Object-oriented concepts

Object-Oriented Object-Oriented Programming Programming

ConceptsConcepts

ContentsContents

1.1. What is OOP?What is OOP?

2.2. Classes and ObjectsClasses and Objects

3.3. Principles of OOPPrinciples of OOP

• InheritanceInheritance

• AbstractionAbstraction

• EncapsulationEncapsulation

• PolymorphismPolymorphism

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What is OOP?What is OOP?

What is OOP?What is OOP?

• Object-oriented programming (OOP) is an Object-oriented programming (OOP) is an engineering approach for building software engineering approach for building software systemssystems

• Based on the concepts of classes and Based on the concepts of classes and objects that are used for modeling the real objects that are used for modeling the real world entitiesworld entities

• Object-oriented programsObject-oriented programs

• Consist of a group of cooperating objectsConsist of a group of cooperating objects

• Objects exchange messages, for the Objects exchange messages, for the purpose of achieving a common objectivepurpose of achieving a common objective

• Implemented in object-oriented languagesImplemented in object-oriented languages4

OOP in a NutshellOOP in a Nutshell

• A program models a world of interacting A program models a world of interacting objectsobjects

• Objects create other objects and “send Objects create other objects and “send messages” to each other (in Java, call each messages” to each other (in Java, call each other’s methods)other’s methods)

• Each object belongs to a classEach object belongs to a class

• A class defines properties of its objectsA class defines properties of its objects

• The data type of an object is its classThe data type of an object is its class

• Programmers write classes (and reuse existing Programmers write classes (and reuse existing classes)classes)

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What are OOP’s Claims To What are OOP’s Claims To Fame?Fame?

• Better suited for team developmentBetter suited for team development

• Facilitates utilizing and creating reusable Facilitates utilizing and creating reusable software componentssoftware components

• Easier GUI programmingEasier GUI programming

• Easier software maintenanceEasier software maintenance

• All modern languages are object-oriented: All modern languages are object-oriented: Java, C#, PHP, Perl, C++, ...Java, C#, PHP, Perl, C++, ...

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Classes and ObjectsClasses and Objects

What Are Objects?What Are Objects?

• Software objects model real-world objects Software objects model real-world objects or abstract conceptsor abstract concepts

• E.g. dog, bicycle, queue E.g. dog, bicycle, queue

• Real-world objects have states and Real-world objects have states and behaviorsbehaviors

• Dogs' states: name, color, breed, hungryDogs' states: name, color, breed, hungry

• Dogs' behaviors: barking, fetching, sleepingDogs' behaviors: barking, fetching, sleeping

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What Are Objects?What Are Objects?

• How do software objects implement real-How do software objects implement real-world objects?world objects?

• Use variables/data to implement statesUse variables/data to implement states

• Use methods/functions to implement Use methods/functions to implement behaviorsbehaviors

• An object is a software bundle of variables An object is a software bundle of variables and related methodsand related methods

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10

checkschecks

peoplepeople

shopping listshopping list

……

numbersnumbers

characterscharacters

queuesqueues

arraysarrays

Things in the Things in the real worldreal world

Things in the computercomputer world

Objects Represent

ClassesClasses

• Classes provide the structure for Classes provide the structure for objectsobjects• Define their prototypeDefine their prototype

• Classes define:Classes define:• Set of Set of attributesattributes

• Also called Also called statestate

• Represented by variables and propertiesRepresented by variables and properties

• BehaviorBehavior• Represented by methodsRepresented by methods

• A class defines the methods and types of A class defines the methods and types of data associated with an objectdata associated with an object 11

ObjectsObjects

• Creating an object from a class is called Creating an object from a class is called instantiationinstantiation

• An An objectobject is a concrete is a concrete instanceinstance of a of a particular classparticular class

• Objects have stateObjects have state

• Set of values associated to their attributesSet of values associated to their attributes

• Example:Example:

• Class: AccountClass: Account

• Objects: Ivan's account, Peter's accountObjects: Ivan's account, Peter's account12

Classes – ExampleClasses – Example

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AccountAccountAccountAccount

