Transcript Unit 9 - University of Nottingham

Introduction to Programming
G50PRO
University of Nottingham
Unit 9 : Classes and objects
Paul Tennent
http://paultennent.wordpress.com/G50PRO.html
[email protected]
Room C7
Overview
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A more close look on classes and objects
A brief introduction to object-oriented
programming in java
What Is a Class?
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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
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Fields : which represent any variables that will represent
the object state
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Constructors: one or more constructor used to initialise an
object on creation
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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?
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Java is object oriented programming
language
Objects are key to understanding objectoriented technology
real-world objects: television set, bicycle, car.
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objects have state and behavior
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Using a Circle object
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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
Methods
(behavior)
move()
//private fields
x = 10
y = 15
radius = 2.5
print()
resize()
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:
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new : is a java key word that is used to create an
instance of a class with a specific state
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Using : Circle c1 = new Circle(); will:
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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
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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:
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http://java.sun.com/docs/books/tutorial/java/concepts/index
.html