Object-Oriented Analysis and Design

References Grady Booch: Object-Oriented Analysis and Design with Applications; Addison Wesley Perdita Stevens, Rob Pooley: Using UML Software Engineering with Objects and Components, Pearson C. Larman : Applying UML and Patterns, Pearson J. Rumbaugh, M. Blaha, W. Premerlani, F. Eddy, W. Lorensen: Object-Oriented Modeling and Design, PHI

Complexity Why software is inherently complex? Complexity of problem domain Difficulty of managing the development process Flexibility possible through software Problems of Characterizing the behavior of Discrete systems

Consequences “The more complex the system, the more open it is to total breakdown” adding a new sub-basement to an existing 100 storey building - builder VS. software system user – simple matter of programming software crisis time delay – over budget deficient specification than requirement squandering of human resource & loss of opportunities

Complete Dismal? successful systems for significant complexity software Space shuttle large business organization (eg. Microsoft) Study, understand, comprehend the complexity first

Complex System Attribute 1. Frequently, complexity takes the form of hierarchy, whereby a complex system is composed of interrelated subsystems that have in turn their own subsystems, and so on, until some lowest level of elementary components is reached.

Complex System Attribute 2. The choice of what components in a system are primitive is rarely arbitrary and is largely up to the discretion of the observer of the system.

Complex System Attribute 3. Intracomponent linkages are generally stronger than intercomponent linkages. This fact has the effect of seperating the high-frequency dynamics of the components – involving the internal structure of the components – from the low frequency dynamics – involving interaction among components.

Complex System Attribute 4. Hierarchic systems are usually composed of only a few different kinds of subsystems in various combinations and arrangements.

Complex System Attribute 5. A complex systems that works is invariably found to have evolved from a simple system that worked …. A complex system designed from a scratch never works and cannot be patched up to make it work. Thus, one has to start over, beginning with a working simple system.

Organized-Disorganized Complexity Canonical form of a complex system little orientation - old pilot – new engine aircraft similar skills –already – new to be trained hierarchy not single - rather generalized one “is a” hierarchy - class concept “part of” hierarchy - object concept collectively aka architecture

Organized-Disorganized Complexity Limitation of the human capacity for dealing with complexity intricacy of interaction capability to think about many things at once ? flight case – location, elevation, speed, heading human comprehension is only at order of seven information human mind takes 5 sec to accept new information

Decomposition algorithmic decomposition object oriented decomposition straight, top-down structured structure chart directly from DFD & follows structured design object oriented decomposition depiction of key abstractions of problem domain set of automated agents – collaboration for higher level behaviors through message direction for a action

Role of Decomposition object oriented decomposition yields smaller systems through the reuse of common mechanism provides economy of expression more resilient to change and thus better able to evolve over time – design based upon stable intermediate forms evolve incrementally from smaller systems having already acquired confidence separation of concerns in a large state-space

Role of Abstraction “The span of absolute judgment and the span of immediate memory impose severe limitations on the amount of information that we are able to receive, process and remember. By organizing the stimulus input simultaneously into several dimensions and successively into a sequence of chunk we manage to break … this informational bottleneck” --- Unable to master the entirety of a complex object - ignore its essential details, dealing instead with the generalized, idealized model of the object semantic content

Role of Hierarchy depiction of different object collaboration pattern of interaction – mechanisms highlight common structure and behavior semantic content exposition

Engg : as a science - an art “The conception of a design for a new structure can evolve as much a leap of the imagination and as much a synthesis experience and knowledge as any artist is required to bring to his canvas or paper. And once that design is articulated by the engineer as artist, it must be analyzed by the engineer as scientist in as rigorous an application of the scientific method as any scientist must make” - Petroski – To Engineer is Human - 1985

Design Meaning satisfies a given (perhaps informal) functional specification conforms to limitations of the target medium meets implicit or explicit requirements on performance and resource usage satisfies implicit or explicit design criteria on the form of artifact satisfies restrictions on the design process itself, such as length or cost or the tools available for doing the design

Model Building Importance Follows the principle of decomposition, abstraction and hierarchy Each model within a design describes a specific aspect of the system under consideration Seek to build new models upon old models (which gained confidence already) Opportunity to evaluate each model under both expected and unusual situations

Design Method Elements Notation – Language for expressing each model Process – The activities leading to the ordinarily construction of the system’s model Tools – The artifacts that eliminate the tedium of model building and enforce rules about the models themselves, so that errors and inconsistencies can be exposed A sound design method is based upon a solid theoretical foundation, yet offers degrees of freedom for artistic innovation.

OO Model Logical Physical Dynamic Model Class & Object Structure Module & Process Architecture Static Model