Introduction to Objects & OMT (Lecture 1)

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Topics -- Software Development, Software Models
(L) 1-5 (B)1-2, 7

Software includes all documentation necessary to:

Install
Use
Develop and
Maintain programs

UNIFIED MODELING LANGUAGE (UML)

a general-purpose visual modeling language designed to specify, visualize, construct and document the artifacts of a software system.

a visual modeling language that depicts the structure of the code at a level just above the code itself.
independent of any particular programming language and development process
with extensibility and specialization mechanisms
that provides a formal basis for understanding the model

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What does UML contribute?

A widely-accepted notational system for modeling object-oriented concepts.
Unified Modeling Language is intended to describe both the real-world and software

Analysis	Design
What	How
Investigate the Problem	Create a Solution

Models -- Increase communication capability by including pictures, diagrams, graphs, etc.

A Model is

an abstraction of something for the purpose of understanding it before building it. (Rumbaugh)

a simplification of reality, created in order to better understand the system being created. (Booch)

A model can be used to:

reduce complexity
aid visualization
aid communication
test before building

Software Engineering Methodology

"A process for the organized production of software

using a collection of predefined techniques and

notational convention." Rumbaugh

Common factors of software engineering (Ian Sommerville)

Software built by teams
Use engineering principals
Consists of technical and non-technical aspects

Software development methods depend on aspects of the Software Project

Small	Large
Individual	Team
Simple	Complex

Software Development Life Cycle (SDLC)

Software is developed in manageable stages

Stage	Activity

Requirements analysis and definition	Take steps to understand the problem The system's services, constraints and goals are identified by agreement with the system users and owners.
System and software design	Partition requirements into hardware/software systems
Implementation	Program coding and unit testing
System testing	Test composite system
Operation and maintenance	Typically the longest part of the life cycle

Try to be correct at the beginning of the software development cycle:

per Alan M Davis "Software Requirements Analysis & Specification"

Many errors remain undetected until well after the stage at which they are made
The later in the life cycle that an error is detected, the more expensive it is to repair
Most software errors can be found by inspection
56% of all detected bugs are traceable to errors made during requirements stage.

The early stages of software development should:

Clarify requirements
provide a basis of agreement with user/owner/co-developers
provide a framework for later design and implementation
help write and maintain the software

Functional Model -- Process oriented methodology

Main emphasis is on capturing Input, Process, and Output
The idea is to study behavior to derive data structures and programs
Develop a "global" knowledge to control procedures by using:

structured analysis
structured design

Structured analysis

defines what a system should do by data flow diagrams (DFDs) that model the

storage of data
flow of data
processes that respond to and change data

Potential pitfalls with structured analysis

analysis paralysis
design of structure chart from data flow model was not obvious nor seamless
any small change in requirements could produce large changes in code
data flow diagrams did not ensure complete and accurate data
inconsistent reports
update anomalies
Hard to map the structured analysis to the programs created. (Trace-ability problem)

Structured design -- factor a program into a top-down hierarchy of independent modules (Structure chart)

Each module should be:

Cohesive -- a module should do a specific function
Loosely coupled -- minimize data and parameter passing

The top-level should show the most important division of work -- the top level manager of the system.

Lower hierarchical levels further subdivide the allocated work to lower level managers.

Design concepts:

Transform analysis -- the input and output streams have one clearly defined central process.

Transaction analysis -- the input and output streams have multiple core processes. The top-level manager examines the content of the input stream to determine which process should be invoked
(bank account -- deposit / withdrawal)

Data Modeling is a data-centered methodology

Study the system's data needs independently of how the data is currently used.

Data modeling technique

identify business entities
attribute the data elements that describe these entities
specify the relationships that exist between entities

Data modeled systems focus on

accurate, up-to-date data
today's output requirements
meeting future output requirements without drastic changes (because data is already there)

Data Modeling did not completely solve certain Problems:

Not clear how to connect process to the data model

An Object-Oriented System

Depends on "local" knowledge of objects to accomplish their tasks.

There are very few things an object can do on its own, in order to accomplish any significant task, objects must communicate with other objects.

Emphasis on Abstraction

Model relevant attributes and behavior of a given entity for a particular purpose Focus on what an object "is" and "does.” Class defines both behavior and state.

Properties of a Good Abstraction

Well named

The name accurately communicates the nature of the abstraction

Coherent

The abstraction contains a related set of attributes and behavior that makes sense and is needed and expected in a given setting

Accurate

The abstraction contains only attributes or behavior that are displayed by the entity being modeled.

Minimal

The abstraction does not contain attributes or behavior extraneous to the purpose for which it is defined.

Complete

The abstraction contains all the attributes and behavior necessary to accomplish its intended purpose.

Three levels of modeling detail

Essential --

concepts of the business without reference to implementation technology.

Specification --

the characteristics of software that will meet the requirements of a business solution.

