I. Introduction

Let's consider the following example:
Class rectangle:
In this class,  where we make acquaintance with an object (here Rectangle),we 
introduce:
An object that are known as instance of that class.
A data type of variables (or instances),
A constructor, 
Some instances (functions or methods),
And a main function.

Recall that:
- The function main is static, because it is not a method that we involve an 
object itself. Inside this function, we call the constructor to build 
a concrete rectangle of certain width and length.
- The function main needs always to be static in order to be compiled and 
public in order to be executed.



class Rectangle {
int width, length; //Statements declaring two local variables

Rectangle (int w, int l){// Constructor of the object Rectangle , with the same name 

than the class
width = w;
length = l;
}

int Perimeter (){//First method
return 2 * (width + length);
}

int Surface (){//Second method
return width * length;
}

void Display (String Name1, String Name2){//Another method to display
System.out.println("\n\t This is the first built " + Name1 + ". It is a " + Name2 + " 

with the following features: \n width = " + width + "\n length = " + length + "\n 

Perimeter = " 
+ Perimeter ()+ "\n Surface = " + Surface ()+ ".");
}

public static void main (String[] args){//Main function
Rectangle Rec = new Rectangle (15, 5);// Rec is a built rectangle
Rec.Display ("object", "rectangle");// How to apply a method to an object
}

}

Constructor Object = new Constructor ();
Object.Method(); 



To compile:
C:\Java>javac Rectangle.java
To execute:
C:\Java>java Rectangle

         This is the first built object. It is a rectangle with the following 

features:
 width = 15
 length = 5
 Perimeter = 40
 Surface = 75.

C:\Java>