Object OrientationObject Orientation

Yaodong Bi, Ph.D.Yaodong Bi, Ph.D.Department of Computer SciencesDepartment of Computer SciencesUniversity of ScrantonUniversity of Scranton

April 19, 2023April 19, 2023

Classes and objects for Classes and objects for modelingmodeling

ClassesClasses– Real world conceptsReal world concepts– Student, course, bicycle, etcStudent, course, bicycle, etc

ObjectsObjects– Instances of real word conceptsInstances of real word concepts– John Smith, Operating Systems, John Smith, Operating Systems,

Mongoose Mountain S100, etcMongoose Mountain S100, etc

04/19/2304/19/23 22

Classes and objects for Classes and objects for modelingmodeling

“Customers John and

Susan entered the MakeMoney

bank and were served by

teller Andy.”

Classes

Objects:

04/19/2304/19/23 33

Classes and objects in Classes and objects in softwaresoftware

ObjectsObjects– Packaging both data and the procedures that Packaging both data and the procedures that

operate on the data into oneoperate on the data into one– Operations are the only way to change the data Operations are the only way to change the data

items – items – encapsulationencapsulation ClassesClasses

– Template of objects of the same typeTemplate of objects of the same type– Write once and use many timesWrite once and use many times

Implementation classes/objectsImplementation classes/objects– Some classes/objects do not match to concepts in Some classes/objects do not match to concepts in

the real worldthe real world List, stack, tree, queue – containersList, stack, tree, queue – containers Exception classesException classes

04/19/2304/19/23 44

Attributes and operationsAttributes and operations

Instance attributes (simply attributes)Instance attributes (simply attributes)– Properties (internal state) of an objectProperties (internal state) of an object

Student: name, major, class, GPAStudent: name, major, class, GPA Course: title, # of credits, descriptionCourse: title, # of credits, description

– Each object has its own value for an Each object has its own value for an attributeattribute

Instance operations (simply operations)Instance operations (simply operations)– Services of an objectServices of an object

Stack: push, pop, isEmpty, isFullStack: push, pop, isEmpty, isFull– EncapsulationEncapsulation

Attributes (private and protected) are not Attributes (private and protected) are not accessible directly by clientaccessible directly by client

Operations are used to read/write the values of Operations are used to read/write the values of attributesattributes04/19/2304/19/23 55

Attributes and OperationsAttributes and Operations

Class attributes (static attributes)Class attributes (static attributes)– Represent properties that can be applied Represent properties that can be applied

to all objects of the same classto all objects of the same class– Only one value for each class attribute Only one value for each class attribute

shared by all objects of the classshared by all objects of the class All accounts of the All accounts of the CheckingAccountCheckingAccount class have class have

the same interest ratethe same interest rate– Can be accessed in both instance and Can be accessed in both instance and

class operationsclass operations Class operations (static operations)Class operations (static operations)

– Used to access class attributesUsed to access class attributes– Cannot access instance attributes and Cannot access instance attributes and

operationsoperations04/19/2304/19/23 66

Class member access modesClass member access modes

PublicPublic– Public attributes and public operationsPublic attributes and public operations– Accessible to all clients or the worldAccessible to all clients or the world

ProtectedProtected– Protected attributes and operationsProtected attributes and operations– Accessible to the class and its subclasses Accessible to the class and its subclasses

PrivatePrivate– Private attributes and operationsPrivate attributes and operations– Only accessible by the operations of the Only accessible by the operations of the

classclass04/19/2304/19/23 77

Member access modes in Member access modes in JavaJava

SpecifierSpecifier ClassClass PackagePackage SubclassSubclass WorldWorld

PrivatePrivate YY NN NN NN

No No SpecifierSpecifier

YY YY NN NN

ProtectedProtected YY YY YY NN

PublicPublic YY YY YY YY

04/19/2304/19/23 88

Abstract and concrete Abstract and concrete classesclasses

Abstract classesAbstract classes– Define a common interface for its Define a common interface for its

subclassessubclasses– Defer some or all of its implementation of Defer some or all of its implementation of

operations to its subclassesoperations to its subclasses– Cannot be instantiatedCannot be instantiated

Concrete classesConcrete classes– That are not abstract classesThat are not abstract classes– Can be instantiatedCan be instantiated– Concrete subclasses implement all the Concrete subclasses implement all the

operationsoperations04/19/2304/19/23 99

InterfacesInterfaces

Signature of an operation:Signature of an operation:– Operation nameOperation name– Objects/classes it takes as parametersObjects/classes it takes as parameters– Return value and typeReturn value and type

Interface:Interface:– The set of signatures of related operationsThe set of signatures of related operations– Representing a capability or a set of Representing a capability or a set of

servicesservices– A class may implement multiple interfacesA class may implement multiple interfaces

Its objects have more than one set of servicesIts objects have more than one set of services04/19/2304/19/23 1010

