Ce cours introduit la programmation orientée objet (encapsulation, abstration, héritage, polymorphisme) en l'illustrant en langage Java. Il présuppose connues les bases de la programmation (variables, types, boucles, fonctions, ...). Il est conçu comme la suite du cours « Initiation à la programmation (en Java) ». Comme son prédécesseur, ce cours s'appuie sur de nombreux éléments pédagogiques : vidéos sous-titrées, quizz dans et hors vidéos, exercices, devoirs notés automatiquement, notes de cours.
Attributs statiques
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From the course by École Polytechnique Fédérale de Lausanne
Introduction à la programmation orientée objet (en Java)
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École Polytechnique Fédérale de Lausanne
139 ratings
From the lesson
Le modificateur static ; Interfaces
Cette semaine nous abordons la notion importante d'interface qui va complémenter vos acquis sur l'orienté objet en Java. Nous étudions aussi le modificateur static.
Meet the Instructors
Jamila SamDr
School of Computer and Communication Sciences
Jean-Cédric ChappelierDr.
School of Computer and Communication Sciences
In the previous episodes, we saw that
the attributes defined inside a class represent
information that is specific too each instance of the class.
If we take our favorite example with the rectangle again,
each instance of "Rectangle", each Rectangle-type object
will have its own width, its own height,
information that is specific to it and which caracterizes it.
However, what happens if many instances of a same class
need to access the same information,
a common information?
That's the topic of this episode.
Until now, we have used different types of variables in our programs.
We've used instance variables,
which are commonly called "attributes".
We've used local variables,
which are variables declared inside a method's body.
We've also used parameters, which we can assimilate
to local variables in the methods,
and which are used to provide data to methods
so that they can work with.
In this episode, you'll discover the notion of "static variables",
which are also called "class variables",
as opposed to instance variables.
They are in fact particular attributes,
the declaration of which is preceded by the reserved word "static".
They look like the standard attributes, the instance variables.
They are declared the same way,
except for the "static" modifier that is added before.
They are declared as instance attributes,
as instance variables, outside of the methods.
As the instance variables, they are visible everywhere in the class,
and are inherited by the subclasses.
In this episode, we'll mainly focus
on static variables,
but it must be noted that there also exist static methods,
which will see later on.
In Java, the static modifier is used
for both variables and methods.
In the two cases, a static member can be used
without having constructed the object beforehand.
Note that in Java, the static modifier
doesn't apply to local variables,
that is, to variables declared inside a method's body.
But what does it actually mean that a static member can be
accessed without constructing any object?
If we take the example of the rectangle again,
what we've used until now were attributes
declared inside classes,
a.k.a. instance variables.
And we have methods,
and in this context it is strictly impossible
to use a member of the rectangle class
without constructing an instance of rectangle beforehand.
So here for example, if I want to use the surface method,
I have to construct an object of rectangle type.
And through this object, I can call the
method that computes the surface.
A static member, whether it's a variable or a method,
is invokable without having created an instance beforehand,
by using only the class name
according to terms that we'll examine later.
One of the facets is that we can use
a static member without constructing any objects.
The other facet, relatively to static variables,
is that they are shared between each instance of the class.
Until now you've learnt that each instance has
its own memory area for each of its attributes.
Each instantiated rectangle in a program will have
its own memory areas for the width and the height.
So here for example, for the rectangle 'r1',
we'll have memory areas storing
the value of its own width and height.
For another rectangle, there'll be other memory areas
that will store the width and the height of that second rectangle.
That's why we say "instance variables":
because each instance owns its own variables.
On the other hand, a static variable would be an unique memory area
associated with the class rather than the instances,
but accessible from each instance.
To sum up, for instance variables, the attributes,
we reserve a memory area
for each object that we instantiate, that we create with "new".
So each object has its own memory area, its own value
for the corresponding instance variable.
For a class variable, that is, a class-attribute,
declared as static,
there's a single allocation of an unique memory area.
This reservation is done as soon as we load the class,
that is, to simplify, as soon as the class is mentioned in the program,
No memory area will be allocated for that attribute
when we create a new instance with "new".
