Originated as a building architectural concept, patterns gained popularity and applicability in computer science with the publication in 1994 of "Design Patterns: Elements of Reusable Object-Oriented Software".

The book's authors are Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides, known as the "Gang of Four" or simply "GoF".

The GoF patterns are considered the cornerstone of Object Oriented Analysis and Design. In depth knowledge of GoF patterns is required for architect certification exams such as Sun Certified Enterprise Architect for J2EE Technology (SCEA).

We compiled a list of several GoF patterns accompanied with real life examples applied to an ecommerce application. Each example is illustrated by a UML diagram and Java code.

Aggregation versus Composition