Illinois Institute of Technology

CS48710-1/36

Illinois Institute of Technology

CS487 - Software Engineering

Object -Oriented Concepts & Principle - Chapt 19

David A. Lash

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Object Oriented Concepts & Principles

Object-Oriented Viewpoint - Characterize the problem domain as a set of objects with specific attributes and behaviors

– Functions and methods manipulate the objects

– Further characterize the objects into classes and subclasses

Leads to libraries of reusable classes and objects.

– Reduces time to develop & higher quality programs

– Easier to change

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Object Approach

Was software development only but now encompasses a complete SE view

– More than OOP (OO programming), should consider OORA (OO Req Analysis) OO D (design) & OO CASE.

– Engineering paradigm is more evolutionary since reuse of classes to emphasize

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Object Approach

The OO development process starts with requirements.

– Would define requirement classes

During engineering and construction & release would follow the path below:

Identifycandidate

classes

Search class

library

Build classes

unavailable

Save newclasses

On to customerevaluation

From Requirements, then planning &

Risk Analysis

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What is an object?

A tangible and/or visible thing– Customer, Part, etc.

Something that may be apprehended intellectually

– Appointment, Task, etc.

Something toward which thought or action is directed

– Order, Process, etc.

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Object Definition

An object is characterized by a number of operations and a state which remembers the effect of operations.

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What are an object’s characteristics?

Identity.– A way to distinguish between occurrences

Behavior. – How the object acts and reacts in terms of

state changes and messages (methods)

State.– The state of an object encompasses all of

the (usually static) properties of the object plus the current (usually dynamic) values of each of these properties

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Example OO Concepts - Classes, Objects & Inheritance

Assume have defined a class called Furniture and

– within that class there is an object called chair

Object: Chair--------------------Attributes:- Cost- Dimension- Location- Color

Class: Furniture--------------------Attributes:- Cost- Dimension- Location- Color

Inheritance

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Example OO Concepts - Operations & more Inheritance

Operations modify 1 or more attributes– E.g., location might be a function of building,

floor & room. Then move might modify 1 or more of building, floor & room to effect location.

Object: Chair--------------------Attributes:- Cost- Dimension- Location- ColorOperations- Buy- Sell- Weigh- Move

Class: Furniture--------------------Attributes:- Cost- Dimension- Location- Color

Operations- Buy- Sell- Weigh- Move

Inheritance

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Principles of Object Orientation

Major Elements– Classes & Objects– Abstraction– Encapsulation– Hierarchy/Inheritance– Polymorphism

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What is a class?

A class represents a template for several objects and describes how these objects are structured internally. Objects of the same class have the same definition both for their operations and for their information structures.

A.K.A an object type

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What is the instance of a class?

An instance is an object created from a class. The class describes the (behavior and information) structure of the instance, while the current state of the instance is defined by the operations performed on the instance.

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Instance of Tom from Persons

Class of persons

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Classes & Objects

An OO concept that encapsulates the data and procedures needed to describe the content and behavior of some real work entity

Attributes that describe the class are walled off by methods that manipulate the data some way

– the class encapsulates the data – the processing use methods to manipulate

data– achieves information hiding & minimizes ripple

effect.

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More On Classes & Objects

Classes also are generalized description that describe a category of objects (AKA template)

– All objects within a class inherent its attributes, and operations

A superclass is a collection of classes and subclass is a specialize instance of the class

Furniture

TableChair

Desk

Chair Instances

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Messages & Classes

OO = objects + classification + inheritance = communication

Messages are means by which objects interact– Provide stimulators to some behavior between

objects– message[dest, operation, parms]

Object: XXX--------------------Attributes:- X1- Y1Operations- Z1- A1

Object: AAA--------------------Attributes:- A2- B2Operations- X2- Y2

Msg(dest, oper, parm)

Msg(dest, oper, parm)

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Encapsulation

Grouping related information together and protecting it from the outside world. (e.g., its structure and behavior)

– E.g., grouping data (variables) and actions (methods) to form objects.

Advantages: – Internal details of data & procedures hidden

(reduces side effects of errors)– Classes contain templates of data structures &

operations factilitating reuse– Interfaces are encapsulated via messages with

details. (tends to reduce coupling).

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Hierarchy & Inheritance

Inheritance - A key difference between OO systems and conventional programming

– Subclass Y inherits all attributes & operations. – All algorithms & data structures immediately

available for reuse.– All changes and additions of the superclass are

immediately available to the subclasses. (change propagation)

Class: Kitchen_furniture--------------------Attributes:- Cost- Dimension- Location- ColorOperations- Buy- Sell- Weigh- Move

Class: Furniture--------------------Attributes:- Cost- Dimension- Location- ColorOperations- Buy- Sell- Weigh- Move

Inheritance

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More On Hierarchy & Inheritance

New attributes and operations can be added at each level of the hierarchyWhen adding a class have several options

– Class can be designed from scratch– Search class hierarchy to find one similar to

inherit from– Can restructure the class hierarchy to enable

inheritance by the new class– Can override characteristics of a superclass and

rewrite them for current class use.

