Object-Oriented Design

1. Object-Oriented Design Fred Kuhns fredk@cse.wustl.edu Applied Research Laboratory, Department of Computer Science and Engineering, Washington University in St. Louis

2. The Object Model • Software system are inherently complex: problem domain, development process, maintenance or just clearly articulating the system’s requirements (there eare endless possibilities) • Manage complexity by Decomposing system into smaller, independent components. • Components, in turn, are composed of one or more objects • Abstraction and hierarchy play fundamental roles CS422 – Operating Systems Concepts

3. Fundamentals: Booch, “OOA&D” • Object-oriented Analysis (OOA) • “Object-oriented analysis is a method of analysis that examines requirements from the perspective of the classes and objects found in the vocabulary” • Object-oriented Design (OOD) • “Object-oriented design is a method of design encompassing the process of object-oriented decomposition and a notation for depicting both logical and physical as well as static and dynamic models of the system under design” • Object-oriented Programming (OOP) • “Object-oriented programming is a method of implementation in which programs are organized as cooperative collections of objects, each of which represents an instance of some class, and whose classes are all members of a hierarchy of classes united via inheritance relationships” CS422 – Operating Systems Concepts

4. Major elements of the Object Model • Abstraction: essential characteristics of an object, that is its behavior or semantics. Separate behavior from implementation. • Kinds of abstractions: Entity, Action, Virtual machine and Coincidental. • Contract model: interface defines contract between (server) and user (client): it defines the objects responsibilities. • Invariants; for each operation associated with an object may have a set of preconditions (required invariants before invocation) and postconditions (invariants satisfied by operation). Exceptions indicate an invariant is not satisfied. • static and dynamicproperties • may be active or passive. Active objects may use callback mechanism to notify client of an event. CS422 – Operating Systems Concepts

5. Major elements of the Object Model • Encapsulation • implementation of abstraction • object abstraction should precede implementation decisions. Implementation is not visible external to object and so implementation may change of time with no impact to clients. • separation of concerns: one level of abstraction should not be concerned with another's implementation. • keep implementation a secret • Attempt to localize aspects of an object that may change • Modularity • design partitioned into a set of components with well defined interfaces. • group logically related classes • “Modularity is the property of a system that has been decomposed into a set of cohesive and loosely coupled modules”, G. Booch CS422 – Operating Systems Concepts

6. Major elements of the Object Model • Hierarchy • “Hierarchy is a ranking or ordering of abstractions”, G. Booch • Class hierarchy define “is-a” relationships (inheritance) • Important to guard against violating encapsulation (Liskov). Control in C++ using public, protected and private • single versus multiple inheritance – see class notes • Containment defines “has-a” relationships. A classes behavior is defined by an other objects. CS422 – Operating Systems Concepts

7. 3 Stages of Developing Software • Analysis: identify problem to be solved and define scope. • This is a critical step, vague or contradictory requirements are defined then the result in software which will likely not satisfy clients. • Vague requirements must be refined • Assumptions must be stated • Design • decompose system into a set of reasonably independent modules (loosely coupled). Use abstraction, encapsulation and hierarchy to manage complexity • Implementation • clarify class roles and relationships • implementation strategy for selected language • test, verify, simplify • revisit all 3 steps • Each step will likely require experimentation, testing, analysis, documentation and refinements. CS422 – Operating Systems Concepts

8. Goals and Quality Metrics • Assume a selfish model: • your time is valuable: minimize required coding, coordination, documentation and justifications. Code reuse, simple implementations, modular. • quality of product reflects on you: meet requirements, bugs, undocumented “features”, performance, efficiency. Test, verify and Characterize. • Avoid late nights at the keyboard: clear, simple and efficient interfaces are unlikely to be abused, misused or broken by yourself or others. Write code once – if you find your self repeating the same code then isolate it in a class or method. Write flexible, portable code. • Minimize frustration when changes are required: ensure classes/components are extensible with minimal effort. When changes are required try to keep it localized. CS422 – Operating Systems Concepts