+Owner: Person+Owner: Person+Ammount: double+Ammount: double+Owner: Person+Owner: Person+Ammount: double+Ammount: double

+suspend()+suspend()+deposit(sum:double)+deposit(sum:double)+withdraw(sum:double)+withdraw(sum:double)


ClassClassClassClassAttributesAttributesAttributesAttributes

OperationsOperationsOperationsOperations

Classes and Objects – Classes and Objects – ExampleExample

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AccountAccountAccountAccount

+Owner: Person+Owner: Person+Ammount: double+Ammount: double+Owner: Person+Owner: Person+Ammount: double+Ammount: double



ClassClassClassClass ivanAccountivanAccountivanAccountivanAccount

+Owner="Ivan Kolev"+Owner="Ivan Kolev"+Ammount=5000.0+Ammount=5000.0+Owner="Ivan Kolev"+Owner="Ivan Kolev"+Ammount=5000.0+Ammount=5000.0

peterAccountpeterAccountpeterAccountpeterAccount

+Owner="Peter Kirov"+Owner="Peter Kirov"+Ammount=1825.33+Ammount=1825.33+Owner="Peter Kirov"+Owner="Peter Kirov"+Ammount=1825.33+Ammount=1825.33

kirilAccountkirilAccountkirilAccountkirilAccount

+Owner="Kiril Kirov"+Owner="Kiril Kirov"+Ammount=25.0+Ammount=25.0+Owner="Kiril Kirov"+Owner="Kiril Kirov"+Ammount=25.0+Ammount=25.0

ObjectObjectObjectObject



MessagesMessages

• What is a message in OOP?What is a message in OOP?

• A request for an object to perform one of its A request for an object to perform one of its operations (methods)operations (methods)

• All communication between objects is done All communication between objects is done via messagesvia messages

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InterfacesInterfaces

• Messages define the interface to the objectMessages define the interface to the object

• Everything an object can do is represented Everything an object can do is represented by its message interfaceby its message interface

• The interfaces provide abstractionsThe interfaces provide abstractions

• You shouldn't have to know anything about You shouldn't have to know anything about what is in the implementation in order to use what is in the implementation in order to use it (black box)it (black box)

• An interface is a set of operations An interface is a set of operations (methods) that given object can perform(methods) that given object can perform

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The Principles of OOPThe Principles of OOP

The Principles of OOPThe Principles of OOP

• InheritanceInheritance

• AbstractionAbstraction

• EncapsulationEncapsulation

• PolymorphismPolymorphism

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InheritanceInheritance

• A class can A class can extendextend another class, inheriting another class, inheriting all its data members and methodsall its data members and methods• The child class can redefine some of the The child class can redefine some of the

parent class's members and methods and/or parent class's members and methods and/or add its ownadd its own

• A class can A class can implementimplement an interface, an interface, implementing all the specified methodsimplementing all the specified methods

• Inheritance implements the “is a” Inheritance implements the “is a” relationship between objectsrelationship between objects

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• TerminologyTerminology

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subclass

or

derived class

superclass

or

base class

extends

subinterface superinterfaceextends

class interfaceimplements


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PersonPersonPersonPerson

+Name: String+Name: String+Address: String+Address: String+Name: String+Name: String+Address: String+Address: String

EmployeeEmployeeEmployeeEmployee

+Company: String+Company: String+Salary: double+Salary: double+Company: String+Company: String+Salary: double+Salary: double

StudentStudentStudentStudent

+School: String+School: String+School: String+School: String

SuperclassSuperclassSuperclassSuperclass

SubclassSubclassSubclassSubclassSubclassSubclassSubclassSubclass

Inheritance in JavaInheritance in Java

• In Java, a subclass can extend only one In Java, a subclass can extend only one superclasssuperclass

• In Java, a subinterface can extend one In Java, a subinterface can extend one superinterfacesuperinterface

• In Java, a class can implement several In Java, a class can implement several interfacesinterfaces

• This is Java’s form of This is Java’s form of multiple inheritancemultiple inheritance

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Interfaces and Abstract Interfaces and Abstract Classes in JavaClasses in Java

