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{{short description|Reusable design for writing code that provides a well-defined function}}In software engineering, a design pattern describes a relatively small, well-defined aspect (i.e. functionality) of a computer program in terms of how to write the code.Using a pattern is intended to leverage an existing concept rather than re-inventing it. This can decrease the time to develop software and increase the quality of the resulting program.Notably, a pattern does not consist of a software artifact. Most development resources that a programmer uses involve configuring the codebase to use an artifact such as a library (to name just one example). In contrast, to use a pattern, a programmer writes code as described by the pattern. The result is unique every time even though the result may be recognizable as based on the pattern.Some consider using patterns to be best practice for software design.Some consider using design patterns as a structured approach to computer programming.Conceptually, design pattern may be described as more specific than programming paradigm and less specific than algorithm.

History

Patterns originated as an architectural concept by Christopher Alexander as early as 1977 (c.f. "The Pattern of Streets," JOURNAL OF THE AIP, September, 1966, Vol. 32, No. 5, pp. 273–278). In 1987, Kent Beck and Ward Cunningham began experimenting with the idea of applying patterns to programming – specifically pattern languages – and presented their results at the OOPSLA conference that year.CONFERENCE Design patterns gained popularity in computer science after the book (Design Patterns|Design Patterns: Elements of Reusable Object-Oriented Software) was published in 1994 by the so-called "Gang of Four" (Gamma et al.), which is frequently abbreviated as "GoF". 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:

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, Although design patterns have been applied practically for a long time, formalization of the concept of design patterns languished for several years.REPORT ,

Practice

Design patterns can speed up the development process by providing proven development paradigms.WEB , Effective software design requires considering issues that may not become apparent until later in the implementation. Freshly written code can often have hidden, subtle issues that take time to be detected; issues that sometimes can cause major problems down the road. Reusing design patterns can help to prevent such issues,BOOK,weblink 636, Software Applications: Concepts, Methodologies, Tools, and Applications: Concepts, Methodologies, Tools, and Applications, 9781605660615, Tiako, Pierre F., Pierre F, Tiako, 31 March 2009, Formal Modeling and Specification of Design Patterns Using RTPA, 10.4018/978-1-60566-060-8, and enhance code readability for those familiar with the patterns.Software design techniques are difficult to apply to a broader range of problems.{{Citation needed|date=August 2014}} Design patterns provide general solutions, documented in a format that does not require specifics tied to a particular problem.

Motif

A pattern describes a design motif, a.k.a. prototypical micro-architecture, as a set of program constituents (e.g., classes, methods...) and their relationships. A developer adapts the motif to their codebase to solve the problem described by the pattern. The resulting code has structure and organization similar to the chosen motif.

Domain-specific patterns

Efforts have also been made to codify design patterns in particular domains, including the use of existing design patterns as well as domain-specific design patterns. Examples include user interface design patterns,WEB, Laakso, Sari A., 2003-09-16, Collection of User Interface Design Patterns,weblink 2008-01-31, University of Helsinki, Dept. of Computer Science, information visualization,JOURNAL, 12, 5, Heer, J.! Name! Description! In Design Patterns! In Code CompleteBOOK ! Other| Lazy initialization

last2 = Agrawala, Software Design Patterns for Information Visualization, IEEE Transactions on Visualization and Computer Graphics, 2006,weblink 10.1109/TVCG.2006.178, 17080809, 853–60, 10.1.1.121.4534, 11634997, secure design,BOOK, Secure Design Patterns, 2009	last1 = Dougherty	last2 = Sayre	last3 = Seacord	last4 = Svoboda	last5 = Togashi,weblink Software Engineering Institute, "secure usability",THESIS, Ph.D. thesis, Design Principles and Patterns for Computer Systems That Are Simultaneously Secure and Usable, 2005	last = Garfinkel,weblink Web design WEB , Yahoo! Design Pattern Library , 2008-01-31 ,weblink , dead ,weblink" title="web.archive.org/web/20080229011119weblink">weblink , 2008-02-29 , and business model design.WEB, How to design your Business Model as a Lean Startup?, 2010-01-06,weblink 2010-01-06, The annual Pattern Languages of Programming Conference proceedings Pattern Languages of Programming, Conference proceedings (annual, 1994—) weblink include many examples of domain-specific patterns. Object-oriented programming Object-oriented design patterns typically show relationships and interactions between classes or objects, without specifying the final application classes or objects that are involved. Patterns that imply mutable state may be unsuited for functional programming languages. Some patterns can be rendered unnecessary in languages that have built-in support for solving the problem they are trying to solve, and object-oriented patterns are not necessarily suitable for non-object-oriented languages. Examples Design patterns can be organized into groups based on what kind of problem they solve. Creational patterns create objects. Structural patterns organize classes and objects to form larger structures that provide new functionality. Behavioral patterns provide communication between objects and realizing these patterns. Creational patterns {| class"wikitable"
Abstract factory pattern>Abstract factory| Provide an interface for creating families of related or dependent objects without specifying their concrete classes.| {{yes}}| {{yes}}| {{n/a}}
Builder pattern>Builder| Separate the construction of a complex object from its representation, allowing the same construction process to create various representations.| {{yes}}| {{yes}}| {{n/a}}
Dependency injection>Dependency Injection| A class accepts the objects it requires from an injector instead of creating the objects directly.| {{n/a}}| {{yes}}| {{n/a}}
Factory method pattern>Factory method| Define an interface for creating a single object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.| {{yes}}| {{yes}}| {{n/a}}
Proxy pattern>Proxy pattern.| {{yes}}| {{yes}}	{{abbr	Patterns of Enterprise Application Architecture}}BOOK , Martin , Fowler , Martin Fowler (software engineer) , 2002 , Patterns of Enterprise Application Architecture , Addison-Wesley , 978-0-321-12742-6 ,weblink , }}
Multiton pattern>Multiton| Ensure a class has only named instances, and provide a global point of access to them.| {{yes}}| {{yes}}| {{yes}}
Object pool pattern>Object pool| Avoid expensive acquisition and release of resources by recycling objects that are no longer in use. Can be considered a generalisation of connection pool and thread pool patterns.| {{yes}}| {{yes}}| {{yes}}
Prototype pattern>Prototype| Specify the kinds of objects to create using a prototypical instance, and create new objects from the 'skeleton' of an existing object, thus boosting performance and keeping memory footprints to a minimum.| {{yes}}| {{yes}}| {{yes}}
Resource Acquisition Is Initialization>Resource acquisition is initialization (RAII)| Ensure that resources are properly released by tying them to the lifespan of suitable objects.| {{yes}}| {{yes}}| {{yes}}
Singleton pattern>Singleton| Ensure a class has only one instance, and provide a global point of access to it.| {{yes}}| {{yes}}| {{yes}}

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Structural patterns {| class"wikitable"

! Name! Description! In Design Patterns! In Code Complete! OtherAdapter pattern>Adapter, Wrapper, or Translator| Convert the interface of a class into another interface clients expect. An adapter lets classes work together that could not otherwise because of incompatible interfaces. The enterprise integration pattern equivalent is the translator.| {{yes}}| {{yes}}| {{yes}}Bridge pattern>Bridge| Decouple an abstraction from its implementation allowing the two to vary independently.| {{yes}}| {{yes}}| {{yes}}Composite pattern>Composite| Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.| {{yes}}| {{yes}}| {{yes}}Decorator pattern>Decorator| Attach additional responsibilities to an object dynamically keeping the same interface. Decorators provide a flexible alternative to subclassing for extending functionality.| {{yes}}| {{yes}}| {{yes}}Delegation pattern>Delegation| Extend a class by composition instead of subclassing. The object handles a request by delegating to a second object (the delegate)| {{yes}}| {{yes}}| {{yes}}| Extension object| Adding functionality to a hierarchy without changing the hierarchy.| {{yes}}| {{yes}}| {{yes}}Facade pattern>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}}| {{yes}}Flyweight pattern>Flyweight| Use sharing to support large numbers of similar objects efficiently.| {{yes}}| {{yes}}| {{yes}}| Front controller| The pattern relates to the design of Web applications. It provides a centralized entry point for handling requests.| {{yes}}| {{yes}}{{abbr|J2EE Patterns|Core J2EE Patterns: Best Practices and Design Strategies (2nd Edition)}}BOOK , {{abbr|PoEAA|Patterns of Enterprise Application Architecture}}BOOK , }}Marker interface pattern>Marker| Empty interface to associate metadata with a class.| {{yes}}| {{yes}}Effective JavaBOOK Module pattern>Module| Group several related elements, such as classes, singletons, methods, globally used, into a single conceptual entity.| {{yes}}| {{yes}}| {{yes}}Proxy pattern>Proxy| Provide a surrogate or placeholder for another object to control access to it.| {{yes}}| {{yes}}| {{yes}}Twin pattern>TwinHTTP://WWW.SSW.JKU.AT/RESEARCH/PAPERS/MOE99/PAPER.PDF >TITLE=TWIN – A DESIGN PATTERN FOR MODELING MULTIPLE INHERITANCE, | Twin allows modeling of multiple inheritance in programming languages that do not support this feature.| {{yes}}| {{yes}}| {{yes}}">

