object oriented modeling concepts and principles

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Chapter 19 -- Assistance -- Lamimi V. KamatFebruary 14, 1999 R. A. Volz 1

OBJECT ORIENTED

MODELING, CONCEPTS

AND PRINCIPLES

Chapter 19


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CONTENTS

What is object-oriented development ?

Object-oriented process model

Object-oriented concepts

Object modeling technique

Unified Modeling Language (UML)

Concepts and Principles of Object Modeling

Object-oriented vs functional approach


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What is OO Development ?New way of thinking about problems using

models organized around real world concepts.

The fundamental construct is the object Combines both data structure and operations in asingle entity called an object.

Leads to reuse, faster software development

and higher quality programs.Easier to maintain

Structure inherently decoupled

Fewer side-effects


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The OO process modelMoves through an evolutionary spiralEmphasizes development of reuse capability


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Object Oriented ConceptsObjects and Object Model

Object: Data and operations relevant to some

real world or significant program entity

encapsulated into a monolithic unit accessibleonly through a well defined interface. For ex.

File in the file system together with operations

such as open, close, read, & write,

Object Model: Describes the structure of theobjects in the system

their identity, relationships to other objects,

attributes and operations.


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Classification & Classes A class describes a group of objects with

similar properties (attributes), commonbehavior (operations), common relationships to

other objects, and common semantics.

Object Modeling


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Object Classes

Thus, a class is an abstraction that describes

relevant properties and hides the rest.

Represented diagrammatically as below.

Class Name

Attributes

Operations


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Object Modeling


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Object Modeling

Attributes: An attribute is a data value held

by the objects in a class. Name, age, and

weightare attributes ofPerson objects.


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Object Modeling

Operations and Methods : Operations : An operation is a function or

transformation that may be applied to or by

objects in a class. Each operation has a targetobject as an implicit argument. The behavior of

the operation depends on the class of its target.

Methods : A methodis the implementation of an

operation for a class.Categories: 1) manipulate data, 2) perform

computation, and 3) monitor for occurrence of

controlling event.


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An operation may have arguments in addition to

its target object. Such arguments parameterize the

operation but do not affect the choice of method.

Object Modeling


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Class and Instance

Polygon

VerticesBorder Color

Fill Color

DrawErase

Move

Polygon

v={(0,0),(0,1),(1,0)}BC = Red

FC = Blue

DrawErase

Move


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Abstraction and Encapsulation


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Abstraction Isolate those aspects that are important and

suppress (or hide) those that are unimportant(e.g., representations).

Focus on what object is and does before

deciding how it should be implemented.

Abstraction allows dealing only with

application domain concepts, not making

design and implementation decision before

problem is understood.



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Encapsulation (Information Hiding)

Separates the external aspects of an object,which are accessible to other objects, from the

internal implementation details of the object,

which are hidden from other objects.

Combining Data and Operations: The OO approach combines the data structure

and operations in a single entity.



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Interfaces

Does not have an implementation of its own.Other classes provide implementations of it.

Client classes are only interested in behavior.

Class Name

Operations


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Inheritance

Sharing of attributes and operations among

classes based on hierarchical relationship.


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Class and Subclass

Polygon

VerticesBorder Color

Fill Color

DrawErase

Move

Right Triangle

Vertices

Hypotenuse length

Border Color

Fill Color

DrawErase

Move


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OperationsPolymorphism

The same operation may behave differently on

different classes. E.g., the move operation behavesdifferently on a Window and ChessPiece.

Operations may be overloadedwhen subclasses

defined.

The compiler can distinguish based on the type of the

operands in method invocations which operation is

actually needed.


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Polymorphism

Car

Paint

Polygon

Paint

TrianglePaint

SquarePaint


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Communication

Message: [destination, operation, params]


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What Does OO Mean?Pressman (Coad & Yourdon)

Objects (identity)

Classification

Inheritance

Communication

Rumbaugh Objects (identity)

Classification

Inheritance

Polymorphism


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Object Modeling TechniqueObject modeling technique (OMT) extends

from analysis thru design to implementation

Analysis model contains objects found in

the application domain, including properties

of object and their operations.

These application domain objects form aframework to the design model.


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The sameseamless notation is used from

analysis to design to implementation.

The system is modeled using three related

but different view points. Object Model : Represents the static, structural,

data aspects of the system.

Dynamic Model : Represents the temporal,

behavioral, control aspects of the system.

Functional Model : Represents transformational,

functional aspects of the system.

Object Modeling Technique


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Object Modeling

Links and Associations

Link: A physical or conceptual connection between

instances. E.g., Joe Smith Works-forSimplex

company. Mathematically, a tuple, i.e., an orderedlist of object instances. A link is an instance of an

association.

Associations : A group of links with common

structure and semantics. E.g., a person Worksfora

company. All the links in an association connect

objects from the same classes.


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Object Modeling

Multiplicity: Specifies how many instances of one class may

relate to a single instance of an associated class

Role Names: One end of an association. Binary association

has two roles.

Link attributes

May be defined for associations, e.g., if theassociation is uses, the link attribute might be

one of permission.


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Binary Association & Multiplicity


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Ternary Association


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Link Associations


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AggregationA part-whole ora-part-of relationship


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OO Software ProcessFramework

Identify major classes and connections.

Do enough design to ensure they are implementable

Extract reusable components and build prototype. Test to uncover errors and get customer feedback.

Iterate on design and refine it.

Engineer special objects (not in library).

Assemble a new prototype. Test and obtain customer feedback.

Iterate until satisfactory product obtained.


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OO MetricsBecause of reuse, LOC not so useful# of Scenario scripts, each a triplet of the form

[initiator, action, participant], where

Initiator = object initiating a request

action = result of request (method invocation)

participant = server object satisfying request

# of key [highly independent] classes# of support classes (and ave. per key class)

# of subsystems


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Possible Estimating Approach

Develop scenario scripts and estimate count

Determine the number of key classes

Categorize key classes

Interface type Multiplier

No GUI 2.0

Text-based user int. 2.25GUI 2.5

Complex GUI 3.0


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Possible Estimating ApproachEstimate # of support classes by multiplying

# key classes in each category by multiplier

Estimate # of person-days per class, e.g.,

15-20

Estimate the number of major iterations

There should be a contract deliverable foreach major iteration


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OO Progress TrackingThis needs to be done for each iteration (see

text for list of specifics in each category)

OO analysis completed

OO design completed

OO programming completed

OO testing completed


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Object-Oriented vs Structured Approach

Easier to maintain

Combines data

structure and behaviorin a single entity

Emphasizes object

structure

Reuse more readilyaccomplished

Harder to maintain

May separate data and

behavior Emphasizes

procedural structure

Reuse limited, hence

possible delay insoftware construction


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Object-Oriented vs Structured Approach

Strong cohesion and

weak coupling

Encapsulation,Inheritance and

Polymorphism are

strong features of OO

software development

Harder to achieve

weak Coupling and

strong cohesion Some languages

support encapsulation

and polymorphism,

but rarely inheritance

object oriented modeling concepts and principles

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