Patterns

A pattern is a three part rule, which expresses a relation between a certain context, a problem and a solution. A pattern addresses a recurring design problem that arises in specific design situations, and presents a solution to it. Patterns are very much effective in identifying and specifying abstractions that are above the level of single classes and instances. Thus patterns provide a common effective vocabulary and understanding for design principles. Context, problem and solution are the three parts of a pattern. Context is a situation which gives rise to a problem. There are three categories in patterns. They are architectural patterns, design patterns and idioms. Architectural patterns which are templates for concrete software architecture provides a set of predefined subsystems, with specific mention of their responsibilities and includes rules and regulations for organizing the relationship between them. Design patterns are medium scale patterns which provide a scheme which refines the subsystems. Idioms deal with the implementation of particular design issues.