Lecture 13: Object- Oriented Concepts Anita S. Malik [email protected] Adapted from Schach...

CS540 Software Design 2Lecture 13

Object-Oriented Object-Oriented ProgrammingProgramming

Objects can be used effectively to Objects can be used effectively to represent real-world entitiesrepresent real-world entities

For instance, an object might For instance, an object might represent a particular employee in a represent a particular employee in a companycompany

Each employee object handles the Each employee object handles the processing and data management processing and data management related to that employeerelated to that employee


ObjectsObjects An object has:An object has:

statestate - descriptive characteristics - descriptive characteristics

behaviorsbehaviors - what it can do (or what can be done to - what it can do (or what can be done to it)it)

The state of a bank account includes its account The state of a bank account includes its account number and its current balancenumber and its current balance

The behaviors associated with a bank account include The behaviors associated with a bank account include the ability to make deposits and withdrawalsthe ability to make deposits and withdrawals

Note that the behavior of an object might change its Note that the behavior of an object might change its statestate


ClassesClasses An object is defined by a An object is defined by a classclass

A class is the blueprint of an objectA class is the blueprint of an object

Multiple objects can be created from the same Multiple objects can be created from the same classclass


Objects and ClassesObjects and Classes

Bank Account

A class(the concept)

John’s Bank AccountBalance: $5,257

An object(the realization)

Bill’s Bank AccountBalance: $1,245,069

Mary’s Bank AccountBalance: $16,833

Multiple objectsfrom the same class


AttributesAttributesContain current state of an object.Contain current state of an object.

Attributes can be classified as simple or Attributes can be classified as simple or complex. complex.

Simple attribute can be a primitive type such Simple attribute can be a primitive type such as integer, string, etc., which takes on literal as integer, string, etc., which takes on literal values.values.

Complex attribute can contain collections Complex attribute can contain collections and/or references. and/or references.

Reference attribute represents relationship. Reference attribute represents relationship. complex object:complex object: contains one or more contains one or more

complex attributescomplex attributes


Methods and messagesMethods and messages

MethodMethod: : Defines behavior of an object, as a set of encapsulated Defines behavior of an object, as a set of encapsulated functions.functions.

Message: Message: Request from one object to another asking second object to Request from one object to another asking second object to execute one of its methods.execute one of its methods.

(a)

(b)

(a) Object showing attributes and methods(b) Example of a method


ClassesClasses

ClassClass: Blueprint for defining a set of similar objects.: Blueprint for defining a set of similar objects.

Objects in a class are called Objects in a class are called instancesinstances..


InheritanceInheritance One class can be used to derive another via One class can be used to derive another via

inheritanceinheritance

Classes can be organized into hierarchiesClasses can be organized into hierarchies

Bank Account

Account

Charge Account

Savings Account

Checking Account


Inheritance (contd)Inheritance (contd)Inheritance allows one class of objects to be defined as a Inheritance allows one class of objects to be defined as a

special case of a more general class.special case of a more general class.

Special cases are Special cases are subclassessubclasses and more general cases and more general cases are are superclassessuperclasses..

Generalization: process of forming a superclass Specialization: forming a subclass •Subclass inherits all properties of its superclass and can define its own unique properties. •Subclass can redefine inherited methods. •All instances of subclass are instances of superclass. •Principle of substitutability: instance of subclass can be used whenever method/construct expects instance of superclass.•A KIND OF (AKO): Name for relationship between subclass and superclass


Types of inheritanceTypes of inheritance

(a)

(b)

(c)

(a) Single(b) Multiple(c) Repeated

(b)


Inheritance (contd)Inheritance (contd) Define Define humanBeinghumanBeing to be a to be a classclass

A A humanBeinghumanBeing has has attributesattributes, such as age, , such as age, height, genderheight, gender

Assign values to attributes when describing Assign values to attributes when describing objectobject

Define Parent to be a Define Parent to be a subclasssubclass of HumanBeing of HumanBeing A Parent has all attributes of a HumanBeing, A Parent has all attributes of a HumanBeing,

plus attributes of his/her own (name of oldest plus attributes of his/her own (name of oldest child, number of children)child, number of children)

AA ParentParent inherits all attributes of inherits all attributes of humanBeinghumanBeing The property of inheritance is an essential feature The property of inheritance is an essential feature

of object-oriented languages such as Smalltalk, of object-oriented languages such as Smalltalk, C++, Ada 95, Java (but not C, FORTRAN)C++, Ada 95, Java (but not C, FORTRAN)


Inheritance (contd)Inheritance (contd)

UML notationUML notation Inheritance is represented by a large open triangleInheritance is represented by a large open triangle


Java implementationJava implementation


Overriding and Overriding and OverloadingOverloading

Overriding: Overriding: Process of redefining a property within a Process of redefining a property within a subclass.subclass.

