Introduction to Object Oriented Programming

CIS 230

01-03-06

Introduction

What Makes an object oriented programming language?

1. Objects include both data and instruction

2. Objects inherit behavior from existing classes

3. Objects determine at runtime how to resond to messages

General Types of OOP languages

1. Hybrids• C++, Objective C, Object Pascal• Provide compatibility with older languages

2. Pure OOP languages• Smalltalk, Actor, Java• Programmer is forced to create object-

oriented code.

Why OOP?

• User Interface– 2/3 of an applications code– (windowing, pull down menus, graphics, etc.)– need to be able to write this easily

Misconceptions

1. Makes everything easy

2. You can reuse everything

Learning curves

1. Must learn the tools provided to create object-oriented programs

2. Must learn to work in an object-oriented programming style

Encapsulation

• The process of combining both properties (data) and behaviors (functions) into one entity.Examples:Integers –

Circle –

Check –

digits+, -, *, /

radius, circumference, areahow to calc circumference, how to calc area, …

amount, check number, date, commentwrite, sign, cash, record

Class

• A definition of an object (or for a group of similar objects)

• A template for creating objects

• Note:Each object belongs to only one class

C++ partial examples

• name might contain:

• circle might have:

what it ishow to get to it

data – radiushow to get the radiuscalculate circumferencecalculate area

C++ form

class ClassName

{

private:

data

public:

functions

};

This defines a class, nothing exists yet

prototypes:to keep the definition compact

Circle

class Circle{ private: float radius; public: void store_radius(float); float calc_circum(void); float calc_area(void); float return_radius(void);};

Class Functions

void Circle::store_radius(float value)

{

radius = value;

}

Class Name two colons

Class Functions

float Circle::calc_circum(void){ float circum; circum = 3.14 * 2 * radius; return circum;}float Circle::calc_area(void){ return (3.14 * radius * radius);}

Local variable

Local variable wasn’t really needed

Instance

• Instance – an actual variable of the class

void main()

{

Circle circle1, circle2;

float x, y, z, w;

…

}

Each has its own data (i.e. radius) but they share the functions (methods)

Invoking methods

• instance.method( );

circle1.store_radius(6);cout << “Please enter a circle’s radius ”;cin >> x;circle2.store_radius(x);y = circle1.calc_circum();z = circle1.calc_area();cout << “ A 6 inch circle has a circumference \n”;cout << “ of “ << y << “ and an area of “ << z << “\n”;

Invoking methods

cout << “A “ << x << “ inch circle has a circumference \n”;

cout << “of “ << circle2.calc_circum();

cout << “and an area of “ << circle2.calc_area() << “\n”;

cout << “The sum of the two areas is “;

w = circle1.calc_area() + circle2.calc_area();

cout << w << ‘\n’;

Message

• A request to an object

• Must have at least two parts:1. an instance name

2. the name of a method

• Example:

first_circle.assign_radius(7);

first_circle.calc_area();

they may require more

instance method

OOP Review

• Class – A definition (template) of an object– Contains: data & methods Encapsulation– Data – private– Methods – public

• Instance (object) – An actual variable of the class– Each instance has its own data but jointly use

the methods

OOP Review

• Data Abstraction – The ability to manipulate the data without knowledge of the data’s internal format– Use methods to assign/retrieve values– Example:

circle1.store_radius(6);

y = circle1.calc_circum();

z = circle1.calc_area();

Assign

Retrieve

Protecting the data

Circle circle1, circle2;

…

cin >> x;

circle.store_radius(x);

y = circle2.calc_circum();

what if x was -3.4?

Protecting the data

• A new store_radius() method:Circle::store_radius(float value){ if (value >= 0) radius = value; else radius = -1 * value;}

Constructor

• Automatically invoked when a new object is created:– when:

– or:

– or:

Constructor

• Default Constructor

• Help establish the link between the specific object and its class’ methods

• Can initialize data

Constructors

• Same name as the class• Can NOT return anything (not

even void)

class Circle{ private: float radius; public Circle(); …};Circle::Circle(){ radius = 1; //Default - unit circle}

Destructor

• Automatically invoked when an object goes out of existence– when:

– or:

– or:

Destructor

• Default Destructor

• May cleanup any possible side effects

• ~ClassName:~Circle()