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 objectoriented 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 –
digits
+, -, *, /
Circle –
radius, circumference, area
how to calc circumference, how to calc area, …
Check –
amount, check number, date, comment
write, 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:
what it is
how to get to it
• circle might have:
data – radius
how to get the radius
calculate circumference
calculate 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
Class Name
two colons
void Circle::store_radius(float value)
{
radius = value;
}
Class Functions
float Circle::calc_circum(void)
{
Local variable
float circum;
circum = 3.14 * 2 * radius;
return circum;
}
float Circle::calc_area(void)
{
Local variable wasn’t
return (3.14 * radius * radius);
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:
instance
method
first_circle.assign_radius(7);
first_circle.calc_area();
they may require more
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()