2

. Object Model

Venkat Subramaniam

UML-1

Benefits Of OO Development

• Models System using Objects • Small Semantic gap between reality & model • Understanding the system is easier • Modifications are localized

Venkat Subramaniam

UML-2

Object-Oriented Paradigm

Collection of Discrete Objects Data & Behavior OO Paradigm • Abstraction • Encapsulation • Hierarchy – Inheritance hierarchy (“is-a”) – Part of hierarchy (“has a”) • Polymorphism

Venkat Subramaniam

UML-3

Abstraction

“A simplified description ... of a system that emphasizes some of the system’s details ... while suppressing others” “An abstraction denotes the essential characteristics perspective of an object that distinguish it from all other kind of objects and thus provide crisply defined conceptual boundaries, relative to the of the viewer”

Venkat Subramaniam

UML-4

Encapsulation

• Information hiding • Interface - Implementation • Behavior & Data “Encapsulation is the process of compartmentalizing the elements of an abstraction that constitute its structure and behavior ; encapsulation serves to separate the contractual interface of an abstraction and its implementation ”

Venkat Subramaniam

UML-5

What is an Object?

• “Concept, abstraction, or thing with crisp boundary & meaning for a problem” • An Object has state and behavior • Objects receive stimuli/messages & respond • Receiving a stimulus, Object may change state Examples: jullie

Venkat Subramaniam

dylan kim charlie charlie.fly() stimulus UML-6

What is a Class?

• Group of Objects with similar –properties (attributes) –behavior –relationships to other objects –semantics • Blueprints of Objects

Parrot Example Venkat Subramaniam Girl

jullie kim

Boy

dylan charlie UML-7

Hierarchy

“Hierarchy is a ranking of abstractions” Inheritance : expresses “

is-a

” or “

Kind-of

” relationship • Extensibility & Reusability Part-of: expresses that object is an aggregate another of

Venkat Subramaniam

UML-8

Polymorphism

Hiding alternative procedures behind a Common Interface Send a Message to an object Polymorphism guarantees that the correct/proper implementation is invoked .

Sender

does not

receiver know specific class of

Venkat Subramaniam

UML-9

Object Model

• Captures static structure of system • Objects, relationships, attributes & operations • Most important • Intuitive graphic representation • Valuable for communication & documentation

Venkat Subramaniam

UML-10

Objects

• Decomposing problem into objects – depends on judgement & nature of problem • No one correct representation • Objects have identity

Venkat Subramaniam

UML-11

Class

• Often appear as nouns in problem descriptions • Has semantic • Interpretation of semantics – depends on application and matter of judgement • Each class may have zero, one or more objects • Each object knows it class

Venkat Subramaniam

UML-12

Class Diagrams

• Provide formal graphic notation for modeling • Concise, easy to understand, practical • Describes many possible instances

Venkat Subramaniam

UML-13

Object Diagrams

• Describes how set of objects relate • Useful for – documenting test cases – Clarification of complex class diagrams • Class Diagram corresponds to infinite set of object diagrams

Venkat Subramaniam

UML-14

Notation for Classes & Objects (UML)

Girl Boy Classes

Venkat Subramaniam

Objects UML-15

Attributes

• Data value held by objects of a class • Objects may have same/different values for attribute • Attribute name unique within a class • Adjectives often represent specific enumerated attribute values : “red car”

Venkat Subramaniam

UML-16

Attributes...

• Attribute is a pure data value - not an object • Internal identifiers must not be shown as attributes • Show only important attributes

Venkat Subramaniam

UML-17

Derived Attributes

Base Attribute : – primitive, not dependent on other attributes Derived Attribute : – computing not considered to change state of an object – dependent on base attributes – may be stored or computed upon a query operation Example : Area of a circle, age of a person

