Topic 1: Abstraction and Encapsulation

Synopsis

• Course Overview
   
• Lecture Notes
   
• Textbooks
   
• Practical Classes and Tutorials
   
• Assessment
   
• Consultation
   
• What is Object-orientation?
   
• Why Program using Object-orientation?
   
• Components of the Object Model
   
• Abstraction^¤ (or "All men are animals")
   
• Abstraction^¤ and Classification^¤
   
• Abstraction^¤ and Relationships
   
• Types of Abstraction^¤
   
• An Example
   
• Encapsulation^¤ (or "Don't show your privates")
   
• Encapsulation^¤ to Partition
   
• Encapsulation^¤ to Protect
   
• An Example
   
• Reading
   

Course Overview

• Object-orientation
   
• Software Engineering^¤
   
• C++
   

Lecture Notes

• Available on-line from http://www.csse.monash.edu.au/~jonmc/CSE2305/
   
• There you will also find other important information (syllabus and timetable, reading lists, on-line resources, glossary, etc.)
   
• Will not be handed out
   

Textbooks

• Lippman, S. & Lajoie, J., "C++ Primer, 3rd Edition", Addison Wesley, 1998.

An excellent tutorial introduction to C++ and Object-Orientation.  Stan Lippman wrote the first C++ compilers.

• Stroustrup, B., "The C++ Programming Language, 3rd Ed.", Addison Wesley, 1997.

Another excellent introductory text, this one from the man who invented most of the C++ language. Written in a more technical style than Lippman's text (which some people prefer).

• Fowler, M. (with Kendall Scott), "UML^¤ Distilled", 2nd Ed., Addison Wesley, 2000.

Provides a good overview of the Unified Modelling Language^¤ in a short book.

Practical Classes and Tutorials

• A chance to practice the skills needed to complete the assignment and pass the exam.
   
• Practice, practice, practice!
   

Assessment

• Assignment = 20%
   
• Exam = 80%
   
• You must pass both to pass the subject.
   

Consultation

• See the consultation hours page
   
• Assistant Lecturers' times on the noticeboard
   
• Try and see the AL's first
   

What is Object-orientation?

• Build systems from co-operating collections of "objects"
   
• Objects communicate by passing "messages", which other objects "react" to
   
• Objects organized into "classes^¤". All objects in a class^¤ have the same behaviour^¤ (but may store different data)
   
• Classes^¤ are organized into "abstraction^¤ hierarchies"
   
• Object-oriented analysis, object-oriented design, object-oriented programming.
   

Why Program using Object-orientation?

• Most of us see the world in terms of the interactions of objects (at some level of abstraction^¤)
   
• Classes^¤ and objects often mirror the tangible components of the problem and/or the solution
   
• An OO approach decomposes the problem into manageable and "intuitive" components
   
• Allows us to focus on the data, rather than on the process
   

Components of the Object Model

• Standard list (Booch, Chapter 2)
   
• Abstraction^¤
   
• Encapsulation^¤
   
• Modularity^¤
   
• Hierarchy^¤
   
• Typing^¤
   
• Genericity^¤
   

Abstraction^¤ (or "All men are animals")

• The world is too complex to comprehend directly
   
• Our brains can juggle 7±2 items at any one time
   

• So we "chunk" things by identifying significant commonalities (whilst ignoring trivial differences)
   

• How we choose to abstract depends on what we consider to be "significant"
   

Abstraction^¤ and Classification^¤

• This leads to classification^¤
   
• "All men are animals"
   
• By which we mean: "All men share certain attributes with all other entities which we also label 'animals'"
   
• Usually (in the Real World) the class^¤ is determined either by its actual membership or by formal rules that determine eligibility
   

Abstraction^¤ and Relationships

• Typically the abstractions^¤ we define form a hierarchy^¤
   

• The relationship between levels is known as "is a" (when going up the hierarchy^¤)
   
• Two classes^¤ may share the same "parent" class^¤, and yet be distinct
   
• The relationship between a class^¤ and one of its members^¤ is known as "is an instance^¤ of" (when going from the member^¤ to the class^¤)
   

Types of Abstraction^¤

• Abstraction^¤ of appearance
   
• Abstraction^¤ of structure
   
• Abstraction^¤ of functionality
   
• Abstraction^¤ of privilege
   
• Abstraction^¤ of purpose
   

An Example

• Video library
   
• Abstraction^¤ of appearance (cassettes)
   
• Abstraction^¤ of structure (plots)
   
• Abstraction^¤ of functionality (cleaning tapes))
   
• Abstraction^¤ of privilege (patrons)
   
• Abstraction^¤ of purpose (genres)
   

Encapsulation^¤ (or "Don't show your privates")

• Public front face vs. private backend!
   
• What you know vs. what you show
   
• "Need-to-know basis" is another mechanism for coping with overwhelming complexity
   

Encapsulation^¤ to Partition

• Encapsulation^¤ gives us another way of breaking up the world into "brain-sized" pieces
   
• Separates what's relevant from what's not
   
• Collects what's relevant in one location
   

Encapsulation^¤ to Protect

• Why build forts, castles^¤, bunkers, customs, etc?
   
• To ensure that what's outside doesn't mess with what's inside
   
• Why put up roadblocks, prisons, asylums?
   
• To ensure that what's inside stays there
   
• Why put all the fuses in one fuse-box?
   
• To localized the problem when it occurs
   

An Example

• Biology
   
• The cell
   
• The organ
   
• The organism
   
• The social unit
   
• The species
   

Reading

• Sommerville: Chapter 12
   
• Lippman & Lajoie

Preface, Chapters 1 and 2.

If you find Sections 2.4 to 2.8 difficult just skim along through them.
Also Chapter 13.


 

• Stroustrup

Chapters 1 and 2.

If you find sections difficult just skim along through them.  Don't stress; all will become clear later!


 

• Pressman: Chapter 20.