Encyclopedia of Computer Science and Technology

142 design patternsdesign patternsDesign patterns are an attempt to abstract and generalizewhat is learned in solving one problem so that it canbe applied to future similar problems. The idea was firstapplied to architecture by Christopher Alexander in hisbook A Pattern Language. Alexander described a pattern asa description of situations in which a particular problemoccurs, with a solution that takes into account the factorsthat are “invariant” (not changed by context). Guidance forapplying the solution is also provided.For example, a bus stop, a waiting room, and a line ata theme park are all places where people wait. A “place towait” pattern would specify the problem to be solved (howto make waiting as pleasant as possible) and suggest solutions.Patterns can have different levels of abstraction orscales on which they apply (for example, an intimate theaterand a stadium are both places of entertainment, butone is much larger than the other).Patterns in turn are linked into a network called a patternlanguage. Thus when working with one pattern, thedesigner is guided to consider related patterns. For example,a pattern for a room might relate to patterns for seatingor grouping the occupants.Patterns in SoftwareThe concept of patterns and pattern languages carries overwell into software design. As with architectural patterns,a software pattern describes a problem and solution, alongwith relevant structures (see class and object-orientedprogramming). Note that patterns are not executable code;they are at a higher level (one might say abstract enough tobe generalizable, specific enough to be applicable).Software patterns can specify how objects are createdand ways in which they function and interface with otherobjects. Patterns are generally documented using a commonformat; one example is provided in the book Design Patterns.This scheme has the following sections:• name and classification• intent or purpose• alternative names• problem—the kind of problem the pattern addresses,and conditions under which it can be used• applicability—typical situations of use• structure description—such as class or interactiondiagrams• participants—classes and objects involved in the patternand the role each plays• collaboration—how the objects interact with oneanother• consequences—the expected results of using the pattern,and possible side effects or shortcomings• implementation—explains a way to implement thepattern to solve the problem• sample code—usually in a commonly used programminglanguage• known uses—actual working applications of thepattern• related patterns—other patterns that are similar orrelated, with a description of how they differAn example given in Design Patterns is the “publishsubscribe”pattern. This pattern describes how a numberof objects (observers) can be dependent on a “subject.” Allobservers are “subscribed” to the subject, so they are notifiedwhenever any data in the subject changes. This patterncould be used, for example, to set up a system where differentreports, spreadsheets, etc., need to be updated whenevernotified by a controlling object that has received new data.Some critics consider the use of patterns to be tooabstract and inefficient. Since a pattern has to be re-implementedfor each use, it has been argued that well-documented,reusable classes or objects would be more useful.Proponents, however, argue that “design reuse” is morepowerful than mere “object reuse.” A pattern provides awhole “language” for talking about a problem and its provensolutions, and can help both the original designer and othersunderstand and extend the design.Further ReadingAlexander, Christopher. A Pattern Language: Towns, Buildings,Construction. New York: Oxford University Press, 1977.“Design Patterns.” IBM Research. Available online. URL: http://www.research.ibm.com/designpatterns/. Accessed September10, 2007.Freeman, Eric, and Elisabeth Freeman. Head First Design Patterns.Sebastapol, Calif.: O’Reilly, 2004.Gamma, Erich, et al. Design Patterns: Elements of Reusable Object-Oriented Software. Upper Saddle River, N.J.: Addison-WesleyProfessional, 1995.Kurotsuchi, Brian T. “Welcome to the Wonderful World of DesignPatterns.” Available online. URL: http://www.csc.calpoly.edu/~dbutler/tutorials/winter96/patterns/. Accessed September10, 2007.desktop publishing (DTP)Traditionally documents such as advertisements, brochures,and reports were prepared by combining typed or printedtext with pasted-in illustrations (such as photographs anddiagrams). This painstaking layout process was necessaryin order to produce “camera-ready copy” from which aprinting company could produce the final product.Starting in the late 1980s, desktop computers becamepowerful enough to run software that could be used to createpage layouts. In addition, display hardware gained ahigh enough resolution to allow for pages to be shown onthe screen in much the same form as they would appear onthe printed page. (This is known by the acronym WYSI-WYG, or “what you see is what you get.”) The final ingredientfor the creation of desktop publishing was the advent ofaffordable laser or inkjet printers that could print near printquality text and high-resolution graphics (see printers).This combination of technologies made it feasible fortrained office personnel to create, design, and produce many