InheritanceInheritance

Specifies Specifies is-ais-a or or a-kind-ofa-kind-of relationshiprelationship

Generalization and specializationGeneralization and specialization Superclasses and subclassesSuperclasses and subclasses Single and multiple inheritanceSingle and multiple inheritance Class and interface inheritanceClass and interface inheritance

04/19/2304/19/23 1111

Superclasses and Superclasses and subclassessubclasses

SuperclassSuperclass– Also called base class or parent classAlso called base class or parent class– All of its members are inherited by All of its members are inherited by

its subclasses its subclasses SubclassesSubclasses

– Also called child classesAlso called child classes– Inherit all the properties of its Inherit all the properties of its

superclassessuperclasses04/19/2304/19/23 1212

Single and multiple Single and multiple inheritanceinheritance

Single inheritanceSingle inheritance– A subclass cannot have more than one A subclass cannot have more than one

parent parent Multiple inheritanceMultiple inheritance

– A subclass may have more than one A subclass may have more than one parentparent

– Java does not allow multiple inheritanceJava does not allow multiple inheritance– Java allows multiple interface Java allows multiple interface

inheritanceinheritance Through interface implementationThrough interface implementation

04/19/2304/19/23 1313

Class inheritance latticeClass inheritance lattice

PersonSSN

NamegetSSN()

getName()

Facultyoffice

getOffice()Staffdept

getDept()

Studentmajor

getMajor()

FullTime

salarygetSalary()

PartTime

PayRategetPayRate(

)

Graduate

PayRategetPayRate(

)

UndergPayRate

getPayRate()

TAlabs

getLabs()04/19/2304/19/23 1414

Implementation inheritanceImplementation inheritance

Also called class inheritanceAlso called class inheritance Define an object’s Define an object’s

implementation in terms of implementation in terms of another object’s implementationanother object’s implementation

Pure class inheritance in C++Pure class inheritance in C++ Pure class inheritance in JavaPure class inheritance in Java

04/19/2304/19/23 1515

Pure class inheritance with Pure class inheritance with C++C++

Class BinaryTree {Class BinaryTree {

getRoot() {;}getRoot() {;}

setRoot() {;}setRoot() {;}

leftTree() {;}leftTree() {;}

rightTree() {;}rightTree() {;}

}}

Class BinSearchTree: Class BinSearchTree: privateprivate BinaryTree BinaryTree

{{

insert() {;}insert() {;}

remove() {;}remove() {;}

find() {;}find() {;}

}}

They use the operations of BinaryTree

The operations of BinaryTree are not

accessible/visible to the clients of BinSearchTree because BinaryTree is

private04/19/2304/19/23 1616

Pure class inheritance with Pure class inheritance with JavaJava

Java does not allow private parentJava does not allow private parent The parent and grandparents are The parent and grandparents are

public and accessible through the public and accessible through the subclasssubclass

Java cannot implement pure Java cannot implement pure implementation inheritance.implementation inheritance.

04/19/2304/19/23 1717

Interface inheritanceInterface inheritance

Describe when an object can be Describe when an object can be used in place of anotherused in place of another

Pure interface inheritance in C++Pure interface inheritance in C++– Superclasses are pure abstract Superclasses are pure abstract

classesclasses Pure interface inheritance in JavaPure interface inheritance in Java

– Superclasses are Java interfacesSuperclasses are Java interfaces

04/19/2304/19/23 1818

Interface inheritance with C+Interface inheritance with C+++

Class Stack {Class Stack {virtual push() = 0;virtual push() = 0;virtual pop() = 0;virtual pop() = 0;virtual isEmpty() = 0;virtual isEmpty() = 0;virtual isFull() = 0;virtual isFull() = 0;

}}

Class MyStack: Class MyStack: publicpublic Stack Stack{{

push() {;}push() {;}pop() {;}pop() {;}isEmpty() {;}isEmpty() {;}isFull() {;}isFull() {;}

}}

Class YourStack: Class YourStack: publicpublic Stack Stack{{

push() {;}push() {;}pop() {;}pop() {;}isEmpty() {;}isEmpty() {;}isFull() {;}isFull() {;}

}}

A C++ Abstract class

04/19/2304/19/23 1919

Interface inheritance with Interface inheritance with JavaJava

Interface Stack {Interface Stack {push();push();pop();pop();isEmpty();isEmpty();isFull() 0;isFull() 0;

}}

Class MyStack Class MyStack implements Stack {implements Stack {push() {;}push() {;}pop() {;}pop() {;}isEmpty() {;}isEmpty() {;}isFull() {;}isFull() {;}

}}

Class YourStackClass YourStack

implements Stack {implements Stack {push() {;}push() {;}pop() {;}pop() {;}isEmpty() {;}isEmpty() {;}isFull() {;}isFull() {;}

}}

A Java Interface

04/19/2304/19/23 2020

PolymorphismPolymorphism

When a client sends a request to a When a client sends a request to a reference, the method executed depends reference, the method executed depends on the object behind the referenceon the object behind the reference

Polymorphism and inheritance are very Polymorphism and inheritance are very powerful tool in software design powerful tool in software design

