1. Introduction to Programming G50PRO University of Nottingham Unit 9 : Classes and objects
Paul Tennent
Room C7

2. Overview A more close look on classes and objects
A brief introduction to object-oriented programming in java

3. 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.

4. 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

5. 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;

6. 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

7. 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

8. 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)

9. 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.resize(3);
System.out.println ("\n Circle 2");
Circle c2 = new Circle(5,7,2.5);
c2.print()
} // end of main
}// end of class

10. 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

11. 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: