Software design patterns
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software engineering Software engineering is a systematic engineering approach to software development. A software engineer is a person who applies the principles of software engineering to design, develop, maintain, test, and evaluate software, computer software. Th ...
, a software design pattern is a general, reusable solution to a commonly occurring problem within a given context in
software design Software design is the process by which an agent creates a specification of a software artifact intended to accomplish goal A goal is an idea of the future or desired result that a person or a group of people envision, Planning, plan an ...
. It is not a finished design that can be transformed directly into
source Source may refer to: Research * Historical document * Historical source * Source (intelligence) or sub source, typically a confidential provider of non open-source intelligence * Source (journalism), a person, publication, publishing institute o ...
or
machine code In computer programming Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programm ...
. Rather, it is a description or template for how to solve a problem that can be used in many different situations. Design patterns are formalized
best practice A best practice is a method or technique that has been generally accepted as superior to other known alternatives because it often produces results that are superior to those achieved by other means or because it has become a standard way of doing ...
s that the programmer can use to solve common problems when designing an application or system.
Object-oriented Object-oriented programming (OOP) is a programming paradigm based on the concept of "Object (computer science), objects", which can contain data and Computer program, code. The data is in the form of Field (computer science), fields (often kno ...
design patterns typically show relationships and interactions between classes or
object Object may refer to: General meanings * Object (philosophy) An object is a philosophy, philosophical term often used in contrast to the term ''Subject (philosophy), subject''. A subject is an observer and an object is a thing observed. For mo ...
s, without specifying the final application classes or objects that are involved. Patterns that imply mutable state may be unsuited for
functional programming In computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied sc ...
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. Design patterns may be viewed as a structured approach to
computer programming Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as anal ...
intermediate between the levels of a
programming paradigm Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mainly with implications for the execution model of the language, suc ...
and a concrete
algorithm In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific Computational problem, problems or to perform a computation. Algorithms are used as specificat ...
.


History

Patterns originated as an architectural concept by
Christopher Alexander Christopher Wolfgang John Alexander (4 October 1936 – 17 March 2022) was an Austrian-born British-American architect and design theorist. He was an emeritus professor at the University of California, Berkeley. His theories about the nature ...
as early as 1977 (c.f. "The Pattern of Streets," JOURNAL OF THE AIP, September, 1977, Vol. 32, No. 3, pp. 273–278). In 1987,
Kent Beck Kent Beck (born 1961) is an American software engineer Software engineering is a systematic engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunne ...
and
Ward Cunningham Howard G. Cunningham (born May 26, 1949) is an American computer programmer who developed the first wiki and was a co-author of the ''Agile software development#The Agile Manifesto, Manifesto for Agile Software Development''. A pioneer in both ...
began experimenting with the idea of applying patterns to programming – specifically
pattern language A pattern language is an organized and coherent set of ''patterns'', each of which describes a problem and the core of a solution that can be used in many ways within a specific field of expertise. The term was coined by architect Christopher Alexa ...
s – and presented their results at the
OOPSLA OOPSLA (Object-Oriented Programming, Systems, Languages & Applications) is an annual Association for Computing Machinery, ACM research conference. OOPSLA mainly takes place in the United States, while the sister conference of OOPSLA, ECOOP, is ty ...
conference that year. In the following years, Beck, Cunningham and others followed up on this work. Design patterns gained popularity in
computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...
after the book ''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 Pattern Languages of Programs is the name of a group of annual conferences sponsored by The Hillside Group. The purpose of these conferences is to develop and refine the art of software design pattern HTTP Switchboard, In software engineering ...
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: * * * * * * * * Although design patterns have been applied practically for a long time, formalization of the concept of design patterns 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. 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 helps to prevent such subtle issues, 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 In computer programming, indirection (also called dereferencing) is the ability to reference something using a name, reference, or container instead of the value itself. The most common form of indirection is the act of manipulating a value throug ...
, 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 In software development (and computer programming in general), code reuse, also called software reuse, is the use of existing software, or software knowledge, to build new software, following the reusability, reusability principles. Code reuse m ...
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. Software design techniques are difficult to apply to a broader range of problems. Design patterns provide general solutions, documented in a format that does not require specifics tied to a particular problem.


Structure

Design patterns are composed of several sections (see 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.


