Design pattern (computer science)

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In software engineering (or computer science), a design pattern is a general repeatable solution to a commonly occurring problem in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that can be used in many different situations. Object-oriented design patterns typically show relationships and interactions between classes or objects, without specifying the final application classes or objects that are involved. Algorithms are not thought of as design patterns, since they solve computational problems rather than design problems.

Contents 1 History 2 Uses 3 Classification 4 Documentation 5 Relation to dynamic languages 6 Criticism 6.1 Unlike components, does not provide reuse 6.2 Does not differ significantly from other abstractions 7 See also 8 References 9 Further reading 10 External links

[edit] History

Patterns originated as an architectural concept by Christopher Alexander (1977/79). In 1987, Kent Beck and Ward Cunningham began experimenting with the idea of applying patterns to programming and presented their results at the OOPSLA conference that year^[1][2]. In the following years, Beck, Cunningham and others followed up on this work.

Design patterns gained popularity in computer science after the book Design Patterns: Elements of Reusable Object-Oriented Software was published in 1994 (Gamma et al). That same year, the first Pattern Languages of Programs conference was held and the following year, the Portland Pattern Repository was set up for documentation of design patterns. The scope of the term remained a matter of dispute into the next decade.

Although the practical application of design patterns is a phenomenon, formalization of the concept of a design pattern languished for several years^[3].

[edit] Uses

Design patterns can speed up the development process by providing tested, proven development paradigms. Effective software design requires considering issues that may not become visible until later in the implementation. Reusing design patterns helps to prevent subtle issues that can cause major problems and improves code readability for coders and architects familiar with the patterns.

Often, people only understand how to apply certain software design techniques to certain problems. These techniques are difficult to apply to a broader range of problems. Design patterns provide general solutions, documented in a format that doesn't require specifics tied to a particular problem.

Design patterns are composed of several sections (see Documentation). Of particular interest are the Structure, Participants, and Collaboration sections. These sections describe a design motif: a prototypical micro-architecture that developers copy and adapt to their particular designs to solve the recurrent problem described by the design pattern. A micro-architecture is a set of program constituents (e.g., classes, methods...) and their relationships. Developers use the design pattern by introducing in their designs this prototypical micro-architecture, which means that micro-architectures in their designs will have structure and organization similar to the chosen design motif.

In addition, patterns allow developers to communicate using well-known, well understood names for software interactions. Common design patterns can be improved over time, making them more robust than ad-hoc designs.

[edit] Classification

Design patterns can be classified in terms of the underlying problem they solve. Examples of problem-based pattern classifications include:

• Fundamental patterns
• Creational patterns which deal with the creation of objects
• Structural patterns that ease the design by identifying a simple way to realize relationships between entities.
• Behavioral patterns that identify common communication patterns between objects and realize these patterns.
• Concurrency patterns

There is also Architectural patterns, which is usually considered different from, and larger in scope than design pattern.

[edit] Documentation

The documentation for a design pattern describes the context in which the pattern is used, the forces within the context that the pattern seeks to resolve, and the suggested solution.^[4] There is no single, standard format for documenting design patterns. Rather, a variety of different formats have been used by different pattern authors. However, according to Martin Fowler certain pattern forms have become more well-known than others, and consequently become common starting points for new pattern writing efforts.^[5] One example of a commonly used documentation format is the one used by Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides (collectively known as the Gang of Four) in their book Design Patterns. It contains the following sections:

• Pattern Name and Classification: A descriptive and unique name that helps in identifying and referring to the pattern.
• Intent: A description of the goal behind the pattern and the reason for using it.
• Also Known As: Other names for the pattern.
• Motivation(Forces): A scenario consisting of a problem and a context in which this pattern can be used.
• Applicability: Situations in which this pattern is usable; the context for the pattern.
• Structure: A graphical representation of the pattern. Class diagrams and Interaction diagrams may be used for this purpose.
• Participants: A listing of the classes and objects used in the pattern and their roles in the design.
• Collaboration: A description of how classes and objects used in the pattern interact with each other.
• Consequences: A description of the results, side effects, and trade offs caused by using the pattern.
• Implementation: A description of an implementation of the pattern; the solution part of the pattern.
• Sample Code: An illustration of how the pattern can be used in a programming language
• Known Uses: Examples of real usages of the pattern.
• Related Patterns: Other patterns that have some relationship with the pattern; discussion of the differences between the pattern and similar patterns.

[edit] Relation to dynamic languages

Many design patterns bring some of the flexibility of dynamic programming languages to "non-dynamic" languages such as C, C++, and Java. According to Norvig, "16 of 23 patterns have qualitatively simpler implementation in Lisp or Dylan than in C++ for at least some uses of each pattern."^[6]

[edit] Criticism

In the field of computer science, there exist some criticisms regarding the concept of design patterns.