• An abstract class can have code for some An abstract class can have code for some of its methodsof its methods

• Other methods are declared Other methods are declared abstractabstract and left and left with no codewith no code

• An interface only lists methods but does An interface only lists methods but does not have any codenot have any code

• A concrete class may extend an abstract A concrete class may extend an abstract class and/or implement one or several class and/or implement one or several interfaces, supplying the code for all the interfaces, supplying the code for all the methodsmethods 23

Inheritance BenefitsInheritance Benefits

• Inheritance plays a dual role:Inheritance plays a dual role:

• A subclass reuses the code from the A subclass reuses the code from the superclasssuperclass

• A subclass inherits the A subclass inherits the data typedata type of the of the superclass (or interface) as its own superclass (or interface) as its own secondary typesecondary type

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Class HierarchiesClass Hierarchies

• Inheritance leads to a hierarchy of classes Inheritance leads to a hierarchy of classes and/or interfaces in an application:and/or interfaces in an application:

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Game

GameFor2

BoardGame

Chess Backgammon

Solitaire


• An object of a class at the bottom of a An object of a class at the bottom of a hierarchy inherits all the methods of all the hierarchy inherits all the methods of all the classes aboveclasses above

• It also inherits the data types of all the It also inherits the data types of all the classes and interfaces aboveclasses and interfaces above

• Inheritance is also used to extend Inheritance is also used to extend hierarchies of library classeshierarchies of library classes

• Allows reusing the library code and Allows reusing the library code and inheriting library data typesinheriting library data types

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AbstractionAbstraction

• Abstraction means ignoring irrelevant Abstraction means ignoring irrelevant features, properties, or functions and features, properties, or functions and emphasizing the relevant ones...emphasizing the relevant ones...

• ... relevant to the given project (with an eye ... relevant to the given project (with an eye to future reuse in similar projects)to future reuse in similar projects)

• Abstraction = managing complexityAbstraction = managing complexity27

““Relevant” to what?Relevant” to what?

AbstractionAbstraction

• Abstraction is something we do every dayAbstraction is something we do every day• Looking at an object, we see those things Looking at an object, we see those things

about it that have meaning to usabout it that have meaning to us

• We abstract the properties of the object, and We abstract the properties of the object, and keep only what we needkeep only what we need

• Allows us to represent a complex reality in Allows us to represent a complex reality in terms of a simplified modelterms of a simplified model

• Abstraction highlights the properties of an Abstraction highlights the properties of an entity that we are most interested in and entity that we are most interested in and hides the othershides the others

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Abstraction in JavaAbstraction in Java

• In Java abstraction is achieved by use ofIn Java abstraction is achieved by use of

• Abstract classes Abstract classes

• InterfacesInterfaces

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+Color : long

AbstractButton

+click()

«interface»IClickable

Button RadioButton ImageButton

Abstract Data TypesAbstract Data Types

• Abstract Data Types (ADT) are data types Abstract Data Types (ADT) are data types defined by a set of operationsdefined by a set of operations

• Examples:Examples:

30LinkedList

+add(in item : Object)+remove(in item : Object)+clear()

«interface»IList

ArrayList

Abstraction in AWT/SwingAbstraction in AWT/Swing

• java.lang.Objectjava.lang.Object

• ||

• +--java.awt.Component+--java.awt.Component

• ||

• +--java.awt.Container+--java.awt.Container

• ||

• +--javax.swing.JComponent+--javax.swing.JComponent

• ||

• +--javax.swing.+--javax.swing.AbstractButtonAbstractButton31

EncapsulationEncapsulation

• Encapsulation means that all data members Encapsulation means that all data members ((fieldsfields) of a class are declared ) of a class are declared privateprivate

• Some methods may be private, tooSome methods may be private, too

• The class interacts with other classes The class interacts with other classes (called the (called the clientsclients of this class) only of this class) only through the class’s constructors and public through the class’s constructors and public methodsmethods

• Constructors and public methods of a class Constructors and public methods of a class serve as the serve as the interfaceinterface to class’s clients to class’s clients