Behavioral patterns {| class"wikitable"

! Name! Description! In Design Patterns! In Code Complete! OtherBlackboard (design pattern)>Blackboard| Artificial intelligence pattern for combining disparate sources of data (see blackboard system)| {{yes}}| {{yes}}| {{yes}}Chain-of-responsibility pattern>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}}| {{yes}}| {{yes}}Command pattern>Command| Encapsulate a request as an object, thereby allowing for the parameterization of clients with different requests, and the queuing or logging of requests. It also allows for the support of undoable operations.| {{yes}}| {{yes}}| {{yes}}|Fluent interface| Design an API to be method chained so that it reads like a DSL. Each method call returns a context through which the next logical method call(s) are made available.| {{yes}}| {{yes}}| {{yes}}Interpreter pattern>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}}| {{yes}}| {{yes}}Iterator pattern>IteratorAggregate pattern>aggregate object sequentially without exposing its underlying representation.| {{yes}}| {{yes}}| {{yes}}Mediator pattern>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 allows their interaction to vary independently.| {{yes}}| {{yes}}| {{yes}}Memento pattern>Memento| Without violating encapsulation, capture and externalize an object's internal state allowing the object to be restored to this state later.| {{yes}}| {{yes}}| {{yes}}Null Object pattern>Null object| Avoid null references by providing a default object.| {{yes}}| {{yes}}| {{yes}}Observer pattern>Observer or Publish/subscribe| Define a one-to-many dependency between objects where a state change in one object results in all its dependents being notified and updated automatically.| {{yes}}| {{yes}}| {{yes}}Design pattern Servant>Servant| Define common functionality for a group of classes. The servant pattern is also frequently called helper class or utility class implementation for a given set of classes. The helper classes generally have no objects hence they have all static methods that act upon different kinds of class objects.| {{yes}}| {{yes}}| {{yes}}Specification pattern>Specificationbusiness logic in a Boolean algebra>Boolean fashion.| {{yes}}| {{yes}}| {{yes}}State pattern>State| Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.| {{yes}}| {{yes}}| {{yes}}Strategy pattern>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}}| {{yes}}Template method pattern>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}}| {{yes}}Visitor pattern>Visitor| Represent an operation to be performed on instances of a set of classes. Visitor lets a new operation be defined without changing the classes of the elements on which it operates.| {{yes}}| {{yes}}| {{yes}}">

Concurrency patterns {| class"wikitable"