Overloading: Overloading: Allows name of a method to be reused with a Allows name of a method to be reused with a class or across classes.class or across classes.

Overriding Example:Might define method in Staff class to increment salary based on commission

method void giveCommission(float branchProfit) {

salary = salary + 0.02 * branchProfit; }

May wish to perform different calculation for commission in Manager subclass:

method void giveCommission(float branchProfit) {

salary = salary + 0.05 * branchProfit; }


Aggregation Aggregation

UML Notation UML Notation


AssociationAssociation

UML NotationUML Notation


Equivalence of Data and Equivalence of Data and ActionAction

Classical paradigmClassical paradigm record_1.field_2record_1.field_2

Object-oriented paradigmObject-oriented paradigm thisObject.attributeBthisObject.attributeB thisObject.methodC()thisObject.methodC()


Example – Chapter 7 Example – Chapter 7 Schach (2002)Schach (2002)

Design of an operating system for a large Design of an operating system for a large mainframe computer. It has been decided that mainframe computer. It has been decided that batch jobs submitted to the computer will be batch jobs submitted to the computer will be classified as high priority, medium priority, or classified as high priority, medium priority, or low priority. There must be three queues for low priority. There must be three queues for incoming batch jobs, one for each job type. incoming batch jobs, one for each job type. When a job is submitted by a user, the job is When a job is submitted by a user, the job is added to the appropriate queue, and when the added to the appropriate queue, and when the operating system decides that a job is ready to be operating system decides that a job is ready to be run, it is removed from its queue and memory is run, it is removed from its queue and memory is allocated to it allocated to it

Design 1 (Next slide)Design 1 (Next slide) Low cohesion—operations on job queues are spread all Low cohesion—operations on job queues are spread all

over productover product


Data Encapsulation — Data Encapsulation — Design 1Design 1


Data Encapsulation — Data Encapsulation — Design 2Design 2


Data EncapsulationData Encapsulation

m_encapsulationm_encapsulation has informational cohesion has informational cohesion m_encapsulation m_encapsulation is an implementation of is an implementation of

data encapsulationdata encapsulation Data structure (Data structure (job_queuejob_queue) together with ) together with

operations performed on that data operations performed on that data structure structure

Advantages Advantages DevelopmentDevelopment MaintenanceMaintenance


Data Encapsulation and Data Encapsulation and DevelopmentDevelopment

Data encapsulation is an example of Data encapsulation is an example of abstractionabstraction

Job queue exampleJob queue example Data structureData structure

job_queuejob_queue Three new functionsThree new functions

iinitialize_job_queuenitialize_job_queue add_job_to_queueadd_job_to_queue delete_job_from_queuedelete_job_from_queue

AbstractionAbstraction Conceptualize problem at higher levelConceptualize problem at higher level

job queues and operations on job queuesjob queues and operations on job queues not lower level not lower level

records or arraysrecords or arrays


Stepwise RefinementStepwise RefinementStep 1: Design in terms of high level concepts Step 1: Design in terms of high level concepts

It is irrelevant how job queues are implementedIt is irrelevant how job queues are implemented

Step 2: Design low level componentsStep 2: Design low level components

Totally ignore what use will be made of themTotally ignore what use will be made of them In 1st step, assume existence of lower levelIn 1st step, assume existence of lower level

Concern is the behavior of the data structureConcern is the behavior of the data structure job_queuejob_queue

In 2nd step, ignore existence of high levelIn 2nd step, ignore existence of high level Concern is the implementation of that behaviorConcern is the implementation of that behavior

In a larger product, there will be many levels of In a larger product, there will be many levels of abstractionabstraction


Data Encapsulation and Data Encapsulation and MaintenanceMaintenance

Identify aspects of product likely to Identify aspects of product likely to changechange

Design product so as to minimize the Design product so as to minimize the effects of changeeffects of change Data structures are unlikely to changeData structures are unlikely to change Implementation may changeImplementation may change

Data encapsulation provides a way to Data encapsulation provides a way to cope with changecope with change


Implementation of Implementation of ClassClass JobQueue JobQueue

C++C++

JavaJava


Implementation of Implementation of queueHandlerqueueHandler

C++ JavaC++ Java


Data Encapsulation and Data Encapsulation and Maintenance (contd)Maintenance (contd)

What happens if queue is now implemented as a What happens if queue is now implemented as a two-way linked list of two-way linked list of JobRecordJobRecord?? Module that uses Module that uses JobRecordJobRecord need not be need not be

changed at all, merely recompiledchanged at all, merely recompiled

C+C+++

JavaJava


Abstract Data TypesAbstract Data Types

Problem with both implementationsProblem with both implementations Only one queueOnly one queue

