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Design pattern (computer science)
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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.
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Encyclopedia
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:
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, researchers have worked to turn patterns into components. Meyer and Arnout claim a two-thirds 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 , Secure Usability and web design.
The Pattern Languages of Programming Conference (annual, 1994—) includes many examples of domain specific patterns.
Classification
Design patterns were originally grouped 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 introduced 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 (book) | In Code Complete |
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| Creational patterns | | Abstract factory | Provide an interface for creating families of related or dependent objects without specifying their concrete classes. | | | | 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. | | | | Builder | Separate the construction of a complex object from its representation so that the same construction process can create different representations. | | | | 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. | | | | Object pool | Avoid expensive acquisition and release of resources by recycling objects that are no longer in use | | | | Prototype | Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. | | | | Singleton | Ensure a class has only one instance, and provide a global point of access to it. | | | | Resource acquisition is initialization | Ensure that resources are properly released by tying them to the lifespan of suitable objects. | | | |
| 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. | | | | Bridge | Decouple an abstraction from its implementation so that the two can vary independently. | | | | Composite | Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly. | | | | Decorator | Attach additional responsibilities to an object dynamically keeping the same interface. Decorators provide a flexible alternative to subclassing for extending functionality. | | | | 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. | | | | Flyweight | Use sharing to support large numbers of fine-grained objects efficiently. | | | | Proxy | Provide a surrogate or placeholder for another object to control access to it. | | | | 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. | | | | Command | Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations. | | | | Interpreter | Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language. | | | | Iterator | Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. | | | | 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. | | | | Memento | Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later. | | | | Null Object | designed to act as a default value of an object. | | | | Observer | Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. | | | | State | Allow an object to alter its behavior when its internal state changes. The object will appear to change its class. | | | | Strategy | Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it. | | | | Specification | Recombinable business logic in a boolean fashion | | | | 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. | | | | 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. | | | | Concurrency patterns | | Active Object | The Active Object design pattern 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. | | | | Event-Based Asynchronous | The event-based asynchronous design pattern addresses problems with the Asychronous Pattern that occur in multithreaded programs. | | | | Balking | The Balking pattern is a software design pattern that only executes an action on an object when the object is in a particular state. | | | | Double checked locking | Double-checked locking is a software design pattern also known as "double-checked locking optimization". The pattern is designed to 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.
The pattern, when implemented in some language/hardware combinations, can be unsafe. It can therefore sometimes be considered to be an anti-pattern. | | | | Guarded | In concurrent programming, guarded suspension is a software design pattern for managing operations that require both a lock to be acquired and a precondition to be satisfied before the operation can be executed. | | | | Monitor object | A monitor is an approach to synchronize two or more computer tasks that use a shared resource, usually a hardware device or a set of variables. | | | | Read write lock | A read/write lock pattern or simply RWL is a software design pattern that allows concurrent read access to an object but requires exclusive access for write operations. | | | | Scheduler | The scheduler pattern is a software design pattern. It is a concurrency pattern used to explicitly control when threads may execute single-threaded code. | | | | Thread pool | In the thread pool pattern in programming, 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. | | | | Thread-specific storage | Thread-local storage (TLS) is a computer programming method that uses static or global memory local to a thread. | | | | Reactor | The reactor design pattern is a concurrent programming pattern for handling service requests delivered concurrently to a service handler by one or more inputs. The service handler then demultiplexes the incoming requests and dispatches them synchronously to the associated request handlers. | | |
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. 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.
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 cited 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
Further reading
Books***********
Web sites*
External links
- at hillside.net.
- Ward Cunningham's Portland Pattern Repository.
- Design Patterns library that aims to provide full or partial componentized version of all known Patterns in Java.
- J2EE Pattern Oriented Framework
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- by Buschmann, Henney, and Schmidt
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- at oodesign.com
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