Chapter 8: Object-Oriented Design

Omar MeqdadiSE 273 Lecture 8

Department of Computer Science and Software Engineering University of Wisconsin-Platteville

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Topics covered

Objects and object classes An object-oriented design process Design evolution

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Object-Oriented Design

Designing systems using self-contained objects and object classes

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Objects and Classes

Class – a generalization of a set of entities with common structure, behavior, and relationships to other classes. An abstract data type. A person, an employee

Object – an instance of a class. It has a state, value, and scope of existence Joe Smith, Jane Doe

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Objects

Objects are entities in a software system which represent instances of real-world and system entities

An object has a state, behavior and identity. Terms instance and object are interchangeable. State – the properties of an object and the current values of these

properties Behavior – how an object acts and reacts in terms of its state change

and message passing

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Types of objects

Boundary – represent the interactions between the system and actors

Control – represent the tasks that are performed by the user and supported by the system

Entity – represent the persistent information tracked by the system

See [Jacobson ’99]

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Objects and object classes

Object classes Templates for objects (used to create objects) The structure and behavior of similar objects are defined in

their class. Includes declarations of all the attributes and services

which should be associated with an object of that class Object classes may inherit attributes and services from

other object classes

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What is a good Class?

Should provide a crisp abstraction of something from the problem (or solution) domain

Embody a small well defined set of responsibilities and carry them out well

Provides clear separation of abstraction, specification, and implementation

Is understandable and simple yet extendable and adaptable.

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Characteristics of OOD

Objects are abstractions of real-world or system entities and manage themselves

Objects are independent and encapsulate state and representation information

System functionality is expressed in terms of object services

Shared data areas are eliminated. Objects communicate by message passing

Objects may be distributed and may execute sequentially or in parallel

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Advantages of OOD

Easier maintenance. Objects may be understood as stand-alone entities

Objects are appropriate reusable components For some systems, there may be an obvious

mapping from real world entities to system objects

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Object Oriented paradigm

OO Analysis – A method of analysis which examines requirements from the perspective of classes and objects found in the vocabulary of the problem domain

OO Design – A method of design encompassing the process of object oriented decomposition.

OO Programming – A method of implementation in which programs are organized as cooperative collections of objects, each an instance of a class whose members are part of a inheritance hierarchy

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A Class in UML

+print()

-name : string(idl)-age : int

PersonClass name

Attributes

Operators

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Class attributes

Person

+ name : String# address : Address# birthdate : Date- ssn : Id

An attribute is a named property of a class that describes the object being modeled

Attributes are usually listed in the form:

attributeName : Type Attributes can be:

+ public # protected - private

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Class operations

Person

name : Stringaddress : Addressbirthdate : Datessn : Id

eatsleepworkplay

Operations describe the class behavior and appear in the third compartment

Specify an operation by stating its signature

Operations can be: + public # protected - private

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An object in UML

object name and class joe : Person

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Example class diagram

+open()+close()+display()

-name : string(idl)window

dialogboxconsolewindow

event

control

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Class relationships in UML

Generalization Dependency Association

These can represent inheritance, using, aggregation, etc.

-Role1

*

-Role2

0..1

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Generalisation and inheritance

Objects are members of classes which define attribute types and operations

Classes may be arranged in a class hierarchy where one class (a super-class) is a generalisation of one or more other classes (sub-classes)

A sub-class inherits the attributes and operations from its super class and may add new methods or attributes of its own

Generalisation in the UML is implemented as inheritance in OO programming languages

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A generalisation hierarchy

Employee

Programmer

projectprogLanguage

Manager

ProjectManager

budgetsControlled

dateAppointed

projects

Dept.Manager

StrategicManager

dept responsibilities

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GeneralizationSensor

+currentValue()+calibrate()

CalibratingSensor

+currentDirection()

WindDirectionSensor

+resetHighest()+resetLowest()

-highestValue-lowestVale

HistoricalSensor

-trend

TrendSensor

+currentHumidity()

-humidity

HumiditySensor

+currentSpeed()

-speed

WindspeedSensor

+currentTemp()

-temp

TemperatureSensor

+currentPressure()

