CS696: Doc 1, Introduction

CS 696: Advanced OO Programming and Design
Spring Semester, 1997
Doc 1, Introduction

[To Lecture Notes Index]
San Diego State University -- This page last updated Jan 27, 1997

Contents of Doc 1, Introduction

Doc 1, Introduction Slide # 1

References

Object-Oriented Software Construction, Bertrand Meyer, Prentice Hall, 1988

Object-Oriented Design Heuristics, Arthur Riel, Addison-Wesley, 1996

Doc 1, Introduction Slide # 2

Introduction
Meyer's Criteria for Evaluating for Modularity

Decomposability

Decompose problem into smaller subproblems
that can be solved separately

Example: Top-Down Design

Counter-example: Initialization Module

Doc 1, Introduction Slide # 3

Meyer's Criteria for Evaluating for Modularity Composability

Freely combine modules to produce new systems

Examples: Math libraries
Unix command & pipes

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Meyer's Criteria for Evaluating for Modularity Understandability

Individual modules understandable by human reader

Counter-example: Sequential Dependencies

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Meyer's Criteria for Evaluating for Modularity Continuity

Small change in specification results in:

Changes in only a few modules

Does not affect the architecture

Example: Symbolic Constants

const MaxSize = 100

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Meyer's Criteria for Evaluating for Modularity Protection

Effects of an abnormal run-time condition is confined to a few modules

Example: Validating input at source

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Principles for Software Development

KISS Keep it simple, stupid
Supports:


Understandablity
Composability

Decomposability

Small is Beautiful
Upper bound for average size of an operation[1]

Language	Lines of Code
Smalltalk	8
C++	24

Supports:

Decomposability
Composability
Understandability

Doc 1, Introduction Slide # 8

Applications of Principles
First program:

   class HelloWorldExample 
   {
      public static void main( String args[] ) 
      {
         System.out.println( "Hello World" );
      }
   }

Grow programs

Start with working program

Add small pieces of code and debug

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What is Object-Oriented Programming

Language Level Definition

Class

Object

Inheritance

Conceptual Level Definition

Abstraction

Encapsulation

Information Hiding

Hierarchy
<hr>

Language Level Definition of OOP

Object

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Language Level Definition of OOP Class

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Language Level Definition of OOP Inheritance

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Conceptual Level Definition of OOP

Abstraction
"Extracting the essential details about an item or group of items, while ignoring the unessential details."
Edward Berard

"The process of identifying common patterns that have systematic variations; an abstraction represents the common pattern and provides a means for specifying which variation to use."
Richard Gabriel

Example

Pattern:    Priority queue

Essential Details:   length 
   items in queue 
   operations to add/remove/find item

Variation:   link list vs. array implementation
   stack, queue
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Conceptual Level Definition of OOP Encapsulation

Enclosing all parts of an abstraction within a container

Example

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Conceptual Level Definition of OOP Information Hiding

Hiding parts of the abstraction

Example

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Conceptual Level Definition of OOP Hierarchy
Abstractions arranged in order of rank or level

Class Hierarchy

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Conceptual Level Definition of OOP Hierarchy

Object Hierarchy

Doc 1, Introduction Slide # 17

Stages of Object-Oriented Expertise [2]

Novice
Duration: 3 to 6 months

"Novices spend their time trying to get their function-oriented heads turned around, so they can see the application object classes. They have a hard time finding the classes and write a lot of function-oriented code."

Apprentice
Duration: 3 to 6 months

"Apprentices begin to understand what object-orientation really means. They still prefer to work with others, who help foster an environment of discovery. Apprentices begin turning out good designs, although not consistently. Gurus and journeymen still need to keep an eye on apprentices."

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Stages of Object-Oriented Expertise Journeyman
Duration: varies greatly, a least a year

"Journeyman have internalized the OO paradigm and work independently. They can provide key inputs during the design reviews. They still have mental roadblocks that may require a guru to break through."

Guru
Duration: lifetime

"For gurus, OO development comes naturally. A guru automatically sees things in an OO perspective, moving quickly through the phases of development during rapid iterations. A guru can move the project over mental hurdles, causing others to see something clearly that eluded them before."

"There are very few gurus in the world today. The rest of us will have to rely on proven processes and methodologies."

"I've yet to see someone successfully make it past the novice stage and even consider thinking about software in other than OO terms."

Doc 1, Introduction Slide # 19

Some Heuristics

A class should capture one and only one abstraction

Class = abstraction

Keep related data and behavior in one place

An abstraction is both data and behavior (methods)

All data should be hidden within its class


Information hiding

Doc 1, Introduction Slide # 20

Heuristics Continued
Beware of classes that have many accessor methods defined in their public interface. Having many implies that related data and behavior is not being kept in one place.

class test
   {
   private int thisData;
   private int thatData;
   private float moreData;

   public void setThisData( int data )  { thisData = data; }
   public void setThatData( int data )  { thatData= data; }
   public void setMoreData( int data )  { moreData= data; }
   public void getThisData(  )  { return thisData; }
   public void getThatData(  )  { return thatData; }
   public void getMoreData(  )  { return moreData; }
   public String toString() { // code deleted }
   }

No work is being done in this class.

Other classes are getting the data in class test and performing some operation on it.

Why is this class not doing the work on the data!

Who is doing the work?

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The God Class Problem
Distribute system intelligence horizontally as uniformly as possible, that is, the top-level classes in a design should share the work uniformly.

Do not create god classes/objects in your system. Be very suspicious of a class whose name contains Driver, Manager, System, or Subsystem

Beware of classes that have too much noncommunicating behavior, that is, methods that operate on a subset of the data members of a class. God classes often exhibit much noncommunicating behavior.

Doc 1, Introduction Slide # 22

Divide noncommunicating behavior into separate classes

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The God Class Problem - Data Form Legacy Non-OO System

Poor Migration to OO System

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Proliferation of Classes Problem
Be sure that the abstractions that you model are classes and not simply roles object play

Do we use a Father class, Mother Class, and a Child class to model a family?

Or do we use a Person class, with Person objects acting as Father, Mother and Child?

Do not turn an operation into a class.

Be suspicious of any class whose name is a verb or is derived from a verb, especially those that have only one piece of meaningful behavior ( i.e. do not count sets, gets, prints).

CS 696: Advanced OO Programming and Design Spring Semester, 1997 Doc 1, Introduction

Contents of Doc 1, Introduction

CS 696: Advanced OO Programming and Design
Spring Semester, 1997
Doc 1, Introduction