SOFTWARE DESIGN AND ARCHITECTURE LECTURE2 DESIGN PRICIPLES ABSTRACTION

SOFTWARE DESIGN AND ARCHITECTURE LECTURE#2

DESIGN PRICIPLES • ABSTRACTION • ENCAPSULATION • DECOMPOSITION • GENERALIZATION

ABSTRACTION • Abstraction breaks a concept down into a simplified description that ignores unimportant details and emphasizes the essentials needed for the concept, within some context. • Rule of astonishment • Choose abstraction that is more appropriate to the context • If situation changes, Reconsider your abstractions.

Encapsulation • The ability to “bundle” attribute values (or data) and behaviours (or functions) that manipulate those values, into a self-contained object. • The ability to “expose” certain data and functions of that object, which can be accessed from other objects, usually through an interface • The ability to “restrict” access to certain data and functions to only within the object.

• Data and functions that manipulate that data are “bundled” into a self-contained object. • Data and functions of the object can be exposed or made accessible from other objects. • Data and functions of the object can be restricted to only within the object

Decomposition • It consists of taking a whole thing, and dividing it into different parts. • The general rule for decomposition is to look at the different responsibilities of a whole and evaluate how the whole can be separated into parts that each have a specific responsibility • “Lifetimes” of whole and part classes should be well defined.

Decomposition • Three types of relationship in decomposition • Association • Aggregation • Composition

Generalization • Generalization helps reduce redundancy when solving problems. • In coding, algorithmic behaviors are often modelled through methods • object-oriented modelling achieves generalization by classes through inheritance

Design Principles(cont’d) • Evaluating Design Complexity • Design complexity applies t both classes and methods • Metrics for complexity is • Coupling • Cohesion