Venkat Subramaniam

UML-18

Attributes Notation

clas s Nam e -attrPrivate #attrProtected +attrPublic $attrStatic /attrDerived

Underline in revised notation

-attrWType: int -attrWInitVal: double=0.0

Girl -name +dob /age: int

Venkat Subramaniam

UML-19

Operations & Methods

• Operation : Function that may be applied to or by objects • Same Operations applying to different classes: Polymorphic • Method is implementation of an operation for a class • Operation has a target object and may have arguments • Same operations on different classes should have – same signature and consistent intent • Query Operation : Does not affect the state of object • Show only important methods

Venkat Subramaniam

UML-20

Operations Notation

className +publicMethod() #protectedMethod() -privateMethod() $staticMethod() +methodWParam(int) +methodWRetVal(): int Girl +sing() +play()

Venkat Subramaniam

UML-21

Venkat Subramaniam

Example with Attributes & Operations

StopWatch -seconds +start() +stop() +reset() +getSeconds(): double UML-22

Associations and Links

• Link is physical or conceptual connection between objects • Link is an instance of an Association Example: Link : Susan is-wife-of Robert Julie is-wife-of John Association: Woman is-wife-of Man Associations and Links appear as Verbs

Venkat Subramaniam

UML-23

Associations & Links...

• Associations are bi-directional –may be implemented as unidirectional • Implemented usually as pointers – important not to think as pointers • Associations may be –one-to-one –one-to-many –many-to-one –many-to-many

Venkat Subramaniam

UML-24

Ternary & High Order Associations

Binary Association : Relates two classes Woman is-wife-of Man Ternary Association : Relates three classes Nancy is-daughter-of Susan and Robert n-ary Association : Relates n classes Higher Order Associations – complicated to draw, implement and think – try to avoid if possible

Venkat Subramaniam

UML-25

Associations Notations

Exactly One * M any 0..1

Optional 1..* One Or M ore 0..* Ze ro Or M ore

Directed Association

*

Company 0..* employs employees 1..* worksFor Person

Venkat Subramaniam

UML-26

Link Attributes & Association Classes

• Attributes that belong to association of object rather than one object • Link Attributes belong in Association Classes • Ex: Salary received by Employee from Company • In an one-to-one association you may try to make it attribute of one of the objects –Leads to extensibility problems

Venkat Subramaniam

UML-27

Venkat Subramaniam

Association Class Notation

Person * * Company Employment salary:double UML-28

association

Role Names

• Name given to either end of an • Helps to navigate from one object to related objects Woman wife 0..1

married husband 0..1

Man • Helps clarify when two classes have several associations between them Woman

Venkat Subramaniam

wife 0..1

* daughter married children husband 0..1

1 father Man UML-29

objects

class

Qualifiers

• Distinguishes among set of associated • Models associative arrays, dictionaries • Qualifiers may be wrongly

modeled as attribute of associated

GradeList * * Student code Unqualified GradeList code * Qualified 1 Student

Venkat Subramaniam

UML-30

Aggregation & Composition

Aggregation: • Part-of or part-whole relationship (by reference) • Example : Car has Engine and Transmission • Assembly of objects with aggregate and component parts • Component existence may or may not depend on aggregate

Venkat Subramaniam

UML-31

Aggregation & Composition...

Composition: • Part belongs to only one whole (by Value) • Part lives and dies with the whole • Whole cannot replace the part

Venkat Subramaniam

UML-32

Aggregation/ Composition Notation

A ggregation

Has By V alue Has By Re fe r e nce

Vehicle

+drive()

Engine

Venkat Subramaniam

VIN UML-33

Aggregation / Composition Example

Person Aggregation

Venkat Subramaniam

Car Composition brain UML-34

Inheritance

• Models is-a relationship • Relationship between a class and its refined versions • Superclass or Base class • Subclass or Derived class • Inheritance is transitive • Discriminator : The property being abstracted by a particular inheritance • Breath Vs. Depth of inheritance

Venkat Subramaniam

UML-35

Inheritance Notation

Generalization

Superclass

Vehicle

+drive()