-pressure

Barometer

•An is-a relationship•Abstract class

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Single inheritance

TelemetryData

TelemetryData

TelemetryData

TelemetryData

TelemetryData

TelemetryData

TelemetryData

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Single inheritance

class TelemetryData {public: TelemetryData(); virtual ~TelemetryData(); virtual void transmit(); Time currentTime() const; private: int id; Time timeStamp;};

class ElectricalData : public TelemetryData {public: ElectricalData(float v1, float v2, float v1, float v2); virtual ElectricalData(); virtual void transmit(); float currentPower() const;private:float fuelCell1Voltage, fuelCell2Voltage;float fuelCell1Amperes, fuelCell2Amperes;};

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Multiple inheritance

InsurableItem

BankAccount

SavingAccount

InterestBearing

CheckingAccount

Asset

RealEstate

Security

Bond

Stock

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Multiple inheritance

class Asset;class InsurableItem;class InterestBearing;

class BankAccount : public Asset, public InsurableItem, public InterestBearing {};

class RealEstate : public Asset, public InsurableItem {};

class Security : public Asset, public InterestBearing {};

class SavingsAccount : public BankAccount {};class CheckingAccount : public BankAccount {};

class Stock : public Security {};class Bond : public Security {};

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Advantages of inheritance

It is an abstraction mechanism which may be used to classify entities

It is a reuse mechanism at both the design and the programming level

The inheritance graph is a source of organisational knowledge about domains and systems

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Problems with inheritance

Object classes are not self-contained. they can not be understood without reference to their super-classes

Designers have a tendency to reuse the inheritance graph created during analysis. Can lead to significant inefficiency

The inheritance graphs of analysis, design and implementation have different functions and should be separately maintained

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Inheritance and OOD

There are differing views as to whether inheritance is fundamental to OOD. View 1. Identifying the inheritance hierarchy or network is

a fundamental part of object-oriented design. Obviously this can only be implemented using an OOPL.

View 2. Inheritance is a useful implementation concept which allows reuse of attribute and operation definitions. Identifying an inheritance hierarchy at the design stage places unnecessary restrictions on the implementation

Inheritance introduces complexity and this is undesirable, especially in critical systems

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Association

Structural relationship between peer classes (or objects). Association can have a name and direction, or be bi-

directional Role names for each end of the association Multiplicity of the relationship

Association code example

class Person {

public:

private:

Company *employer;

};

class Company {

public:

private:

Person **employee;

};

Each instance of Person has a pointer to its employer

Each instance of Company has a collection of pointers denoting its employees

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Examples of association

person company

person company

Works For

-employee

1..*

-employer

*

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An association model

EmployeeDepartment

Manager

is-member-of

is-managed-by

manages

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Association Relationships

A class can have a self association.

LinkedListNode

next

previous

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Link Attributes

Associations may have properties in the same manner as objects/classes.

Salary and job title can be represented as

person company

-salary-title

1..*

-employee

*

-employer

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Aggregation & Composition

Special type of association Part of relationship (is-part-of, is-made-of, contains) Can use roles and multiplicity

university department

1 *

Aggregation vs. Composition

Aggregation is a shared containment. No strong life time dependency.

Composition is constrained aggregation that implies ownership. The life time of the aggregates are determined by the object.

1 *

1 *

Aggregation code example

A part-of (has-a) relationship (physical containment)

class Team{public: Team(); Team(vector<*members>); private: vector<*department> members[n];};

Composition code example

A part-of (has-a) relationship (physical containment)

class university {public: university(); university(vector<*department>); private: vector<*department> dept[n];};

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Dependency

Represents a using relationship If a change in specification in one class effects another

class , then there is a dependency

windchill

windspeedSensor

tempatureSensor

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Dependency relationships

CourseSchedule

add(c : Course)remove(c : Course)

Course

A dependency indicates a semantic relationship between two ormore elements. The dependency from CourseSchedule to Course exists because Course is used in both the add and remove operations of CourseSchedule.

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Which Relation is Right?

Aggregation – aka is-part-of, is-made-of, contains Use association when specific (persistent) objects have

multiple relationships (e.g., there is only one Bill Gates at MS)

Use dependency when working with static objects, or if there is only one instance

Do not confuse part-of with is-a

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Object modeling

Given the high-level requirements (use cases) Define the object model

Identify objects Compile a data dictionary Identify association and aggregations Identify attributes of objects Generalize objects into classes Organized and abstract using inheritance Iterate and refine model Group classes into modules/components

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Example: Weather Monitoring Station

This system shall provide automatic monitoring of various weather conditions. Specifically, it must measure: wind speed and direction temperature barometric pressure humidity

The system shall also proved the following derived measurements: wind chill dew point temperature temperature trend barometric pressure trend

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Weather Monitoring System Requirements

The system shall have the means of determining the current time and date so that it can report the highest and lowest values for any of the four primary measurements during the previous 24 hour period.