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 In the industrial design field of human–computer interaction, a user interface (UI) is the space where interactions between humans and machines occur. The goal of this interaction is to allow effective operation and control of the machine fr ...
design patterns,
information visualization Information is an Abstraction, abstract concept that refers to that which has the power to Communication, inform. At the most fundamental level information pertains to the Interpretation (logic), interpretation of that which may be sensed. ...
, secure design, "secure usability", Web design and business model design. The annual
Pattern Languages of Programming Pattern Languages of Programs is the name of a group of annual conferences sponsored by The Hillside Group. The purpose of these conferences is to develop and refine the art of software design pattern HTTP Switchboard, In software engineering ...
Conference proceedings Pattern Languages of Programming, Conference proceedings (annual, 1994—

/ref> include many examples of domain-specific patterns.


Classification and list

Design patterns had originally been categorized into 3 sub-classifications based on what kind of problem they solve. Creational patterns provide the capability to create objects based on a required criterion and in a controlled way. Structural patterns are about organizing different classes and objects to form larger structures and provide new functionality. Finally, behavioral patterns are about identifying common communication patterns between objects and realizing these patterns.


Creational patterns


Structural patterns


Behavioral patterns


Concurrency pattern In software engineering, concurrency patterns are those types of design pattern (computer science), design patterns that deal with the multi-threaded programming paradigm. Examples of this class of patterns include: * Active Object * Balking patte ...
s


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 John Matthew Vlissides (August 2, 1961 – November 24, 2005) was a software engineer known mainly as one of the four authors (referred to as the Gang of Four) of the book '' Design Patterns: Elements of Reusable Object-Oriented Software''. Vlis ...
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 diagram In software engineering, a class diagram in the Unified Modeling Language (UML) is a type of static structure diagram that describes the structure of a system by showing the system's class (computer science), classes, their attributes, operations ...
s 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

It has been observed that design patterns may just be a sign that some features are missing in a given programming language (
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
or
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language In computer software, a general-purpose programming language (GPL) is a programming language for building software in a wide variety of application Domain (so ...
for instance).
Peter Norvig Peter Norvig (born December 14, 1956) is an American computer scientist and Fellow, Distinguished Education Fellow at the Stanford Institute for Human-Centered AI. He previously served as a director of research and search quality at Google. Norv ...
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 A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech. Types * A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lisping ...
or Dylan. Related observations were made by Hannemann and Kiczales who implemented several of the 23 design patterns using an aspect-oriented programming language (AspectJ) and showed that code-level dependencies were removed from the implementations of 17 of the 23 design patterns and that aspect-oriented programming could simplify the implementations of design patterns. See also Paul Graham's essay "Revenge of the Nerds". Inappropriate use of patterns may unnecessarily increase complexity.


See also

* Abstraction principle *
Algorithmic skeleton In computing, algorithmic skeletons, or parallelism patterns, are a high-level parallel programming model for parallel and distributed computing. Algorithmic skeletons take advantage of common programming patterns to hide the complexity of parall ...
*
Anti-pattern An anti-pattern in software engineering, project management, and business processes is a common response to a recurring problem that is usually ineffective and risks being highly counterproductive. The term, coined in 1995 by computer programmer An ...
*
Architectural pattern An architectural pattern is a general, reusable solution to a commonly occurring problem in software architecture within a given context. The architectural patterns address various issues in software engineering, such as computer hardware perform ...
* Canonical protocol pattern * Debugging patterns *
Design pattern A design pattern is the re-usable form of a solution to a design problem. The idea was introduced by the architect Christopher Alexander and has been adapted for various other disciplines, particularly software engineering. The "Design Patterns ( ...
* Distributed design patterns * Double-chance function *
Enterprise Architecture framework An enterprise architecture framework (EA framework) defines how to create and use an enterprise architecture. An architecture framework provides principles and practices for creating and using the architecture description of a system. It structu ...
* GRASP (object-oriented design) * Helper class *
Idiom An idiom is a phrase or expression that typically presents a Literal and figurative language, figurative, non-literal meaning (linguistic), meaning attached to the phrase; but some phrases become figurative idioms while retaining the literal meani ...
in programming * Interaction design pattern *
List of software development philosophies This is a list of approaches, styles, methodologies, philosophies in software development and engineering. It also contains programming paradigm Programming paradigms are a way to classify programming languages based on their features. Lang ...
* List of software engineering topics *
Pattern language A pattern language is an organized and coherent set of ''patterns'', each of which describes a problem and the core of a solution that can be used in many ways within a specific field of expertise. The term was coined by architect Christopher Alexa ...
* Pattern theory * Pedagogical patterns * Portland Pattern Repository *
Refactoring In computer programming Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming ...
*
Software development methodology In software engineering Software engineering is a systematic engineering approach to software development. A software engineer is a person who applies the principles of software engineering to design, develop, maintain, test, and evaluate so ...


References


Further reading

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