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Object and Classes

Lesson #3

Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

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Contents

Objects Messages Classifications Classes Instances Metaclasses

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Object-based Programming

One way to view the world is as a collection of autonomous, interacting objects. Examples: people, animals, plants,

buildings, rooms, stairs, ...

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Real-World Object Properties

Active, autonomous behavior not directly controlled by the outside

Communicative can "send messages" to other objects

Collaborative long-term relationships between objects will

arise

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Real-World Object Properties Nested

complex objects have other objects as components (which in turn may have object components...)

Uniquely named (though not always – trade-off between

short names and unambiguous names) Creation/Destruction

May be created and destroyed

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Examples

A person Thinks (most of the time) Communicates with others Collaborates and works with others Has a Name (and a SSN) Is born/then dies

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Examples

A room Autonomous and Communicative (Arguable)

“open” is a message sent to a person to open a door, “open” is part of the room’s attributes.

Collaborative Accept the messages from other objects, and sends

messages to other objects based on your view on the room

Room No. Built/Destroyed

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Examples

A computer Autonomous: it can do many things Communicative (with people, with other

computers) A serial No. Built/Destroyed

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Virtual Objects

Objects that exist in programs only: Virtual objects consist of the above

features. Virtual objects are much more precise in

their name, borders, interactions, etc. Virtual objects are the basic components

for you object-oriented programs.

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Examples of Virtual Objects

A Bank Account Has a balance; responds to messages for

deposits, withdrawals, and balances.

Set Elements can be added, deleted, and queried.

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Examples of Virtual Objects

E-Ticket record that customer has paid for service

in advance of flight Payment

an event (transaction?) in which money is exchanged

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Virtual Object Example: a Set

The external view (signature): aSet understands the messages

aSet.add(anElement) aSet.remove(anElement) aSet.contains(anElement) aSet.size()

Messages have both an effect (internal changes or induced messages) and a return value.

Users only need external view of aSet to use it.

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Internal view Instance variables for aSet:

hashArray, valueArray Instance variables are private for just

that object and have indefinite extent For each message, there exists a

method to perform the action

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Sets in non-object-based languages

The Set itself is a (passive) struct with an array and a hash table

Operations on the set are active but stateless functions: procedure add(s:Set,e:Element)... procedure remove(s:Set,e:Element)... procedure contains(s:Set,e:Element) (stateless functions don't remember

anything from one call to the next)

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Sets in non-object-based languages

Note there is no encapsulation: The programmer could directly manipulate

the data, possibly putting the data in a compromised state.

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What are Objects?

Booch: Something that “has state, behavior, and identity.”

Martin/Odell” “Anything, real, or abstract, about which we store

data and those methods (operations) that manipulate the data.”

Peter Müller: An object is an instance of a class. It can be

uniquely identified by its name and it defines a state which is represented by the values of its attributes at a particular time.

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Message (Definition )

A message is a request to an object to invoke one of its methods. A message therefore contains the name of the method and the arguments of the method.

Consequently, invocation of a method is just a reaction caused by receipt of a message. This is only possible when the method is actually known to the object.

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The Interface

The operation name list open to other objects.

If an object has a function but does not show it to the public, this is useless for the public (called private function).

In C++, only public functions (belong to the interface) can be called (through messages such as a.fun()) by other objects.

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Again, Examples of objects

ID: SSN

Operations:EatWalkWork…

A person:

States:

Name

Sex

Age

Interface:What is in a person’s resume…

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Examples of objects

ID: ISBN

A book OperationsBuyOpenGet infoCloseDiscard

States:TitleAuthorPublisher Interface

All the above operation names

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Properties of Objects

Encapsulation (Data & Operations) Information Hiding Data Abstraction (with classes) Abstract Data Type (with classes)

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ADVANTAGES OF OBJECTS

Same as abstract data types Information hiding Data abstraction Procedural abstraction

Inheritance provides further data abstraction Easier and less error-prone product

development Easier maintenance

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Classification

Classification is not unique to object-oriented systems. On the contrary, classification is applied in many other domains.

Peter Wegner gave the description about classification (1986).

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Classification An object can be "of" a certain class, but not

its creator. An apple is of Fruit, but is not the instance of Fruit.

In this view, classes are like sets. John is a man. Jane is a woman. Spot is a dog. All men are people. All women are people. All people are mammals. All dogs are mammals.