Subclass1 Subclass2

Car

Venkat Subramaniam

-sunRoof Truck UML-36

Inheritance Example

Child Animal Girl

Venkat Subramaniam

Boy Dog Cat UML-37

Grouping Mechanism : Package

• Grouping classes together into higher-level units • Package diagram with dependency • Dependency between packages exists if – class in one package depends on a class in the other – definition change of one package may change other

Venkat Subramaniam

UML-38

Package Notation

componentName VehicleComponent

Venkat Subramaniam

Accessories

Aggregation Vs. Association

• Special form of Association • Confusing • Aggregation represents “part-of” relationship • Some operations on whole automatically applied to its parts • Aggregate is asymmetric : part is subordinate to the whole • Association is symmetric : objects involved are of equal stature

Venkat Subramaniam

UML-40

Aggregation Vs. Inheritance

• Aggregation represents part-of relationship • Inheritance represents kind-of relationship • Aggregation refers to object relationships • Inheritance refers to class relationships

Venkat Subramaniam

UML-41

Fixed, Variable & Recursive Aggregates Fixed : – Fixed structure – Number & types of parts pre-defined Variable : – Finite number of levels - Number of parts vary Recursive : – Contains instances of the same kind of aggregate – number of potential levels unlimited

Venkat Subramaniam

UML-42

Layout

Example : Fixed, Variable, Recursive Aggregation

* ElectronicComponent * NAND

Venkat Subramaniam

2 AND FlipFlop UML-43

Operations & Aggregation

• Operation or Triggering : – automatic application of an operation to network of objects when applied to some starting object • ShallowCopy • DeepCopy

Venkat Subramaniam

UML-44

Abstract Classes

Representing an Abstraction that is Abstract.

• Abstract classes represent – concepts – not real objects • ABCs used only to create other “Concrete” classes

Venkat Subramaniam

UML-45

Abstract Classes ...

Example: Shape, Employee, Animal Whether a class in Abstract or not depends on – judgement – application on hand

Venkat Subramaniam

UML-46

Inheritance : Extension & Restriction

Extension : – Subclass adds new features – Subclass inherits all properties & operations of ancestor Restriction : – Subclass constrains ancestor attributes – Subclass may not inherit all properties & operations of its ancestor – Often leads to trouble (Liskov Substitutability Principle)

Venkat Subramaniam

UML-47

Inheritance : Extension & Restriction ...

• Proper Extension: – A Subclass may override the internal implementation of an operation – No problem as long as external protocol remains the same

Venkat Subramaniam

UML-48

Metadata & Metaclass

Metadata : Data that is description of other data Metaclass : Class descriptor objects that describe other classes

Venkat Subramaniam

UML-49

Constraints

Restricts values that entities can assume Entity : objects, classes, attributes, associations Better to capture constraints in structure rather than using constraints constructs ElectronicsClass

Venkat Subramaniam

* {E E Discipline} Students UML-50

Multiple Inheritance

Allows a class to have more than one Superclass • Join class TerrestrialBeing AquaticBeing Amphibian

Venkat Subramaniam

UML-51

Multiple Inheritance Poses Problems

gender Person discipline ComputerScientist CivilEngineer Male Female FemaleComputerScientist • Ambiguous function collisions • Class proliferation

Venkat Subramaniam

UML-52

Workarounds for MI

• Delegation using aggregation of roles • Single inherit important class & delegate rest • Nested Generalization

Venkat Subramaniam

UML-53

Delegation using Aggregation of Roles

Person gender discipline * PersonsDiscipline PersonsGender Male Female ComputerScientist • Inheritance of operations across aggregation not automatic CivilEngineer • Join class must handle that operations and delegate to appropriate component

Venkat Subramaniam

UML-54

Single Inherit Important Class & Delegate Rest

Person gender discipline * PersonsDiscipline Male Female ComputerScientist CivilEngineer • Preserves identity and inheritance across at least one generalization

Venkat Subramaniam

UML-55

Nested Generalization

Person gender Male Female MaleComputerScientist MaleCivilEngineer FemaleComputerScientist FemaleCivilEngineer • Violates the OO spirit • Not practical for large number of combinations

Venkat Subramaniam

UML-56