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