Software design pattern

In software engineering, a design pattern is a general reusable 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.

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. 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 Programming Conference was held and the following year, the Portland Pattern Repository was set up for documentation of design patterns. The scope of the term remains a matter of dispute. Notable books in the design pattern genre include:

• Fowler, Martin (2002). Patterns of Enterprise Application Architecture. Addison-Wesley. ISBN 978-0321127426.
• Gamma, Erich (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. ISBN 0-201-63361-2. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Hohpe, Gregor (2003). Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions. Addison-Wesley. ISBN 0-321-20068-3. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

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

Practice

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.

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

By definition, a pattern must be programmed anew into each application that uses it. Since some authors see this as a step backward from software reuse as provided by components, researches have worked to turn patterns into components. Meyer and Arnout claim a 2/3rds success rate in componentizing the best-known 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.

Structure

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.

Domain specific patterns

Efforts have also been made to codify design patterns in particular domains, including use of existing design patterns as well as domain specific design patterns. Examples include User Interface design patterns, Information Visualization and web design.

The Pattern Languages of Programming Conference(annual,1994->) proceedings includes many examples of domain specific patterns.

Classification

Design Patterns originally grouped design patterns into the categories Creational Patterns, Structural Patterns, and Behavioral Patterns, and described them using the concepts of delegation, aggregation, and consultation. For further background on object-oriented design, see coupling and cohesion. For further background on object-oriented programming, see inheritance, interface, and polymorphism. Another classification has also introducted the notion of architectural design pattern which may be applied at the architecture level of the software such as the Model-View-Controller pattern.

Name	Description	In Design Patterns	In Code Complete
Creational patterns
Abstract factory	Provide an interface for creating families of related or dependent objects without specifying their concrete classes.	Yes	Yes
Factory method	Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.	Yes	Yes
Builder	Separate the construction of a complex object from its representation so that the same construction process can create different representations.	Yes	No
Lazy initialization	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.	No	No
Object pool	Avoid expensive acquisition and release of resources by recycling objects that are no longer in use	No	No
Prototype	Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.	Yes	No
Singleton	Ensure a class only has one instance, and provide a global point of access to it.	Yes	Yes
Utility	A class with a private constructor that contains only static methods.
Structural patterns
Adapter	Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.	Yes	Yes
Bridge	Decouple an abstraction from its implementation so that the two can vary independently.	Yes	Yes
Composite	Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.	Yes	Yes
Decorator	Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.	Yes	Yes
Facade	Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.	Yes	Yes
Flyweight	Use sharing to support large numbers of fine-grained objects efficiently.	Yes	No
Proxy	Provide a surrogate or placeholder for another object to control access to it.	Yes	No
Behavioral patterns
Chain of responsibility	Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.	Yes	No
Command	Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.	Yes	No
Interpreter	Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.	Yes	No
Iterator	Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.	Yes	Yes
Mediator	Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.	Yes	No
Memento	Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.	Yes	No
Observer	Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.	Yes	Yes
State	Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.	Yes	No
Strategy	Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.	Yes	Yes
Specification	Recombinable Business logic in a boolean fashion	No	No
Template method	Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.	Yes	Yes
Visitor	Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.	Yes	No
Single-serving visitor	Optimize the implementation of a visitor that is allocated, used only once, and then deleted	No	No
Hierarchical visitor	Provide a way to visit every node in a hierarchical data structure such as a tree.	No	No
Concurrency patterns
Active Object		No	No
Balking		No	No
Double checked locking		No	No
Guarded		No	No
Leaders/followers		No	No
Monitor object		No	No
Read write lock		No	No
Scheduler		No	No
Thread pool		No	No
Thread-specific storage		No	No
Reactor		No	No

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. 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. 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, or GoF for short) 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.

Criticism

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

Workarounds for missing language features

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, Self and JavaScript.

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

Does not differ significantly from other abstractions

Some authors allege that design patterns don't differ significantly from other forms of abstraction, 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. 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.

See also

• 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)
• Service-Oriented Modeling Framework (SOMF)

Further reading

Books

• Alexander, Christopher (1977). A Pattern Language: Towns, Buildings, Construction. New York: Oxford University Press. ISBN 978-0195019193. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Beck, Kent (2007). Implementation Patterns. Addison-Wesley. ISBN 978-0321413093. {{cite book}}: Cite has empty unknown parameter: |coauthors= (help); Unknown parameter |month= ignored (help)
• Beck, Kent (1996). Proceedings of the 18th International Conference on Software Engineering. pp. 25–30. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
• Borchers, Jan (2001). A Pattern Approach to Interaction Design. John Wiley & Sons. ISBN 0-471-49828-9.
• Coplien, James O. (1995). Pattern Languages of Program Design. Addison-Wesley. ISBN 0-201-60734-4. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Coplien, James O. (1996). Pattern Languages of Program Design 2. Addison-Wesley. ISBN 0-201-89527-7. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Fowler, Martin (2002). Patterns of Enterprise Application Architecture. Addison-Wesley. ISBN 978-0321127426.
• Freeman, Eric (2004). Head First Design Patterns. O'Reilly Media. ISBN 0-596-00712-4. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Gabriel, Richard (1996). Patterns of Software: Tales From The Software Community (PDF). Oxford University Press. p. 235. ISBN 0-19-512123-6.
• Gamma, Erich (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. ISBN 0-201-63361-2. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Hohpe, Gregor (2003). Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions. Addison-Wesley. ISBN 0-321-20068-3. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Holub, Allen (2004). Holub on Patterns. Apress. ISBN 1-59059-388-X.
• Kerievsky, Joshua (2004). Refactoring to Patterns. Addison-Wesley. ISBN 0-321-21335-1.
• Kircher, Michael (2005). Remoting Patterns: Foundations of Enterprise, Internet and Realtime Distributed Object Middleware. John Wiley & Sons. ISBN 0-470-85662-9. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Larman, Craig (2005). Applying UML and Patterns. Prentice Hall. ISBN 0-13-148906-2.
• Manolescu, Dragos (2006). Pattern Languages of Program Design 5. Addison-Wesley. ISBN 0-321-32194-4. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Marinescu, Floyd (2002). EJB Design Patterns: Advanced Patterns, Processes and Idioms. John Wiley & Sons. ISBN 0-471-20831-0.
• Martin, Robert Cecil (1997). Pattern Languages of Program Design 3. Addison-Wesley. ISBN 0-201-31011-2. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Mattson, Timothy G (2005). Patterns for Parallel Programming. ISBN 0-321-22811-1. {{cite book}}: Unknown parameter |Publisher= ignored (|publisher= suggested) (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Shalloway, Alan (2001). Design Patterns Explained, Second Edition: A New Perspective on Object-Oriented Design. Addison-Wesley. ISBN 0-321-24714-0. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Vlissides, John M. (1998). Pattern Hatching: Design Patterns Applied. Addison-Wesley. ISBN 0-201-43293-5.
• Weir, Charles (2000). Small Memory Software: Patterns for systems with limited memory. Addison-Wesley. ISBN 0201596075. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

Web sites

• "History of Patterns". Portland Pattern Repository. Retrieved 2005-07-28.
• "Are Design Patterns Missing Language Features?". Cunningham & Cunningham, Inc. Retrieved 2006-01-20.
• "Show Trial of the Gang of Four". Cunningham & Cunningham, Inc. Retrieved 2006-01-20.

External links

• 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 Curlie
• 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
• Printable Design Patterns Quick Reference Cards
• 101 Design Patterns & Tips for Developers