COMP 325 Lecture 6: Modules and Objects
major resources: Object-Oriented and Classical Software Engineering 6ed, Schach 2005, Object-Oriented Software Engineering, Schach 2008.

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Introduction

• need for modular rather than monolithic structure
• top-down design
• affect of modularity on quality attributes
• evolution from module to object

Definition of Module

• matter of debate
• easy to define in language-dependent or paradigm-dependent way
• more difficult to define in general way
• Schach likes Yourdon definition: lexically contiguous sequence of program statements, bounded by boundary elements, having an aggregate identifier
• Yourdon definition general enough to cover functions and procedures in procedural paradigm (Pascal, C) in addition to classes and methods in OO paradigm (C++, Java)
• Does Yourdon definition cover functional paradigm? (Lisp, Scheme) Logic paradigm? (Prolog)

Definition of Object

• not particularly controversial but hard to define concisely
• instance of a class (begs the question)
• something that has attributes, behaviors, identity, encapsulation and state
• what do all those terms mean? Illustrate with example

Cohesion

• a measure of how well a module "fits together"
• a module should implement a single logical entity or function
• cohesion is desirable : changes are localized in a single cohesive component
• various levels of cohesion have been identified
• Coincidental cohesion (weak): Component parts are simply bundled together without rhythm or reason
• Logical cohesion (weak): Components which perform similar functions are grouped
• Temporal cohesion (weak): Components which are activated at the same time are grouped
• Procedural cohesion (weak): Components that make up a single control sequence are grouped
• Communicational cohesion (medium): components that operate on the same data are grouped
• Functional cohesion (strong): module performs one function and each operation is necessary
• Informational (Object) cohesion (strong): Each operation in module has separate identity and operates independently on common data structure

Coupling

• A measure of the strength of the inter-connections between system components
• Loose coupling desirable: component changes are unlikely to affect other components
• various degrees of coupling have been identified
• Content coupling (tight): one directly references content of other
• Common coupling (tight): shared variables or control information
• Control coupling (relaxed fit): control information shared via parameters "with intent"
• Stamp coupling (relaxed fit): data shared via parameter but only part of parameter needed
• Data coupling (loose): data shared via parameter and all parts of parameter needed

Cohesion and coupling affect quality

• in general, the stronger the cohesion the higher the quality
• in general, the looser the coupling the higher the quality
• consider corrective maintainability
• consider adaptive maintainability
• consider perfective maintainability
• consider reusability
• consider these quality measures: robustness, understandability, portability, performance
• what about quality measures such as usability and learnability?

Encapsulation

• gathering all aspects of modeled entity into one module (object)
• OO language is not required to achieve this
• neither is this guaranteed by use of OO language
• how do you achieve this using OO language?
• what are implications for cohesion and coupling (and thus quality)?

Information Hiding

• module hides implementation details of its encapsulated properties from other modules
• OO language is not required to achieve this (e.g. Ada package)
• neither is this guaranteed by use of OO language
• how do you achieve this using OO language?
• what are implications for cohesion and coupling (and thus quality)?

Object Oriented Paradigm

• paradigm (PAIR-a-dime): a pattern or model (Oxford dictionary), theoretical framework (science), conceptual framework for thinking about programs (Sanderson)
• associations
• aggregation
• inheritance
• polymorphism
• what are implications of each for cohesion and coupling (and thus quality)?