! Name! Description! In POSA2BOOK last3 = Rohnert last4 = Buschmann , ! OtherActive object>Active Objectasynchronous method invocation and a scheduling (computing)>scheduler for handling requests.| {{yes}}| {{n/a}}Balking pattern>Balking| Only execute an action on an object when the object is in a particular state.| {{no}}| {{n/a}}Binding properties pattern>Binding properties| Combining multiple observers to force properties in different objects to be synchronized or coordinated in some way.Binding Properties| {{no}}| {{n/a}}| Compute kernel| The same calculation many times in parallel, differing by integer parameters used with non-branching pointer math into shared arrays, such as GPU-optimized Matrix multiplication or Convolutional neural network.| {{no}}| {{n/a}}Double checked locking pattern>Double-checked locking| Reduce the overhead of acquiring a lock by first testing the locking criterion (the 'lock hint') in an unsafe manner; only if that succeeds does the actual locking logic proceed.Can be unsafe when implemented in some language/hardware combinations. It can therefore sometimes be considered an anti-pattern.| {{yes}}| {{n/a}}Event-Based Asynchronous Pattern>Event-based asynchronous LAST1 = NAGEL LAST2 = EVJEN LAST3 = GLYNN LAST4 = WATSON LAST5 = SKINNERPUBLISHER = WILEYISBN = 978-0-470-19137-8, Event-based Asynchronous Pattern, | {{no}}| {{n/a}}| Guarded suspension| Manages operations that require both a lock to be acquired and a precondition to be satisfied before the operation can be executed.| {{no}}| {{n/a}}Join-pattern>Join| Join-pattern provides a way to write concurrent, parallel and distributed programs by message passing. Compared to the use of threads and locks, this is a high-level programming model.| {{no}}| {{n/a}}Lock (computer science)>Lock| One thread puts a "lock" on a resource, preventing other threads from accessing or modifying it.Lock Pattern| {{no}}PoEAA}}Messaging pattern>Messaging design pattern (MDP)| Allows the interchange of information (i.e. messages) between components and applications.| {{no}}| {{n/a}}Monitor (synchronization)>Monitor object| An object whose methods are subject to mutual exclusion, thus preventing multiple objects from erroneously trying to use it at the same time.| {{yes}}| {{n/a}}Reactor pattern>Reactor| A reactor object provides an asynchronous interface to resources that must be handled synchronously.| {{yes}}| {{n/a}}Read/write lock pattern>Read-write lock| Allows concurrent read access to an object, but requires exclusive access for write operations. An underlying semaphore might be used for writing, and a Copy-on-write mechanism may or may not be used.| {{no}}| {{n/a}}Scheduler pattern>Scheduler| Explicitly control when threads may execute single-threaded code.| {{no}}| {{n/a}}| Service handler patternLAST2=TABONE TITLE=ELIXIRST: A SESSION-BASED TYPE SYSTEM FOR ELIXIR MODULES DATE=OCTOBER 2023 DOI=10.1016/J.JLAMP.2023.100891thread-per-session.SCHMIDT LAST2=VINOSKI TITLE=OBJECT INTERCONNECTIONS: COMPARING ALTERNATIVE PROGRAMMING TECHNIQUES FOR MULTI-THREADED CORBA SERVERS (COLUMN 7) JOURNAL=SIGS C++ REPORT S2CID=2654843, | {{no}}| {{n/a}}Thread pool pattern>Thread pool| A number of threads are created to perform a number of tasks, which are usually organized in a queue. Typically, there are many more tasks than threads. Can be considered a special case of the object pool pattern.| {{no}}| {{n/a}}Thread-Specific Storage>Thread-specific storage| Static or "global" memory local to a thread.| {{yes}}| {{n/a}}| Safe Concurrency with Exclusive Ownership| Avoiding the need for runtime concurrent mechanisms, because exclusive ownership can be proven. This is a notable capability of the Rust language, but compile-time checking isn't the only means, a programmer will often manually design such patterns into code - omitting the use of locking mechanism because the programmer assesses that a given variable is never going to be concurrently accessed.| {{no}}| {{n/a}}| CPU atomic operationC Sharp (programming language)>C# language uses the Interlocked class for these capabilities.| {{no}}| {{n/a}}

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.WEB , One example of a commonly used documentation format is the one used by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides 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

Some suggest that design patterns may be a sign that features are missing in a given programming language (Java or C++ for instance). Peter Norvig demonstrates that 16 out of the 23 patterns in the Design Patterns book (which is primarily focused on C++) are simplified or eliminated (via direct language support) in Lisp or Dylan.CONFERENCE See also Paul Graham's essay "Revenge of the Nerds".WEB Inappropriate use of patterns may unnecessarily increase complexity.BOOK 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, researchers have worked to turn patterns into components. Meyer and Arnout were able to provide full or partial componentization of two-thirds of the patterns they attempted.JOURNAL In order to achieve flexibility, design patterns may introduce additional levels of indirection, which may complicate the resulting design and decrease runtime performance.

See also

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• Abstraction principle
• Algorithmic skeleton
• Anti-pattern
• Architectural pattern
• Canonical protocol pattern
• Debugging patterns
• Design pattern
• Distributed design patterns
• Double-chance function
• Enterprise Architecture framework
• GRASP (object-oriented design)
• Helper class
• Idiom in programming
• Interaction design pattern
• List of software development philosophies
• List of software engineering topics
• Pattern language
• Pattern theory
• Pedagogical patterns
• Portland Pattern Repository
• Refactoring
• Software development methodology

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References

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Further reading

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• BOOK, Designing Distributed Control Systems: A Pattern Language Approach, Eloranta, Veli-Pekka, Koskinen, Johannes, Leppänen, Marko, Reijonen, Ville, Wiley, 2014, 978-1118694152,
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