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.

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.

Not all software patterns are design patterns. For instance, algorithms solve computational problems rather than software design problems.

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 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:

• 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)
• Buschmann, Frank (1996). Pattern-Oriented Software Architecture, Volume 1: A System of Patterns. John Wiley & Sons. ISBN 0-471-95869-7. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Schmidt, Douglas C. (2000). Pattern-Oriented Software Architecture, Volume 2: Patterns for Concurrent and Networked Objects. John Wiley & Sons. ISBN 0-471-60695-2. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Fowler, Martin (2002). Patterns of Enterprise Application Architecture. Addison-Wesley. ISBN 978-0321127426.
• 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 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. 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 were able to provide full or partial componentization of two-thirds of the patterns they attempted.

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 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.

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", web design and business model design.

The annual Pattern Languages of Programming Conference proceedings include many examples of domain specific patterns.

Classification and list

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, 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	In POSA2	In PoEAA
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 allowing the same construction process to create various 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.	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.	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.	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

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.

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.

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" that 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 that is often ignored in the literature.

See also

• Abstraction principle (programming)
• Algorithmic skeleton
• Anti-pattern
• Architectural pattern
• Business pattern
• Distributed design patterns
• Double-chance function
• GRASP (object-oriented design)
• 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

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)
• Alur, Deepak (2003). Core J2EE Patterns: Best Practices and Design Strategies (2nd Edition). Prentice Hall. ISBN 0131422464. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (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 (1997). Analysis Patterns: Reusable Object Models. Addison-Wesley. ISBN 0-201-89542-0.
• 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)
• Hohmann, Luke (2003). Beyond Software Architecture. Addison-Wesley. ISBN 0-201-77594-8. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Alur, Deepak (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.
• 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.
• Liskov, Barbara (2000). Program Development in Java: Abstraction, Specification, and Object-Oriented Design. Addison-Wesley. ISBN 0-201-65768-6. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• 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. Addison-Wesley. ISBN 0-321-22811-1. {{cite book}}: 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.
• "Design Patterns in Modern Day Software Factories (WCSF)". XO Software, Ltd. Retrieved 2010-02-22.

External links

• Directory of websites that provide pattern catalogs at hillside.net.
• Ward Cunningham's Portland Pattern Repository.
• Messaging Design Pattern
• Template:Dmoz
• PerfectJPattern Open Source Project Design Patterns library that aims to provide full or partial componentized version of all known Patterns in Java.
• Lean Startup Business Model Pattern Example of design pattern thinking applied to business models
• Jt J2EE Pattern Oriented Framework
• Printable Design Patterns Quick Reference Cards
• 101 Design Patterns & Tips for Developers
• On Patterns and Pattern Languages by Buschmann, Henney, and Schmidt
• Patterns for Scripted Applications
• Design Patterns Reference at oodesign.com
• Design Patterns for 70% programmers in the world by Saurabh Verma at slideshare.com