1 - Introduction to Java Objects and Classes
The purpose of Java object-oriented programming, is to facilitate programming by creating classes,Through which we can Create a new object, whenever we need, by using what is called instantiation.
Exemple
String myString = new String ( “ Hello ! “)
Here we create an object of the string type named myString using the String model. The method by which we created the object is:
object myObject = new object();is called the instantiation. An object has properties and methods that provide informations about the object.
Exemple
myString.lenght);
Which here gives the length of the string myString = "Hello" which is equal to 5.
To create a class in Java we use the syntax:
class class_name { // code of class_name here ! }
2 - Object and Java Class
2 - 1 Declaration of a Java class
To facilitate learning we choose a simple example of a class to generate cars objects all having the same brand and the same price:class Car { // class attributes String brand ; Doble price; }
2 - 2 Constructor without parameters (default constructor)
In order to create objects using a Java class, you must associate a method (function) with the same name of class called constructor :class Car { // class attributes String brand ; Doble price; Car() { } }
And now to create a car object, just do the instantiation: for example if we want to create a car object named my Car, you can use the code :
Car myCar = new Car();
2 - 3 - Constructor with parameters
It is also possible to give the user the option of choosing the brand and the price of the car by adding parameters to the constructor :Car (String br, double pr) { This.brand = br; This.prix = pr; }
And now the user is free to choose his own brand and price that at the time of instanciation, for example if he wants to create a car of Peugeot brand and price 25000 Euro Just instantiate with these parameters:
Car hisCar = new car ("Peugeot", 25000);
Here is the final code of the class:
import java.util.Scanner; Class car { String brand; Double price; Car (String br, double pr) { this.brand = br; this.prix = pr; } Public static void main (String [] args) { Car hisCar = new car ("Peugeot", 25000); System.out.println ("The Brand of his Car is:" + hisCar.brand); System.out.println ("The price his Car is:" + hisCar.price + "Euro"); } }
What will display :
The brand of his Car is: Peugeot
The price of his car is: 25000.0 Euro