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Classification

Mammal

People

Bird

Dog

Animal

man woman

John Jane

. . . . .

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Class

A class is either a classification or an abstraction of objects.

Class is not only an concept, but also a practical mechanism of programs.

Object-Oriented programming is actually programming-with-classes.

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Classes ”Class - a definition of an

implementation (methods and data structures) shared by a group of objects"

”Class - a template from which objects may be created. It contains a definition of the state descriptors and methods for the object."

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Classes

Classes serve two distinct purposes Factories that create new objects

(code plus specification) Classifications of objects

(specification only: e.g. sizable class is any object with a size method)

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Intentional notion of Class

New objects are instances of a class. Their state and behavior are determined by the class definition.

This is the so-called intentional notion of a class.

The intentional notion determines the structure of instances of that class.

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Extensional Notion of Class

A class consists of object-warehouse and object-factory.

Object warehouse means that a class implicitly maintains a class extent.

A class extent consists of all instances of one class. Object-factory means that there exists a constructor for each class.

The purpose of this constructor is to generate new instances of a class.

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Understanding Classes

A class provides a definition of the structure of instances of that class.

The class defines the names of attributes (state) and methods (behavior) of an object belonging to this class.

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What is a Class

A class is a user-defined type. Booch:

“A set of objects that share a common structure and common behavior”

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Class ::=<Id, Ds, Ops, Intfc>, where Id is the Identification or name of the

class; Dt is the space description (template) for

memory; Op is the set of the operations’

implementation the objects of this class can make;

Int is the unify interface of the objects of the class.

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A class(Similar to C++)

class Integer {Ds:

int IOps:

setValue(int n)Integer addValue(Integer j)

}

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Examples of class

class horse{ Ds:

Age Weight Color

Ops: Drag Run ride

}

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Examples of class class building{ Ds:

Name Architecture Height Time built Builder

Ops: Open Enter Leave …

}

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Relationships among Classes

Link(use-a) A link relationship exists between two

classes that need to communicate (an instance of one class sends a message to an instance of another class).

Composition (has-a) A composition relationship exists when a

class contains data members that are also class class objects.

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Instantiation

The mechanism of creating new objects from a class definition is called instantiation.

Every class has such a mechanism.

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Instantiation

Static instantiation and Dynamic instantiation. instantiation at compile time; instantiation at run time.

Dynamic instantiation requires run-time support for allocation and de-allocation of memory.

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Making objects from class templates

aSet = new Set() /*Set is the class; this makes an object*/

anotherSet = new Set() /*Same factory, but different contents*/

Each time an object created, it is new and unique

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Object ::= <Oid, Cid, Body>, where Oid is the identification or name of the

object; Cid is the identification or name of the

class of this object; Body is the actual space for memory;

Note: the operations of the object are implemented in the class.

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Instantiation Examples

bk = new Book ( Actually in the Publishing House);

bldg = new Building ( Actually by workers)

child = new Person ( Actually by parents)

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Examples of Class in C++

#include <iostream.h> // Stream I/O Class Library

#include <stdio.h> // Standard Header File

#include <math.h> // Math Library: need acos() declaration

const double PI = acos( -1.0 ) ;

typedef float Radius ;

typedef float Coordinate ;

typedef bool Boolean ;

#define NUM_SAMPLES 4

//ex3class.cpp

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ex3class.cppclass Point

{

private:

Coordinate x , y ;

public:

Point() ; // default constructor

Point( Coordinate new_x ) ;

Point( Coordinate new_x, Coordinate new_y ) ;

Coordinate x_coordinate( void ) const ;

Coordinate y_coordinate( void ) const ;

Point add( const Point *p ) const ;

Point add( const Point &p ) const ;

} ;

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ex3class.cpp

Point:: Point(Coordinate new_x, Coordinate new_y)

: x ( new_x ) , y( new_y )

{}

Point:: Point(): x( 0 ) , y ( 0 ) // initialize to default values

{}

Coordinate Point:: x_coordinate( void ) const

{ return( x ) ;}

Coordinate Point:: y_coordinate( void ) const

{ return( y ) ;}

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ex3class.cpp

Point Point:: add( const Point *p ) const{ Coordinate new_x = x_coordinate() + p->x_coordinate() ;

Coordinate new_y = y_coordinate() + p->y_coordinate() ;Point result( new_x, new_y ) ;return ( result ) ; // return result of the point addition

