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A description of an object-oriented designtechnique which names, abstracts and identifies aspects of adesign structure that are useful for creating anobject-oriented design. The design pattern identifiesclasses and instances, their roles, collaborations andresponsibilities. Each design pattern focuses on a particularobject-oriented design problem or issue. It describes when itapplies, whether it can be applied in the presence of otherdesign constraints, and the consequences and trade-offs of itsuse.Home(http://st-www.cs.uiuc.edu/users/patterns/patterns.html).["Design Patterns: Elements of Reusable Object-OrientedSoftware", Erich Gamma, Richard Helm, Ralph Johnson, and JohnVlissides].