wiley, 1998 ISBN 0-471-19713-0).

The Antipatterns newsgroup

Essential Elements

A design pattern has four essential elements:
• Pattern Name allow developers to discuss patterns without
• Problem - when it makes sense to use a pattern.
• Solution - the object designs and collaborations that solves a problem.
• Consequences - the trade-offs involved in the solution and other patterns that may also be helpful.

Types of Patterns

Type of Pattern	Conventional Patterns	Description
Structural Patterns describe the architecture and how objects interact with each other. They allow designers to organize classes and objects into larger structures.	adapter (wrapper) bridge composite decorator facade flyweight proxy
Behavioral Patterns describes how objects are used to change the behavior of a system at run time. They assign responsibilities to objects.	mediator chain of responsibility command interpreter iterator visitor template hook memento observer state strategy
Creational Patterns are used when creating objects. They describe techniques to instantiate objects (or groups of objects).	factory builder singleton prototype

Structural Patterns

The adapter pattern allows two objects to communicate even though they have incompatible interfaces. It is also known as the wrapper pattern. It uses an intermediary object that is compliant with the client object and delegates the execution of the object requests to the incompatible server object.

A bridge pattern separates an interface from its implementation. Used by Microsoft's COM (ADO, RDO) for flexibility so that the two can vary independent (uncoupled) from each other at run-time. This pattern allows standarization of an implementation across multiple classes by bridging all interfaces classes to one implementation object.

The Decorator pattern allows additional responsibilities to be assigned to an object dynamically, without subclassing or modifying the interface of the original object.

The facade pattern is used when a system provides its services through multiple subsuystems or through calling multiple procedures. The facade shields clients from the intricacies of subsystems by making calls to its constituents to provide services.

The Proxy pattern provides a surrogate object that delegates method calls to another object. It acts like a placeholder for an object that can be used to efficiently control access to the other object. The proxy mimics the other object to the extent that the client does not know it is communicating with a proxy. This pattern is used whenever an object needs to receive method calls on behalf of another object.

The Composite pattern eliminates the need to write case statements that depend on the type of class when traversing a hierarchy. Client code can recursively iterate through a hierarchy without having to write separate code to distinguish between collections and primitives.

The Flyweight pattern provides a method to pool and share a large number of contexts. It allows a single object to be used in several contexts simultaneously.

Behavioral Patterns

CreationalPatterns

Resources

• The original Design Patterns FAQ from Oswego College

• Ward Cunningham's Portland Pattern Respository at c2.com and the original Wiki Web.
• The Hillside Patterns Library calls itself the “Patterns Home Page”
• Pattern Depot
• Pattern Stories at the University of Illinois at Urbana-Champaign hosts postings of experiences with using patterns in development projects. Their mailing lists:
  • patterns for presenting and describing software patterns
  • business-patterns
  • general discussions about patterns
• conferences on Pattern Languages of Programs (PLoP)