Patterns are Object-Oriented reusable units. The principal idea behind patterns is to capture and reuse the abstractions that have been formed by expert programmers and designers to solve problems that occur in particular contexts. These abstractions capture the valuable experiences of experts in solving problems. Although patterns are currently being used successfully, there is no general agreement among the software community as to how patterns should be formalized or represented. Various formal specification schemes have been proposed to complement the natural language description of patterns in order to alleviate the ambiguities inherent in the natural language description by rigorously reasoning about the structural and behavioral aspects of patterns. Existing formal specification languages of design patterns have generally failed to provide a standard definition, specification, or representation for patterns because there is no general agreement as to how patterns should be formalized. Also, each formal specification is generally based on a different mathematical formalism and when pattern users want to understand a pattern, first they have to understand the respective mathematical formalism. In addition to comparing three existing formal specification schemes, the main objective of this research work was to lay the foundation for developing a formal specification scheme that could be understandable without having to delve into the details of the underlying formalism. This research work attempted to capture and represent the structural aspects of design patterns since capturing the behavioral aspects of design patterns is a semantic issue and is beyond the scope of this work. Two-Level Grammar (TLG) was used to capture and represent the structural aspects of design patterns. This study was conducted using the GoF design patterns [Gamma et al. 1995]. It has already been demonstrated that TLGs have the capability to represent the building blocks of object-oriented software systems. The primary advantage of TLGs in defining design patterns is that specifications written in TLGs are understandable due to their natural-language-like vocabulary [Edupuganty 1987] [Lee 2003] [Maluszynski 1984]. The TLG representation of the observer pattern was developed to gauge the feasibility of the proposed pattern representation scheme. TLGs could help pattern users understand the formalized version of patterns more readily compared to other formal specification methods that are difficult to understand due to their arcane mathematical notations.Computer Science Departmen
