class : set of variables, constructors and methods. A class holds a name that 
it will be compiled as javac name.java and executed as java name (with JDK : java 
Developpement Kit compiler).


Concstructor:  a function, with the same name as the class (name (...)), and that 
contains or not the declared variables. These variables will be used to construct 
the object of the class.


Method:  a function or a procedure that manipulate the object. In this case, the 
method is satatic. If it doesn't deal with the object it is not static.


Variable:  If a variable deals with the object, then it's a variable object. If 
not, it's then a class variable.


Field: All the separated members of the class holding data of the object 
or the class are called fields.


Instance:  An istance is the fresh duilt object: the related sytax is: 
Constructor Instance = new Constructor(...);
A class is instantiated when its constructor is called. 
Making an object is equivalent to create an instance of a class. 
Instance variable and instance methods are the variable and methods used 
when an instance is declared.

Modifiers (access modifiers): To set visibilty, a the name of class may be preceded 
by a modifier such as:
- public:  visible from everywhere. In the directory (or package), where it is made, 
a class can be compiled and executed without the public modifier. The related fields 
followa the same rule. 
- private:  Fields (variables or methods) may be used only by an instance of the 
same class. This class which declares the variable or method, 
- protected:  Field are available to all subclasses of the class and all the 
classes in the same package.
Without any access modifier, the related class becomes a default class.
NB. main mehod must be always public. Class is either public 
abstract. Fields are public, private or protected.


Type:  Describes the kind of a variable or method like int, double, boolean, String, 
Object,.... A method (function) that returns nothing takes void as type.


extends:  A class extends (inherits) from its super (mother) class, but it implements 
another class which is abstract ( with all fields abstract). A class can extends one 
class and implements many classes from the set called Interface. An abstarct class 
cannot be instantiated.

static: Field associated with a class rather than an object. Static fields are attached 
to a class, not an object.


final: Field that can not be changed. A class is declared as final if it can 
not be extended.