So powerful that if they are used So powerful that if they are used properly, they can hurt really badproperly, they can hurt really bad– If something goes wrong, it is hard to tell If something goes wrong, it is hard to tell

which class/object caused the problemwhich class/object caused the problem

04/19/2304/19/23 2121

Dynamic (late) bindingDynamic (late) binding

Dynamic binding is a way of implementing Dynamic binding is a way of implementing polymorphismpolymorphism

It means that a function call is not linked to It means that a function call is not linked to the actual function until execution timethe actual function until execution time

Since a reference may be used to point to Since a reference may be used to point to different types (subtypes or subclasses) of different types (subtypes or subclasses) of objects at execution time, so the actual objects at execution time, so the actual function to be executed may not be known function to be executed may not be known when the program is compiled. when the program is compiled.

The opposite of dynamic binding is static The opposite of dynamic binding is static binding which links a function call to the binding which links a function call to the actual function at compilation time.actual function at compilation time.

04/19/2304/19/23 2222

Method OverridingMethod Overriding

A subclass overrides the operation(s) A subclass overrides the operation(s) defined in the parent/grandparent classes. defined in the parent/grandparent classes.

This is for specialized subclasses to This is for specialized subclasses to override the behavior defined in the super override the behavior defined in the super class(es).class(es).

For example:For example:– Superclass Employee’s Superclass Employee’s printprint method prints method prints

employee’s SSN, Name, Address.employee’s SSN, Name, Address.– Its subclass Engineer’s Its subclass Engineer’s printprint method prints method prints

engineer’s type and level in addition to engineer’s type and level in addition to employee’s normal information.employee’s normal information.

04/19/2304/19/23 2323

Function overloadingFunction overloading

The same name is used for more than The same name is used for more than functionfunction

For example, a function of adding two For example, a function of adding two integers may be named the same as the integers may be named the same as the one of adding two floats:one of adding two floats:– Add(int x, int y);Add(int x, int y);– Add(float x, float y);Add(float x, float y);– Two functions use the same name for the Two functions use the same name for the

same purpose with different data same purpose with different data parametersparameters

04/19/2304/19/23 2424

Inheritance vs. CompositionInheritance vs. Composition

Two common techniques for reuseTwo common techniques for reuse Class Inheritance Class Inheritance

– White-box reuseWhite-box reuse– Defined at compile-time, staticallyDefined at compile-time, statically– Cannot change parent at rum-timeCannot change parent at rum-time

Object CompositionObject Composition– Black-box reuseBlack-box reuse– A reference/pointer to the objectA reference/pointer to the object– Can be changed at run-timeCan be changed at run-time

Favor composition over inheritanceFavor composition over inheritance04/19/2304/19/23 2525

DelegationDelegation

A way to make composition as A way to make composition as powerful as class inheritancepowerful as class inheritance

A A receiving objectreceiving object delegates delegates requests to its requests to its delegatesdelegates== subclass defers request to parent class== subclass defers request to parent class

In class inheritance In class inheritance – parent can use parent can use thisthis to access the child to access the child

In delegationIn delegation– Receiving object can pass itself to its Receiving object can pass itself to its

delegatedelegate04/19/2304/19/23 2626

Three techniques for reuseThree techniques for reuse

Class InheritanceClass Inheritance– can provide default operations and let subclass can provide default operations and let subclass

override themoverride them– Cannot be changed at run-timeCannot be changed at run-time

Object CompositionObject Composition– Can change the behavior being composed at run-Can change the behavior being composed at run-

timetime– Requires redirection and so becomes less efficientRequires redirection and so becomes less efficient

Generics Generics – Changes the types that a class can useChanges the types that a class can use– Cannot change at run-timeCannot change at run-time

04/19/2304/19/23 2727

ToolkitsToolkits

A set of predefined classes A set of predefined classes Provides useful, general-purpose Provides useful, general-purpose

functionalityfunctionality– Collection classes (list, stack, etc)Collection classes (list, stack, etc)– GWT (Google Web Toolkit)GWT (Google Web Toolkit)

Does NOT impose any design level Does NOT impose any design level decisiondecision

Code ReuseCode Reuse04/19/2304/19/23 2828

FrameworksFrameworks

A reusable design for a type of A reusable design for a type of applicationsapplications

Dictates the architecture of your appDictates the architecture of your app– Overall structureOverall structure– Key components and their responsibilitiesKey components and their responsibilities– How classes/objects interact with each otherHow classes/objects interact with each other

Design reuseDesign reuse Inversion of Control Inversion of Control

– Dependency InjectionDependency Injection

04/19/2304/19/23 2929

FrameworksFrameworks

Framework examples:Framework examples:– Web AppsWeb Apps

J2EE, Spring Framework, .NetJ2EE, Spring Framework, .Net

– ORM (object to relation mapping)ORM (object to relation mapping) Hibernate, OpenJPA, MyBatisHibernate, OpenJPA, MyBatis

– Mobile AppsMobile Apps AndroidAndroid iOSiOS

04/19/2304/19/23 3030