Principles of Systems Design • Abstraction • Modularity: Decomposition • Hierarchy • Encapsulation
Abstraction • A good abstraction is one that emphasizes details that are significant to the reader or user and suppresses details that are, at least for the moment, immaterial or diversionary. [Shaw, 43] • An abstraction denotes the essential characteristics of an object that distinguish it from all other kinds of objects and thus provide crisply defined conceptual boundaries, relative to the perspective of the viewer.
Modularity: Decomposition • Modularity is the property of a system that has been decomposed into a set of cohesive and loosely coupled modules.
Hierarchy • Hierarchy is a ranking or ordering of abstractions. • Inheritance
Encapsulation • Encapsulation is most often achieved through information hiding. • Encapsulation is the process of compartmentalizing the elements of an abstraction that constitute its structure and behavior; encapsulation serves to separate the contractual interface of an abstraction and its implementation.
Primary Objectives of Structured Systems Design • Achieve high-quality programs of predictable behavior • Create programs that are easily modifiable (maintainable) • Simplify programs and program development process • Achieve more predictability and control in the development process • Speed up system development • Lower the cost of system development Source: p.4, Structured Techniques by J. Martin