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.
Design patterns reside in the domain of modules and interconnections. At a higher level there are Architectural patterns that are larger in scope, usually describing an overall pattern followed by an entire system.[1]
Not all software patterns are design patterns. For instance, algorithms solve computational problems rather than software design problems.
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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.[2][3] 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 by the so-called "Gang of Four" (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:
Although design patterns have been applied practically for a long time, formalization of the concept of design patterns languished for several years.[4]
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, researchers have worked to turn patterns into components. Meyer and Arnout claim a two-thirds success rate in componentizing the best-known patterns.[5]
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 to this, 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.
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,[6] information visualization [7], "secure usability"[8], web design [9] and business model design.[10]
The annual Pattern Languages of Programming Conference proceedings [11] include many examples of domain specific patterns.
Design patterns were originally grouped into the categories: creational patterns, structural patterns, and behavioral patterns, and described 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 introduced the notion of architectural design pattern that 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[12] | In POSA2[13] | In PoEAA[14] |
|---|---|---|---|---|---|
| Creational patterns | |||||
| Abstract factory | Provide an interface for creating families of related or dependent objects without specifying their concrete classes. | Yes | Yes | No | No |
| Builder | Separate the construction of a complex object from its representation so that the same construction process can create different representations. | Yes | No | No | No |
| 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 | No | 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 | No | Yes |
| Multiton | Ensure a class has only named instances, and provide global point of access to them. | No | No | No | No |
| 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. | No | No | No | No |
| Prototype | Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. | Yes | No | No | No |
| Resource acquisition is initialization | Ensure that resources are properly released by tying them to the lifespan of suitable objects. | No | No | No | No |
| Singleton | Ensure a class has only one instance, and provide a global point of access to it. | Yes | Yes | No | No |
| Structural patterns | |||||
| Adapter or Wrapper | 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 | No | No |
| Bridge | Decouple an abstraction from its implementation so that the two can vary independently. | Yes | Yes | No | No |
| Composite | Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly. | Yes | Yes | No | No |
| Decorator | Attach additional responsibilities to an object dynamically keeping the same interface. Decorators provide a flexible alternative to subclassing for extending functionality. | Yes | Yes | No | No |
| 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 | No | No |
| Flyweight | Use sharing to support large numbers of fine-grained objects efficiently. | Yes | No | No | No |
| Proxy | Provide a surrogate or placeholder for another object to control access to it. | Yes | No | No | No |
| Behavioral patterns | |||||
| Blackboard | Generalized observer, which allows multiple readers and writers. Communicates information system-wide. | No | No | No | No |
| 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 | No | 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 | No | 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 | No | No |
| Iterator | Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. | Yes | Yes | No | No |
| 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 | No | 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 | No | No |
| Null object | Avoid null references by providing a default object. | No | No | No | No |
| Observer or Publish/subscribe | 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 | No | No |
| Specification | Recombinable business logic in a boolean fashion | No | No | No | No |
| State | Allow an object to alter its behavior when its internal state changes. The object will appear to change its class. | Yes | No | No | 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 | 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 | No | No |
| 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 | No | No |
| Concurrency patterns | |||||
| Active Object | Decouples method execution from method invocation that reside in their own thread of control. The goal is to introduce concurrency, by using asynchronous method invocation and a scheduler for handling requests. | No | No | Yes | No |
| Balking | Only execute an action on an object when the object is in a particular state. | No | No | No | No |
| Binding Properties | Combining multiple observers to force properties in different objects to be synchronized or coordinated in some way.[15] | No | No | No | No |
| Messaging pattern | The messaging design pattern (MDP) allows the interchange of information (i.e. messages) between components and applications. | No | No | No | No |
| 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 lock proceed.
Can be unsafe when implemented in some language/hardware combinations. It can therefore sometimes be considered an anti-pattern. |
No | No | Yes | No |
| Event-based asynchronous | Addresses problems with the Asynchronous pattern that occur in multithreaded programs.[16] | No | No | No | No |
| 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 | No | No | No |
| Lock | One thread puts a "lock" on a resource, preventing other threads from accessing or modifying it.[17] | No | No | No | Yes |
| 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. | No | No | Yes | No |
| Reactor | A reactor object provides an asynchronous interface to resources that must be handled synchronously. | No | No | Yes | No |
| Read-write lock | Allows concurrent read access to an object but requires exclusive access for write operations. | No | No | No | No |
| Scheduler | Explicitly control when threads may execute single-threaded code. | No | No | No | No |
| 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 | No | No | No |
| Thread-specific storage | Static or "global" memory local to a thread. | No | No | Yes | No |
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.[18] 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.[19] 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:
In the field of computer science, there exist some criticisms regarding the concept of design patterns.
Users of dynamic programming languages have discussed many design patterns as workarounds for the limitations of languages such as C++ and Java.[20]
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. In C++, a class is declared as a syntactic structure with a specific and closed set of methods. In a language with multimethods, such as Common Lisp, methods for a class are outside of the class structure, and one may add new methods without modifying it. Similarly, the Decorator pattern amounts to implementing dynamic delegation, as found in Common Lisp, Objective-C, Self and JavaScript.
Many patterns imply object-orientation or more generally mutable state, and so are meaningless in functional programming style, in which data is immutable or treated as such. For example, the Iterator pattern is a generalisation of 'for' loops, an inherently imperative notion.
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 cited as an example of a "pattern" that predates the concept of "design patterns" by several years.[21] 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 that is often ignored in the literature.
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