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Hierarchy & Multiple InheritanceClass: X1--------------------Attributes:- Char1- Char2- Char3

+char4 +char5

Class: X2--------------------Attributes:- Char1- Char2- Char3- Char4- Char5

Class: X3--------------------Attributes:- Char1- Char2- Char3- Char4- Char5- Char6


+char6 +Char7

Suppose wanted X1 to inherit Chart1, 2, 3, 4, 8

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Hierarchy & InheritanceClass: X1--------------------Attributes:- Char1- Char2- Char3

+char4 +char5


Class: X3--------------------Attributes:- Char1- Char2- Char3- Char4- Char5- Char6


+char6 +Char7

Suppose wanted X1 to inherit Chart1, 2, 3, 4, 8

Class: X1--------------------Attributes:- Char1- Char2- Char3

+char4

Class: X2--------------------Attributes:- Char1- Char2- Char3- Char4

Class: X2a--------------------Attributes:- Char1- Char2- Char3- Char4- Char5

+char5

Class: X2b--------------------Attributes:- Char1- Char2- Char3- Char4- Char8

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Hierarchy & InheritanceSuppose added classes benglishcar & englishmotorcycle

Class:Vehicle

Class: EnginePoweredVehicle

Class: PersonPoweredVehicle

Class: 2wheeledVehicle

Class: 4wheeledVehicle

Class: car

Class: Motorcycle

Both need operations leakOil() & eltronicFailure()

EnglishCarEnglishMotorcyle

Different parts of hierarchy? Restructure hierarchy? Copy code method in each?

enginlishthing

Multi-inheritence generally not allowed

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Single Inheritance w/ Java

Java handles single inheritance problem by:– creating a separate hierarchy with mixed

behavior classes called an interface hierarchy.– When you create a new class you can pick from

the superclass also from the other hierarchy.– The interface hierarchy is a collection of

methods (or behaviors), definitions and constants. (E.g., mathematical functions)

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Polymorphism

A facility for the same method name to be used with different objects.

Suppose had 4 graphical shapes and wanted to enable them to grow in size on the screen.

Set all shapes to be subclasses of a general class called shape.

Each object provides a grow method Therefore can call shape.grow(size); The run-time system invokes the version of grow

from the object being used at the time.

Circle circle;

Oval oval;

Rectangle rectangle;

Triangle t;

Circle.grow(change);

oval.grow(change);

rectangle(change);

t.change(change);

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More On Objects

Objects tend to be– external entities - other systems, devices,

people, – things - letters, reports, displays, – Events - sensor alert, robot movement– Roles - manager, engineer, salesperson– Organization unit - division, group, team– Places - loading dock, manufacturing floor– Structure - sensors, 4-wheeled vehicles that

define categories of objects

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More On Objects - II

From the homesafe example– extract nouns from system description and create a

list of potential objects

Potential object/class General Class homeowner role or external entitysensor external entitycontrol panel external entityinstallation occurrencesystem (AKA security sys) thingnumber, type attributes of sensormaster password thingtelephone number thingsensor event occurrenceaudible alarm external entity monitoring service external entity

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More On Objects - III

Six characteristics that should be used on each potential object:

– Retained Information - information about it must be remembered

– Needed services - have a set of identifiable operations that can change attribute’s value

– Multiple Attributes - Are the attributes “major” and useful?

– Common Attributes - can define a set that apply to all occurrences of object

– Essential requirements - External entity in problem and produces information essential to solution

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More On Objects - IV

Apply criteria to potential objects

Potential object/class General Class homeowner Fail 1,2, fail 6 OKsensor OK: All OKcontrol panel OK: all OKinstallation Rejected system (AKA security sys) OK: all OKnumber, type Fail: 3 failsmaster password Fail: 3 failstelephone number Fail: 3 failssensor event OK: All OKaudible alarm Ok 2-6 OK monitoring service Fail: 1-2 fail 6 OK

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More On Attributes

They describe the object and define & clarify it.– E.g., player - couple have

name, position, batting aver, games played name, average salary, peak salary, years left on

contract Study narrative and identify & select things

reasonably belong to object– Data items needed to define object in problem– Security system has set of attributes allow to

configure sensor_info = sensor_type + sensor_numb + alarm_threshold Alarm_response_info = delay_time + phone_numb _ alarm_type activation/deact_info = master_passwd + number_of_tries +

temp_passwd

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More On Operations

Define define the behavior of an object & change the attributes some way

– manipulate data in some way (add, delete, change)

– performance computation– monitor occurrence of an event

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More On Operations - II

Study narrative and identify reasonably belong to object

– study the “verbs” of narrative “Sensor is assigned a number & type” “Master passwd is programmed for arming &

disarming ...”– Tells us,

– assign operation for sensor object– program operation for system object– arm/disarm -> system (maybe system_status =

armed | disarmed)

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More On Operations - III

Would also consider the communication between objects:

What know activities occur during the life of the object?

– Communication between objects -> clues sensor event - message -> sensor to

display location and number cntl panel -> send -> system a reset

message to update system status audible alarm sent query message

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More On Operations

Object: System--------------------Attributes: -SystemID - phone number - system status - sensor table sensor type sensor number alarm threshold - alarm display time - telephone number - alarm threshold - master passwor - temporary password - number of tries----------------------Operations: program - display - Reset - query - modify - call

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Object-Oriented Programming

Object-oriented programming is a method of implementation in which

- programs are organized as cooperative collections of objects,

- each of which represents an instance of some class,

- and whose classes are all members of a hierarchy of classes united via inheritance relationships.

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Object-Oriented Analysis

Object-oriented analysis is a method of analysis that

– examines requirements form the perspective of the classes and objects found in the vocabulary of the problem domain.

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Object-Oriented Design

Object-oriented design is a method of design – encompassing the process of object-oriented

decomposition and – a notation for depicting both logical and

physical as well as – static and dynamic models of the system under

design.

Documents

Illinois Institute of Technology