Introduction to Classes and Objects in Java Programming

1. Introduction to ProgrammingG50PROUniversity of NottinghamUnit 9 : Classes and objects Paul Tennent http://paultennent.wordpress.com/G50PRO.html Paul.tennent@nottingham.ac.uk 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. //private fields x = 10 y = 15 radius = 2.5 Methods(behavior) move() print() resize() 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

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.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

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