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Concepts

Object-Oriented Programming

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i h c Cng ngh . HQG H N i 2Basic concepts

OutlineHistory of programming languagesProcedural programming vs. Object-oriented programming

Basics concepts of OOP

Readings:Core Java 2 , Section 4.1

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History of programming languagesEarly programming languages such as Basic, Fortran... are

unstructured and allow spaghetti code. Programmers used gotoand gosub to jump to anywhere in the programs.

10 k=120 gosub 10030 if y > 120 goto 6040 k = k + 150 goto 2060 print k, y70 stop100 y = 3*k*k + 7*k - 3110 return

Hard to understand, error-prone, difficult to modify.

Jump to an arbitrary position inthe program

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History of programming languagesNext came the new structural

programming paradigm withlanguages like Algol, Pascal,C...

int func(int j){

return (3*j*j + 7*j-3;}

int main(){

int k = 1while (func(k) < 120)

k++;printf("%d\t%d\n", k, func(k));return(0);

}

Characteristics of structural / procedural programming paradigm:

Use of looping structures: for, while, repeat, do-whilePrograms as a series of functions/proceduresProgram source code focus on representing algorithms: how to do things.

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Limitation of procedural programmingstruct Date{

int day;int month;int year;

};

Data are separated from

processesPassive data, active processesNo guarantee of data

consistancy and constrainstsHard to prevent client code frommodifying librarys data

Difficulty in code maintenanceProcesses are scattered, butEach must understand the datastructures

What if

someDay, someMonth,someYearmake an invalid date?

void someProcess(){

...date.day = someDay;date.month = someMonth;date.day = someYear;...

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Object-oriented programmingclass Date

{public:void setDate(int newDay, int newMonth, int newYear);int getDay() { return day; }...

private:int day;int month;int year;

};

void Date::setDate(int newDay, int newMonth, int newYear){

//check validity of newDay, newMonth, newYear...//set new values...

}

OOP helps remove those limitations

Data come together with related processing code

Allow for guarantee of data consistency and constrainstEasier maintenance

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What is OOP?

Object-Oriented ProgrammingMap your problem in the real worldDefine things (objects) which can either do something or have

something done to themCreate a type (class) for these objects so that you dont have toredo all the work in defining an objects properties and behavior

STUDENT

* studentNo- name- birthday

+ enroll()...

* *enrols in

* courseID- lecturer- instructor

+ getStudents()+ setLecturer()...

CLASS

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The goals of object-oriented designRobustness : software is capable of handling

unexpected inputs that are not explicitly defined forits application.

Nuclear plant control softwareAirplane control software

Adaptability : software that can evolve over time inresponse to changing conditions in its environment.Web browsers and Internet search engines typicallyinvolve large programs that are used for many years.

Reusability : the same code should be usable as acomponent of different systems in variousapplications.

Save time and money

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Important OO concepts

AbstractionObjects & Class

Object state and behaviorObject identityMessages

EncapsulationInformation/implementation hiding

InheritancePolymorphism

abstraction

encapsulation

inheritance polymorphism

"P.I.Etriangle

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AbstractionAbstraction: to distill a complicated systemdown to its most fundamental parts anddescribe these parts in a simple, precise

language.naming the partsexplaining their functionality

Examples:Design of data abstract data types (ADT)

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AbstractionFive basic characteristics of Smalltalk, the first

successful object-oriented languageEverything is an object.A program is a bunch of objects telling each other

what to do by sending messages.Each object has its own memory made up of otherobjects.

Every object has a type.All objects of a particular type can receive the samemessages.

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ObjectsThree components:

State :Given by objects internal data

Behaviour :Produced by objects methods

Identity :Objects in any of its states are unique entities (with uniqueaddresses in memory)

Different objects could have same values for data (same state)

An object is manipulated via its handleIn C++: pointers and referencesIn Java: object references

class Date{public:

void setDate();int getDay();...

private:int day;int month;int year;

};

Date today = new Date();

today.setDate(2, 2, 2010);

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Messagestoday.setDate(2, 2, 2010);

A means for object A to request object B to perform one method of Bs.A message consists of:

Handle of the destination object host ( today )Name of the method to perform ( setDate )Other necessary information arguments ( 2, 2, 2010 )

In fact, a message is a function call with the host object as the implicitargument (method invocation)However, the concept of messages has great significance to OOP

Data become activeContrast to the old view of programming that

All actions are centrally controlledData are passive, procedures manipulate data

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Object classes

Classesare the templates to create objects (instantiate).Each object has the same structure and behaviour as the classfrom which it was created

Data type Variable relationClasses are what we design and codeObjects are what are created (from a class) at run-time

class Date{public:

void setDate();...

private:int day;...

};

Date today = new Date();

today.setDate(2, 2, 2010);

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Encapsulation / Information hidingEncapsulation: to group related things together,so as to use one name to refer to thewhole group.

Functions/procedures encapsulate instructionsObjects encapsulate data and related procedures

Information hiding: encapsulate to hideinternal implementation details from outsiders

Outsiders see only interfacesProgrammers have the freedom in implementing

the details of a system.The only constraint on the programmer isto maintain the interfacepublic , private , and protected

class Date{public :

void setDate(...);int getDay()

...private:

int day;int month;int year;

};

void Date::setDate(...){

//check validity//set new values...

}

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Inheritanceis-a relations

The general classescan be specialized tomore specific classesReuse of interfaces & implementation

Mechanism to allow derived classes to possessattributes and operations of base class, as if they weredefined at the derived classWe can design generic services before specialising them

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Polymophism

Polymorphism:Ability to assumedifferent forms or shapes.To exist in more than one form

Object polymorphism:Objects of different derived classes can be treated as if they areof the same class their common base classObjects of different classes understand the same message in

different waysexample: on receiving message draw() , Rectangle and Triangleobjects perform different methods

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OOP languages

Some OOP features can be implemented in C or otherprocedural programming languages, but not enforced bythese languagesOOP languages: OOP concepts are embeded in andenforced by the languages.OOP languages vary in degrees of object-oriented

Pure: Smalltalk, Eiffel, Ruby, JADE..

Original OO plus some procedural features: Python, Java (veryhigh), C++ (mixed), C#..OO features as extension: VB.NET, Fortran 2003, PHP, Perl..

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