User:JarlvonHoother

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Yes this is JarlvonHoother. Who do you be?

Vail Place is a non-profit agency that provides essential recovery oriented services for people in Hennepin County with long term mental illnesses. Vail Place is a clubhouse built around the The Clubhouse Model of Psychosocial Rehabilitation.

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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. 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[edit]

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.[1][2] 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.[3]

Uses[edit]

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.

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, 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[edit]

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[4], Information Visualization [5] and web design.[6]

The Pattern Languages Of Programming Conference(annual,1994->) proceedings includes many for examples of domain specific patterns.

Caveats[edit]

In order to achieve flexibility, design patterns usually introduce additional levels of indirection, which might complicate the resulting designs and hurt application performance.

Classification[edit]

Design Patterns originally grouped design patterns 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.

Name Description In Design Patterns In Code Complete[7]
Creational patterns
Abstract factory Provide an interface for creating families of related or dependent objects without specifying their concrete classes. Yes Yes
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
Builder Separate the construction of a complex object from its representation so that the same construction process can create different representations. Yes 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
Object pool Avoid expensive acquisition and release of resources by recycling objects that are no longer in use No No
Prototype Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. Yes No
Singleton Ensure a class only has one instance, and provide a global point of access to it. Yes Yes
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. Yes Yes
Bridge Decouple an abstraction from its implementation so that the two can vary independently. Yes Yes
Composite Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly. Yes Yes
Decorator Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality. Yes Yes
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
Flyweight Use sharing to support large numbers of fine-grained objects efficiently. Yes No
Proxy Provide a surrogate or placeholder for another object to control access to it. Yes No
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. Yes 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
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
Iterator Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. Yes Yes
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
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
Observer 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
State Allow an object to alter its behavior when its internal state changes. The object will appear to change its class. Yes No
Strategy Define a family of algorithms, encapsulate each one, and make them interchangable. Strategy lets the algorithm vary independently from clients that use it. Yes Yes
Specification Recombinable Business logic in a boolean fashion 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
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
Single-serving visitor Optimize the implementation of a visitor that is allocated, used only once, and then deleted No No
Hierarchical visitor Provide a way to visit every node in a hierarchical data structure such as a tree. No No
Concurrency patterns
Active Object No No
Balking No No
Double checked locking No No
Guarded No No
Leaders/followers No No
Monitor object No No
Read write lock No No
Scheduler No No
Thread pool No No
Thread-specific storage No No
Reactor No No

Documentation[edit]

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.[8] 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.[9] 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) 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[edit]

In the field of computer science, there exist some criticisms regarding the concept of design patterns.

Unlike components, does not provide reuse[edit]

A pattern must be programmed anew into each application that uses it. Some authors see this as a step backward from software reuse as provided by components. This observation has led to work on "componentization": turning patterns into components, in particular by Meyer and Arnout, who claim a 2/3rds success rate in componentizing the best-known patterns.[10]

Workarounds for missing language features[edit]

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 -- but in a language with multimethods, methods are not part of the class structure. Similarly, the Decorator pattern amounts to implementing dynamic delegation, as found in Objective-C and Self.

Peter Norvig, in Design Patterns in Dynamic Programming, discusses the triviality of implementing various patterns in dynamic languages. [11] 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[edit]

Some authors allege that design patterns don't differ significantly from other forms of abstraction[citation needed], 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 touted as an example of a "pattern" which predates the concept of "design patterns" by several years.[citation needed] 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. [citation needed]

See also[edit]

References[edit]

  1. ^ Smith, Reid (October 1987). "Panel on design methodology". OOPSLA '87 Addendum to the Proceedings. OOPSLA '87. doi:10.1145/62138.62151.  , "Ward cautioned against requiring too much programming at, what he termed, 'the high level of wizards.' He pointed out that a written 'pattern language' can significantly improve the selection and application of abstractions. He proposed a 'radical shift in the burden of design and implementation' basing the new methodology on an adaptation of Christopher Alexander's work in pattern languages and that programming-oriented pattern languages developed at Tektronix has significantly aided their software development efforts."
  2. ^ Beck, Kent; Ward Cunningham (September 1987). "Using Pattern Languages for Object-Oriented Program". OOPSLA '87 workshop on Specification and Design for Object-Oriented Programming. OOPSLA '87. Retrieved 2006-05-26. 
  3. ^ Baroni, Aline Lúcia; Yann-Gaël Guéhéneuc and Hervé Albin-Amiot (June 2003). "Design Patterns Formalization" (PDF). Nantes: École Nationale Supérieure des Techniques Industrielles et des Mines de Nantes. Retrieved 2007-12-29. 
  4. ^ Laakso, Sari A. (2003-09-16). "Collection of User Interface Design Patterns". University of Helsinki, Dept. of Computer Science. Retrieved 2008-01-31. 
  5. ^ Heer, J.; M. Agrawala (2006). "Software Design Patterns for Information Visualization". IEEE TRANSACTIONS ON VISUALIZATION AND COMPUTER GRAPHICS 12 (5): 853. 
  6. ^ "Yahoo! Design Pattern Library". Retrieved 2008-01-31. 
  7. ^ McConnell, Steve (June 2004). "Design in Construction". Code Complete (2nd edition ed.). Microsoft Press. p. 104. ISBN 978-0735619678. "Table 5.1 Popular Design Patterns" 
  8. ^ Gabriel, Dick. "A Pattern Definition". Retrieved 2007-03-06. 
  9. ^ Fowler, Martin (2006-08-01). "Writing Software Patterns". Retrieved 2007-03-06. 
  10. ^ Meyer, Bertrand; Karine Arnout (July 2006). "Componentization: The Visitor Example". IEEE Computer (IEEE) 39 (7): 23–30. 
  11. ^ Norvig, Peter (1998-03-17). "Design Patterns in Dynamic Programming". Retrieved 2007-12-29. 

Further reading[edit]

Books
Web sites

External links[edit]

{{Design Patterns Patterns}}

[[Category:Software design patterns|*]]JarlvonHoother (talk) 16:48, 6 February 2008 (UTC)