[edit] Unlike components, does not provide reuse

A pattern must be programmed anew into each application that uses it. Some authors see this as a step backward from software reuse as provided by components. This observation has led to work on "componentization": turning patterns into components, in particular by Meyer and Arnout, who claim a 2/3rds success rate in componentizing the best-known patterns. ^[7]

[edit] Does not differ significantly from other abstractions

Some authors allege that design patterns don't differ significantly from other forms of abstraction^{[citation needed]}, and that the use of new terminology (borrowed from the architecture community) to describe existing phenomena in the field of programming is unnecessary. The Model-View-Controller paradigm is touted as an example of a "pattern" which predates the concept of "design patterns" by several years.^{[citation needed]} It is further argued by some that the primary contribution of the Design Patterns community (and the Gang of Four book) was the use of Alexander's pattern language as a form of documentation; a practice which is often ignored in the literature.^{[citation needed]}

[edit] See also

• Anti-pattern
• Interaction design pattern
• Pedagogical patterns
• Programming practice
• Refactoring
• List of software engineering topics
• List of software development philosophies
• Pattern theory

[edit] References

[edit] Further reading

• Alexander, Christopher; et al (1977). A Pattern Language: Towns, Buildings, Construction. New York: Oxford University Press. 
• Beck, K.; R. Crocker, G. Meszaros, J.O. Coplien, L. Dominick, F. Paulisch, and J. Vlissides (March 1996). Proceedings of the 18th International Conference on Software Engineering, 25-30. 
• Borchers, Jan (2001). A Pattern Approach to Interaction Design. John Wiley & Sons. ISBN 0-471-49828-9. 
• Buschmann, Frank; Regine Meunier, Hans Rohnert, Peter Sommerlad, and Michael Stal (1996). Pattern-oriented Software Architecture, Volume 1: A System of Patterns. John Wiley & Sons. ISBN 0-471-95869-7. 
• Cooper, James W. (October 1998). The Design Patterns Java Companion. Addison-Wesley. 
• Coplien, James O.; Douglas C. Schmidt (1995). Pattern Languages of Program Design. Addison-Wesley. ISBN 0-201-60734-4. 
• Coplien, James O.; John M. Vlissides, and Norman L. Kerth (1996). Pattern Languages of Program Design 2. Addison-Wesley. ISBN 0-201-89527-7. 
• Freeman, Eric; Elisabeth Freeman, Kathy Sierra, and Bert Bates (2004). Head First Design Patterns. O'Reilly Media, 637. ISBN 0-596-00712-4. 
• Gabriel, Richard (1996). Patterns of Software: Tales From The Software Community. Oxford University Press, 235. ISBN 0-19-512123-6. 
• Gamma, Erich; Richard Helm, Ralph Johnson, and John Vlissides (1995). Design Patterns: Elements of Reusable Object-Oriented Software, hardcover, 395 pages, Addison-Wesley. ISBN 0-201-63361-2. 
• Gamma, Erich; Richard Helm, Ralph Johnson, and John Vlissides (1997). Design Patterns CD. ISBN 0-201-63498-8. 
• Goldfedder, Brandon (2002). The Joy of Patterns. Addison-Wesley. ISBN 0-201-65759-7. 
• Harrison, Neil; Brian Foote, and Hans Rohnert (1999). Pattern Languages of Program Design 4. Addison-Wesley. ISBN 0-201-43304-4. 
• Holub, Allen (2004). Holub on Patterns. Apress Publishers. ISBN 1-59059-388-X. 
• Kerievsky, Joshua (2004). Refactoring to Patterns. Addison-Wesley. ISBN 0-321-21335-1. 
• Kircher, Michael; Prashant Jain (2004). Pattern-oriented Software Architecture. Volume 3: Patterns for Resource Management. John Wiley & Sons. ISBN 0-470-84525-2. 
• Kircher, Michael; Markus Völter and Uwe Zdun (2005). Remoting Patterns: Foundations of Enterprise, Internet and Realtime Distributed Object Middleware. John Wiley & Sons. ISBN 0-470-85662-9. 
• Larman, Craig (2005). Applying UML and Patterns, Third Edition. Prentice Hall. ISBN 0-13-148906-2. 
• Marinescu, Floyd (2002). EJB Design Patterns: Advanced Patterns, Processes and Idioms. John Wiley & Sons. ISBN 0-471-20831-0. 
• Martin, Robert Cecil; Dirk Riehle, and Frank Buschmann (1997). Pattern Languages of Program Design 3. Addison-Wesley. ISBN 0-201-31011-2. 
• Schmidt, Douglas C.; Michael Stal, Hans Rohnert, and Frank Buschmann. Pattern-oriented Software Architecture. Volume 2: Patterns for Concurrent and Networked Objects. John Wiley & Sons. ISBN 0-471-60695-2. 
• Shalloway, Alan; James R. Trott (2001). Design Patterns Explained: A New Perspective on Object-Oriented Design. Addison-Wesley. ISBN 0-201-71594-5. 
• Vlissides, John M. (1998). Pattern Hatching: Design Patterns Applied. Addison-Wesley. ISBN 0-201-43293-5. 
• History of Patterns. Portland Pattern Repository. Retrieved on July 28, 2005.
• Are Design Patterns Missing Language Features?. Cunningham & Cunningham, Inc.. Retrieved on January 20, 2006.
• Show Trial of the Gang of Four. Cunningham & Cunningham, Inc.. Retrieved on January 20, 2006.

[edit] External links

• Directory of websites that provide pattern catalogs at hillside.net.
• Ward Cunningham's Portland Pattern Repository.
• Patterns and Anti-Patterns at the Open Directory Project (suggest site)