Introduction to Programming

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18 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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Introduction to Programming


G50PRO

University of Nottingham

Unit 9 : Classes and objects

Paul Tennent

http://paultennent.wordpress.com/G50PRO.html

Paul.tennent@nottingham.ac.uk

Room C7

Overview


A more close look on classes and objects


A brief introduction to object
-
oriented
programming in java


What Is a Class?


A building unit of a java program


Your program may consist of multiple classes


i.e.: you have been using the UserInput and String
classes in many of your programs



In object oriented programming
a class is a blueprint
or prototype from which
objects

are created.






Class Circle Example


A class have three basic section


Fields : which represent any variables that will represent
the object state



Constructors: one or more constructor used to initialise an
object on creation



Methods: which should provide any functionality
(behaviour) required by the object






Class Circle Example

public class Circle {




//class fields



private int x, y;


private double radius;



//constructors


public Circle() {



x = 0;



y = 0;



radius = 0;


}


public Circle (int ax, int ay, double aradius ) {



x = ax;



y = ay;



radius = aradius;


}



Cont. Class Circle Example



//methods


public void move(int newx, int newy){



x = newx;



y = newy;


}




public void resize(double newRadius){



radius = newRadius;


}




public void print(){



System.out.println("x:"+ x +" y:"+ y +"
\
nradius:"+
radius);



}

} // end of class

What Is an Object?


Java is object oriented programming
language


Objects are key to understanding
object
-
oriented

technology


real
-
world objects: television set, bicycle, car.



objects

have
state

and
behavior

Using a Circle object


A circle object is an instance of the circle class with an actual
state


We can change the state of the object using the offered methods

//private fields

x = 10

y = 15

radius = 2.5

move()

print()

resize()

Methods

(
behavior
)

Using a Circle object

public class CircleDemo {


public static void main (String[] args){




System.out.println ("Circle 1");



Circle c1 = new Circle();



c1.print();






c1.move(2,2);



c1.print();






c1.resize(3);



c1.print();






System.out.println ("
\
n Circle 2");



Circle c2 = new Circle(5,7,2.5);



c2.print()



} // end of main

}// end of class

Key points:


new

: is a java key word that is used to create an
instance of a class with a specific state



Using :
Circle c1 = new Circle();
will:



Call the Circle class constructor


The constructor will initialize the Circle class fields (assign
state to the object)


c1 is a reference variable, it holds a reference that points to
a circle object in memory


We may use c1 to access any public fields / methods of the
class

More on OOP in java


object
-
oriented programming (OOP) is a
programming paradigm that make use of "objects"
to design applications and computer programs


It is commonly used in mainstream software
application development since the early 1990s


A more detailed study of OOP is out of the scope of
this course, for more details:


http://java.sun.com/docs/books/tutorial/java/concepts/index
.html