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EncapsulationEncapsulation

• Ensures that structural changes remain Ensures that structural changes remain locallocal::

• Usually, the internal structure of a class Usually, the internal structure of a class changes more often than the class’s changes more often than the class’s constructors and methodsconstructors and methods

• Encapsulation ensures that when fields Encapsulation ensures that when fields change, no changes are needed in other change, no changes are needed in other classes (a principle known as “locality”)classes (a principle known as “locality”)

• Hiding implementation details reduces Hiding implementation details reduces complexity complexity easier maintenance easier maintenance 33

Encapsulation – ExampleEncapsulation – Example

• Data Fields are privateData Fields are private

• Constructors and accessor methods are Constructors and accessor methods are defineddefined

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PolymorphismPolymorphism

• Ability to take more than one formAbility to take more than one form

• A class can be used through its parent A class can be used through its parent class's interfaceclass's interface

• A subclass may override the implementation A subclass may override the implementation of an operation it inherits from a superclass of an operation it inherits from a superclass (late binding)(late binding)

• Polymorphism allows abstract operations Polymorphism allows abstract operations to be defined and usedto be defined and used

• Abstract operations are defined in the base Abstract operations are defined in the base class's interface and implemented in the class's interface and implemented in the subclassessubclasses

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• Why use an object as a more generic type?Why use an object as a more generic type?

• To perform abstract operationsTo perform abstract operations

• To mix different related types in the same To mix different related types in the same collectioncollection

• To pass it to a method that expects a To pass it to a method that expects a parameter of a more generic typeparameter of a more generic type

• To declare a more generic field (especially in To declare a more generic field (especially in an abstract class) which will be initialized an abstract class) which will be initialized and “specialized” laterand “specialized” later

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Polymorphism – ExamplePolymorphism – Example

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Square::calcSurface() {Square::calcSurface() { return size * size;return size * size;}}

-x : int-y : int-size : int

Square

+calcSurface() : double

Figure

-x : int-y : int-radius : int

Circle

Circle::calcSurface() {Circle::calcSurface() { return PI * radius * return PI * radius * raduis;raduis;}}

Abstract Abstract classclass

Abstract Abstract classclass

Abstract Abstract actionaction

Abstract Abstract actionaction

Concrete Concrete classclass

Concrete Concrete classclass

Overriden Overriden actionaction

Overriden Overriden actionactionOverriden Overriden

actionactionOverriden Overriden

actionaction


• Polymorphism ensures that the appropriate Polymorphism ensures that the appropriate method is called for an object of a specific method is called for an object of a specific type when the object is disguised as a more type when the object is disguised as a more generic type:generic type:

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Figure f1 = new Square(...);Figure f1 = new Square(...);Figure f2 = new Circle(...);Figure f2 = new Circle(...);

// This will call Square::calcSurface()// This will call Square::calcSurface()int surface = f1.calcSurface();int surface = f1.calcSurface();

// This will call Square::calcSurface()// This will call Square::calcSurface()int surface = f2.calcSurface();int surface = f2.calcSurface();

Polymorphism in JavaPolymorphism in Java

• Good news: polymorphism is already Good news: polymorphism is already supported in Javasupported in Java

• All you have to do is use it properlyAll you have to do is use it properly

• Polymorphism is implemented using a Polymorphism is implemented using a technique called technique called latelate method bindingmethod binding: :

• Exact method to call is determined at run Exact method to call is determined at run time before performing the calltime before performing the call

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QuestionsQuestions??

OOP ConceptsOOP Concepts

ProblemsProblems

1.1. Describe the term Describe the term objectobject in OOP. in OOP.

2.2. Describe the term Describe the term classclass in OOP. in OOP.

3.3. Describe the term Describe the term interfaceinterface in OOP. in OOP.

4.4. Describe the term Describe the term inheritanceinheritance in OOP. in OOP.

5.5. Describe the term Describe the term abstractionabstraction in OOP. in OOP.

6.6. Describe the term Describe the term encapsulationencapsulation in OOP. in OOP.

7.7. Describe the term Describe the term polymorphismpolymorphism in OOP. in OOP.

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Object-oriented concepts