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In software engineering, 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.

Not all software patterns are design patterns. Design patterns deal specifically with problems at the level of software design. Other kinds of patterns, such as architectural patterns, describe problems and solutions that have alternative scopes.

Contents 1 History 2 Uses 3 Caveats 4 Classification 5 Documentation 6 Criticism 6.1 Unlike components, does not provide reuse 6.2 Workarounds for missing language features 6.3 Does not differ significantly from other abstractions 7 See also 8 References 9 Further reading 10 External links

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]

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 it also improves code readability for coders and architects who are 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 below). 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.

In order to achieve flexibility, design patterns usually introduce additional levels of indirection, which might complicate the resulting designs and hurt application performance.

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

• Fundamental patterns
  • Delegation pattern: an object outwardly expresses certain behaviour but in reality delegates responsibility
  • Functional design: assures that each modular part of a computer program has only one responsibility and performs that with minimum side effects
  • Interface pattern: method for structuring programs so that they're simpler to understand
  • Proxy pattern: an object functions as an interface to another, typically more complex, object
  • Facade pattern: provides a simplified interface to a larger body of code, such as a class library.
  • Composite pattern: defines Composite object (e.g. a shape) designed as a composition of one-or-more similar objects (other kinds of shapes/geometries), all exhibiting similar functionality. The Composite object then exposes properties and methods for child objects manipulation as if it were a simple object.

• Creational patterns which deal with the creation of objects. Abstract Factory and Factory Method are creation patterns
  • Abstract factory pattern: centralize decision of what factory to instantiate
  • Factory method pattern: centralize creation of an object of a specific type choosing one of several implementations
  • Builder pattern: separate the construction of a complex object from its representation so that the same construction process can create different representations
  • Lazy initialization pattern: tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed
  • Object pool: avoid expensive acquisition and release of resources by recycling objects that are no longer in use
  • Prototype pattern: used when the inherent cost of creating a new object in the standard way (e.g., using the 'new' keyword) is prohibitively expensive for a given application
  • Singleton pattern: restrict instantiation of a class to one object

• Structural patterns that ease the design by identifying a simple way to realize relationships between entities. Adapter is a structural pattern
  • Adapter pattern: 'adapts' one interface for a class into one that a client expects
  • Aggregate pattern: a version of the Composite pattern with methods for aggregation of children
  • Bridge pattern: decouple an abstraction from its implementation so that the two can vary independently
  • Composite pattern: a tree structure of objects where every object has the same interface
  • Decorator pattern: add additional functionality to a class at runtime where subclassing would result in an exponential rise of new classes
  • Facade pattern: create a simplified interface of an existing interface to ease usage for common tasks
  • Flyweight pattern: a high quantity of objects share a common properties object to save space
  • Proxy pattern: a class functioning as an interface to another thing
  • Pipes and filters: a chain of processes where the output of each process is the input of the next
  • Private class data pattern: restrict accessor/mutator access

• Behavioral patterns that identify common communication patterns between objects and realize these patterns. Interpreter is a behavioral pattern
  • Chain of responsibility pattern: Command objects are handled or passed on to other objects by logic-containing processing objects
  • Command pattern: Command objects encapsulate an action and its parameters
  • Interpreter pattern: Implement a specialized computer language to rapidly solve a specific set of problems
  • Iterator pattern: Iterators are used to access the elements of an aggregate object sequentially without exposing its underlying representation
  • Mediator pattern: Provides a unified interface to a set of interfaces in a subsystem
  • Memento pattern: Provides the ability to restore an object to its previous state (rollback)
  • Null Object pattern: Designed to act as a default value of an object
  • Observer pattern: aka Publish/Subscribe or Event Listener. Objects register to observe an event which may be raised by another object
  • State pattern: A clean way for an object to partially change its type at runtime
  • Strategy pattern: Algorithms can be selected on the fly
  • Specification pattern: Recombinable Business logic in a boolean fashion
  • Template method pattern: Describes the program skeleton of a program
  • Visitor pattern: A way to separate an algorithm from an object
  • Single-serving visitor pattern: Optimise the implementation of a visitor that is allocated, used only once, and then deleted
  • Hierarchical visitor pattern: Provide a way to visit every node in a hierarchical data structure such as a tree.

• Concurrency patterns
  • Active Object
  • Balking pattern
  • Double checked locking pattern
  • Guarded suspension
  • Leaders/followers pattern
  • Monitor Object
  • Read write lock pattern
  • Scheduler pattern
  • Thread pool pattern
  • Thread-Specific Storage
  • Reactor pattern

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.

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

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.^[6]

Users of dynamic programming languages have discussed many design patterns as workarounds for the limitations of languages such as C++ and Java. For instance, the Visitor pattern need not be implemented in a language that supports multimethods. The purpose of Visitor is to add new operations to existing classes without modifying those classes -- but in a language with multimethods, methods are not part of the class structure. Similarly, the Decorator pattern amounts to implementing dynamic delegation, as found in Objective C and Self.

Peter Norvig, in "Design Patterns in Dynamic Programming", discusses the triviality of implementing various patterns in dynamic languages. [1] Norvig and others have described language features that encapsulate or replace various patterns that a C++ user must implement for themselves.

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]}

• Anti-pattern
• Interaction design pattern
• Pattern theory
• Pedagogical patterns
• Refactoring
• List of software engineering topics
• List of software development philosophies
• Christopher Alexander
• Portland Pattern Repository
• GRASP (Object Oriented Design)

• 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. 
• Alexander, Christopher; et al (1977). A Pattern Language: Towns, Buildings, Construction. New York: Oxford University Press. 
• Beck, K. (October 2007). Implementation Patterns. Pearson Education, 246. 
• 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. 
• 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. 
• Hohpe, Gregor; Bobby Woolf (2003). Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions. ISBN 0-321-20068-3. 
• 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; 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. 
• Manolescu, Dragos; Markus Voelter, and James Noble (2006). Pattern Languages of Program Design 5. Addison-Wesley. ISBN 0-321-32194-4. 
• 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. 
• Shalloway, Alan; James R. Trott (2001). Design Patterns Explained, Second Edition: A New Perspective on Object-Oriented Design. Addison-Wesley. ISBN 0-321-24714-0. 
• Vlissides, John M. (1998). Pattern Hatching: Design Patterns Applied. Addison-Wesley. ISBN 0-201-43293-5. 
• History of Patterns. Portland Pattern Repository. Retrieved on 2005-07-28.
• Are Design Patterns Missing Language Features?. Cunningham & Cunningham, Inc.. Retrieved on 2006-01-20.
• Show Trial of the Gang of Four. Cunningham & Cunningham, Inc.. Retrieved on 2006-01-20.

• Directory of websites that provide pattern catalogs at hillside.net.
• Explanation of design pattern in some simple examples at go4expert.com.
• Ward Cunningham's Portland Pattern Repository.
• Patterns and Anti-Patterns at the Open Directory Project
• PerfectJPattern Open Source Project Design Patterns library that aims to provide full or partial componentized version of all known Patterns in Java.
• Jt J2EE Pattern Oriented Framework

v • d • e Design patterns in Design Patterns
Creational: Abstract factory • Builder • Factory • Prototype • Singleton Structural: Adapter • Bridge • Composite • Decorator • Façade • Flyweight • Proxy Behavioral: Chain of responsibility • Command • Interpreter • Iterator • Mediator • Memento • Observer • State • Strategy • Template method • Visitor