}Point Point:: add( const Point &p ) const{ Coordinate new_x = x_coordinate() + p.x_coordinate() ;

Coordinate new_y = y_coordinate() + p.y_coordinate() ;Point result( new_x, new_y) ;return ( result ) ; // return result of the point addition

}

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ex3class.cpp

class Circle{

private:Point center ; // Using Point classRadius r ;

public:Circle( ) ;Circle( Coordinate x , Coordinate y , Radius radius

) ;Radius radius( void ) const ;double area( void ) const ;double circumference( void ) const ;

} ;

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ex3class.cppCircle:: Circle( Coordinate x, Coordinate y , Radius radius )

: center( x , y ) , r( radius )

{ if( r < 0 )

{ r = fabs( r ) ; }

}

Circle:: Circle( ): center( ) , r( 0 ) // initialization list

{}

Radius Circle:: radius( void ) const

{ return( r ) ;}

double Circle:: area( void ) const

{ return( PI * r * r ) ;}

double Circle:: circumference( void ) const

{ return( 2 * PI * r ) ;}

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ex3class.cpp

void interactive_test( void ) ;Boolean test_circle( void ) ;float sample_list[NUM_SAMPLES] ={ -12.0 , 4.0 , 3.0 , 0.0} ;int main( int , char ** ){// interactive_test() ;

if( test_circle() ){cout << " \n\t Functional Tests ............... Passed\n" ; }else{cout << " \n\t Functional Tests ............... Failed\n" ; }return( 0 ) ;

}

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ex3class.cppvoid interactive_test( void )

{Circle c1( 1 , 3 , 4.0 ) ;

cout << "\n ** Created a Circle at (1,3) with radius = " << c1.radius( ) ;

printf("\n -- Using Standard I/O to Show radius: %f", c1.radius( ) ) ;

Radius radius ;

cout << "\n\n ** To create a new Circle Object, input value for radius: " ;

cin >> radius ;

cout << "\n\n ** Value of PI = " << PI ; // output results

Circle c2( 0 , 0 , radius ) ;

cout << "\n ** Created a Circle at origin with radius = " << c2.radius( ) ;

cout << "\n ** Circle's area = " << c2.area() ;

return ;

}

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ex3class.cpp

Boolean test_circle( void )

{ Boolean test_passed = true ;

double current_area , current_circum ;

float r ;

for( int i = 0 ; i < NUM_SAMPLES ; i++ )

{ Circle c( 0 , 0 , sample_list[i] ) ;

r = (float) fabs( sample_list[i] ) ;

current_area = PI * r * r ;

current_circum = 2 * PI * r ;

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ex3class.cpp

if( current_circum < 0 )

{current_circum = -current_circum ;}

if( c.area() != current_area )

{test_passed = false ;

cout << "\n\t " << i + 1 << " . Circle's radius = " << r ;

cout << "\n\t Expected Area: " << current_area

<< "\n\t Actual Area : " << c.area() ;

break ;

}

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ex3class.cpp

else if( c.circumference() != current_circum )

{test_passed = false ;

cout << "\n\t " << i + 1 << " . Circle's radius = " << r ;

cout << "\n\t Expected Circum: " << current_circum

<< "\n\t Actual Circum : " << c.circumference() ;

break ;

}

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ex3class.cpp

else if( c.radius() != r )

{test_passed = false ;

cout << "\n\t " << i + 1 << " . " ;

cout << "\n\t Expected r : " << sample_list[i]

<< "\n\t Actual r : " << c.radius() ;

break ;

}

}

return( test_passed ) ;

}

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The Relations between Class and Objects

A class is an object? An object is an instance of a class? “Then, what is the class of a class?

The metaclass

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Meta class

A meta class is the class of a class. The attributes of a meta class can be

used to describe a class (e.g. # of instances of a class).

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Meta class

Explicit support of meta classes means that objects, classes and meta classes are treated uniformly.

Classes can be created at run-time by explicitly sending a message to a special metaclass.

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Metaclass

Metaclasses are hidden from the user. A metaclass hierarchy is maintained

parallel to the class hierarchy.

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Metaclass

In C++, there is a metaclass in the compiler. Because each class definition is an instance of a class in the compiler which demonstrates the structures of classes.

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Readings

Chapter 2 Section 2.1