The system shall have a display that continuously indicates all eight primary and derived measurements, as well as current time and date.

Through he use of a keypad the user may direct the system to display the 24 hour low or high of any one primary measurement, with the time of the reported value.

The system shall allow the user to calibrate its sensors against known values, and set the current time and date.

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Identify objects

From the vocabulary of the domain User, clock, sensor, temperature, LCDDisplay, Keypad, time, date,

wind speed, humidity, barometer, calibrator, metric units, English units, input manager, sensor sampler, wind direction, display manager, trend, pressure, current time, current date, current temp, high temp, low temp, change temp, change time, power up, power down, input buffer, trend, key, running, selecting

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Eliminate terms

Refine the model by eliminating Redundancy – classes that represent same concept Irrelevant classes – things you don’t care about Vague classes – ill defined boundaries Attributes – describe parts of objects Operators – sequence of actions are often mistaken for classes Roles – what it is not the role it plays Implementation details – save it for later

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New Data Dictionary

Time & Date Sensors: Temperature, Pressure, Humidity, Wind Speed, Wind Direction Keypad Input Manager Display (LCD Device) Display Manager Timer (clock) Sensor Sampler

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Relationships

sensors

samplerinputManager

displayManager

«interface»keypad

«interface»LCDDevice «interface»

timer

-time-date

timeDate

windChill

1

1

1

1

1

1

11

1

1

dewPoint

windDirection

sensor1

1

1

1

1

1..*

1

1

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Packages

Compiler

A package is a container-like element for organizing other elements into groups.

A package can contain classes and other packages and diagrams.

Packages can be used to provide controlled access between classes in different packages.

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Packages: Example

Classes in the FrontEnd package and classes in the BackEnd package cannot access each other in this diagram.

FrontEnd BackEnd

Compiler

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Packages: Example

Classes in the BackEnd package now have access to the classes in the FrontEnd package.

FrontEnd BackEnd

Compiler

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Packages

JavaCompiler

We can model generalizations and dependencies between packages.Compiler

Java

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Subsystem models

Shows how the design is organised into logically related groups of objects or packages

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Weather station subsystems : example1

«subsystem»Interface

CommsController

WeatherStation

«subsystem»Data collection

«subsystem»Instruments

Air thermometer

WeatherData

Ground thermometer

Anemometer

WindVane

RainGauge

InstrumentStatus

Barometer

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Weather station subsystems : example2 (layer architecture

«subsystem»Data collection

«subsystem»Instruments

«subsystem»Interface

Weather station

Manages allexternal

communications

Collects andsummarisesweather data

Package ofinstruments for raw

data collections

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Concurrent objects

The nature of objects as self-contained entities make them suitable for concurrent implementation

The message-passing model of object communication can be implemented directly if objects are running on separate processors in a distributed system

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Object communication

Conceptually, objects communicate by message passing.

Messages The name of the service requested by the calling object. Copies of the information required to execute the service

and the name of a holder for the result of the service. In practice, messages are often implemented

by procedure calls Name = procedure name. Information = parameter list.

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Message examples

// Call a method associated with a buffer // object that returns the next value // in the buffer

v = circularBuffer.Get () ;

// Call the method associated with a// thermostat object that sets the // temperature to be maintained

thermostat.setTemp (20) ;

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Servers and active objects

Servers. The object is implemented as a parallel process (server)

with entry points corresponding to object operations. If no calls are made to it, the object suspends itself and waits for further requests for service

Active objects Objects are implemented as parallel processes and the

internal object state may be changed by the object itself and not simply by external calls

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Active transponder object

Active objects may have their attributes modified by operations but may also update them autonomously using internal operations

Transponder object broadcasts an aircraft’s position. The position may be updated using a satellite positioning system. The object periodically update the position by triangulation from satellites

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An active transponder object

class Transponder extends Thread { Position currentPosition ; Coords c1, c2 ; Satellite sat1, sat2 ; Navigator theNavigator ; public Position givePosition () { return currentPosition ; } public void run () { while (true) { c1 = sat1.position () ; c2 = sat2.position () ; currentPosition = theNavigator.compute (c1, c2) ; } }

} //Transponder