Implementation --

the construction of the software

Difference between Class and Object

Class --

A description of a set of objects which have something in common.

Class is also a programming concept used to define objects that have the same attributes, operations, relationships and semantics.

Object --

Is an instance of a Class.

Objects are created from a Class at run-time. Since Classes have state, an object retains information about its attributes for some indefinite time. This is different than functions or subprograms.

Object-Oriented Concepts

Abstraction Model relevant attributes and behavior of a given entity for a particular purpose	Focus on what an object "is" and "does." Class defines both behavior and state. "Class" concept is more powerful that either "record" or "procedure."
Encapsulation Grouping of related ideas into one unit referred to by a single name "Information hiding" Information is hidden about data and implementation so that clients only have to know the interface of the class	Class defines an interface and an implementation -- operations and attributes. Attributes and operations are accessible only through the interface. De-couple content of information from its form of representation. Separate "what" an entity does from the "how" so less has to be known in order to use the class. The implementation can be changed without affecting a client's knowledge of the interface. Coding revisions to a class should have only a local effect and not cascade to other dependent objects. Loose coupling enforced
Generalization - Specialization Define the state and behavior properties once in a general super-class (Vehicle) then reuse them in a more specific sub-class (Car, Truck)	Use inheritance to reduce repetition for hierarchical relationships within designs and within programs Inheritance enables a class to receive all the attributes and operations of its parents A specialized sub-class may also add its own properties and override some inherited properties
Object Identity every object has its own identity (regardless of its class or attribute values).	Usually implemented with a pointer reference (object-handle) The object-handle is unique for every object.
Composition use components that interact to accomplish a task	Build complex "whole" systems by assembling simpler "parts" by association or aggregation
Type promotion OO languages support substitution of a specialized object as a more general type.	Can refer to an Car instance as either a Vehicle or a Car. De-couples generic code from the existence of a particular specialization.
Message Passing Objects communicate to other objects via message passing.	Very few objects can accomplish the necessary tasks by themselves. They have to work with other objects. Messages contain the object-handle, name of the operation that is to be performed and any additional arguments that should be used in the operation. The message sender does not have to know the exact class of the receiver
Polymorphism Each class may perform the same operation differently.	At execution time, objects respond based on what they are. Tell a vehicle to move and it will move like a car if it is a Car or a truck if it is a Truck. De-couples generic code from the existence of a particular specialization.
Representation Continuity Same types of diagrams are used in both analysis and design.	Design elaborates the diagrams discovered during analysis. (Particularly true for Class diagrams, Use Case Diagrams and Interaction Diagrams)

Properties of a good abstraction

Well Named	The name accurately communicates the nature of the abstraction
Coherent	The abstraction contains a related set of attributes and behavior that makes sense and is needed and expected in a given setting
Accurate	The abstraction contains only attributes or behavior that are displayed by the entity being modeled
Minimal	The abstraction does not contain attributes or behavior extraneous to the purpose for which it is defined.
Complete	The abstraction contains all the attributes and behavior necessary to accomplish its intended purpose.

Object oriented approach to analysis

the most appropriate programming structures should be based on relevant objects of the application domain.
Organize software as a collection of objects containing

data structure
behavior

Build an object model of an application domain
Add implementation details to the model during design

OO compared to traditional software development (Rumbaugh OMT)

Remove programming language concepts

and emphasize the language of the domain.

OO Analysis should emphasize

front-end conceptual issues (analysis)
data structure before function
be iterative not sequential
language of the application domain

OO Analysis should not emphasize

back-end implementation (code)

Object-oriented development (Rumbaugh OMT)

A seamless progression of objects by

identifying real-world object classes in a domain
using same notation & language throughout development stages (trace-ability)
adding detail to the object classes with each development stage
adding necessary implementation objects at design stages.

Software Development Stages (Rumbaugh OMT)

Stage	Activity
Analysis produce a document specifying what the system must do, not how to do it	start with a problem statement build a real-world models of application domain concepts verify, iterate and refine the models
System Design high-level design of the architectural structure to solve the problem	strategy for allocating software/hardware resources to the problem organize problem components into subsystems consider performance and concurrency characteristics
Object Design produce a design document using the same classes defined by analysis.	Avoid using any particular programming language Objects derived from analysis may require additional details before implementation. Where necessary add: data structures algorithms computer domain concepts
Implementation tranform classes into a particular hardware and software code	programming language database hardware

Use Prototypes & Models to help clarify requirements

Use when requirements are vague/ambiguous
user interface languages and/or high level 4GL
iterate through successive changes until the users can describe the target system

What is "Design"?

Sketch a design of a small, one-branch bank.

(the whole thing, not just the computer part)

The design can include images and/or text.

Limit the design to a single sheet of paper.

You should be able to show this piece of paper to pretty much anyone on the street, and they would recognize the bank's basic design.

Work with someone else.

Take no more than 15 - 20 minutes.