NeedNeed We need: We need:

Data Data typetype + operations performed on + operations performed on instantiations of that data typeinstantiations of that data type

Abstract data typeAbstract data type


Abstract Data Type (contd)Abstract Data Type (contd)

(Problems caused by (Problems caused by publicpublic attributes solved later) attributes solved later)


Information HidingInformation Hiding Data abstractionData abstraction

Designer thinks at level of an ADTDesigner thinks at level of an ADT Procedural abstractionProcedural abstraction

Define a procedure—extend the language Define a procedure—extend the language Instances of a more general design concept, Instances of a more general design concept,

information hidinginformation hiding Design the modules in way that items likely to Design the modules in way that items likely to

change are hiddenchange are hidden Future change is localizedFuture change is localized Changes cannot affect other modulesChanges cannot affect other modules


Information Hiding (contd)Information Hiding (contd)

C++ C++ abstract data abstract data type type implementatiimplementation with on with information information hidinghiding


Information Hiding (contd)Information Hiding (contd)

Effect of information hiding via Effect of information hiding via privateprivate attributes attributes


Polymorphism and Polymorphism and Dynamic BindingDynamic Binding

Classical paradigmClassical paradigm Must explicitly invoke correct Must explicitly invoke correct

versionversion


Polymorphism and Dynamic Polymorphism and Dynamic Binding (contd)Binding (contd)

Object-oriented paradigmObject-oriented paradigm



All that is needed is All that is needed is myFile.open()myFile.open() Correct method invoked at run-time Correct method invoked at run-time

(dynamically)(dynamically) Method Method openopen can be applied to objects can be applied to objects

of different classesof different classes PolymorphicPolymorphic


Polymorphism and dynamic Polymorphism and dynamic binding (contd)binding (contd)

MethodMethod checkOrder (b : Base)checkOrder (b : Base) can be applied to objects of any subclass ofcan be applied to objects of any subclass of BaseBase



Can have a negative impact on Can have a negative impact on maintenancemaintenance Code is hard to understand if there are Code is hard to understand if there are

multiple possibilities for a specific multiple possibilities for a specific methodmethod

Polymorphism and dynamic bindingPolymorphism and dynamic binding Strength and weakness of the object-Strength and weakness of the object-

oriented paradigmoriented paradigm


Polymorphism and dynamic Polymorphism and dynamic binding (contd)binding (contd)

Polymorphism: Polymorphism: Means ‘Means ‘many formsmany forms’. ’.

Dynamic Binding:Dynamic Binding: Runtime process of selecting appropriate Runtime process of selecting appropriate method based on an object’s type.method based on an object’s type.

Example: With list consisting of an arbitrary no. of objects from the Staff hierarchy, we can write: list[i]. print

and runtime system will determine which print() method to invoke depending on the object’s (sub)type.


Cohesion and Coupling of Cohesion and Coupling of ObjectsObjects

No such thing!No such thing! Object-oriented cohesion and coupling always Object-oriented cohesion and coupling always

reduces to classical cohesionreduces to classical cohesion The only feature unique to the object-oriented The only feature unique to the object-oriented

paradigm is inheritanceparadigm is inheritance Cohesion has nothing to do with inheritanceCohesion has nothing to do with inheritance Two objects with the same functionality have the Two objects with the same functionality have the

same cohesionsame cohesion It does not matter if this functionality is inherited It does not matter if this functionality is inherited

or notor not Similarly, so-called object-oriented coupling Similarly, so-called object-oriented coupling

always reduces to classical coupling always reduces to classical coupling


Object-Oriented Metrics Object-Oriented Metrics (contd)(contd)

Two types of so-called object-oriented Two types of so-called object-oriented metricmetric Not related to inheritanceNot related to inheritance

Reduces to a classical metricReduces to a classical metric Inheritance-relatedInheritance-related

May reduce to a classical metricMay reduce to a classical metric No problemNo problem

Classical metrics work just fineClassical metrics work just fine But don’t mislead others by calling them But don’t mislead others by calling them

object-orientedobject-oriented


Advantages of ObjectsAdvantages of Objects Same as as advantages of abstract data Same as as advantages of abstract data

typestypes Information hidingInformation hiding Data abstractionData abstraction Procedural abstractionProcedural abstraction

Inheritance provides further data Inheritance provides further data abstractionabstraction Easier and less error-prone product Easier and less error-prone product

developmentdevelopment Easier maintenanceEasier maintenance

Objects are more reusable than modules Objects are more reusable than modules with functional cohesionwith functional cohesion


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Lecture 13: Object- Oriented Concepts Anita S. Malik [email protected] Adapted from Schach...

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