However, that attribute remains accessible
like instance attributes used up to now,
via all the objects of the class,
which will refer to the same memory area,
the unique memory area allocated for that static attribute.
Let's see this with a concrete example.
We have the case of a class A that contains an instance variable,
an attribute as we have always used them until now,
and a class variable, that is, an attribute
whose declaration is preceded of the reserved word 'static'.
The class A also contains a modified method,
which can access all the attributes of the class,
whether they're static or not.
Now let's examine step by step what happens
when we execute the following code.
In fact, before the execution,
as soon as the class A is used in a program,
as it contains a static variable "c",
there'll be an allocation of a memory space for this static variable "c",
which will be initialized with the value 10 because of this line.
If we examine this first line of code,
the first thing that we see is that the class variable
is accessible independently of the creation of any instance.
We can access the 'c' attribute through the name of the class only,
without creating any instance.
When we execute this line of code,
we increment the class variable 'c',
which is a class variable of the class A, which has already been initialized.
We'll have this situation,
the value of 'c' will go from 10 to 11.
The two following lines declare an instance of A,
and initialize it using the default constructor.
Each instance of A has its own instance variables,
here, in this case, the variable instance 'b',
which means that when this instruction line's execution has finished,
the memory situation will be the following:
we'll have a variable "v1"
containing a reference to an object of type A
which has an attribute 'b',
initialized to 1 with the help of this line.
The modified method is then called on the instance "v1".
This method increments the value of the instance variable 'b',
we'll therefore be in this situation
at the end of the instruction line's execution.
The modified method then increments the class variable 'c'.
You'll note by the way that the methods of the class A
can access the static attributes
in the same way they access the non-static attributes,
the instance variables.
The execution of the second line of this modified method
will therefore result in an additional incrementation
of the class variable 'c', which will be 12.
So we were able to modify the value of the class variable
of the static attribute both by passing by the name
of the class uniquely, and by using an instance.
So here through the "v1" instance,
we were able to modify the class-variable 'c'.
The modified variable, whether its through the name of the class,
or through an instance, is an unique memory area.
With this example we saw
that when we modify an instance variable,
the value changes only for the current object.
But when we modify a class variable,
the value changes for all the objects of the class.
Indeed, the class variable is unique and accessible to all the objects.
As we have just seen in the previous example,
a static variable can be used through the name of the class
without passing by the creation of any object.
We can therefore use static members
and do completely without objects,
which goes against the "object-oriented" approach.
Using static members to avoid the "object-oriented" approach
is of course a bad reason to use static variables.
A good reason is to represent a value
common to all the objects of a class.
Let's have a look at this with a concrete example.
Let's imagine that in a program, we have an "Employe" (= employee) class.
We want to represent the fact that 65 years old
is the official retirement age for all the employees.
Let's consider two possible versions to implement this.
So a version where we use an instance variable
to represent the retirement age,
and another version where we use a class variable,
a static variable.
First version, an "Employe" class
that represents the retirement age as an instance variable.
The constructor of the class "Employe1" would be in charge
of initializing each attribute with values passed as parameters.
Now let's suppose that an "Entreprise" class
uses this "Employe1" type, in particular to create an array
containing a number of employees.
Each element of the array
would be filled by creating an instance of Employe.
As the official retirement age is represented
with an instance variable,
each Employe will store a memory area that contains this official age,
which happens to be the same for everybody.
Therefore, we reserve a memory area for each Employe,
to contain the exact same information.
So if we have thousands of employees,
we'll end up with thousands of times the value 65 stored,
which of course is an unncessary duplication.
Moreover, this duplication causes major maintenance problems.
Let's suppose for example that the law changes,
and that the official retirement age is now 67 years old.
We would have to iterate over all the possible instances
to modify this age in a proper way.
If the instances were stored in an array, it's still feasible,
but imagine if the instance were created independently.
This would create a major update issue.
In the second version, where we now use a static variable
for the official retirement age,
so where we precede the declaration of the attribute with the reserved word "static",
we don't have this problem anymore.
Indeed, there would be an unique memory area
accessible by each instance,
which would contain the official retirement age,
which is the same for everybody.
This retirement age can be accessed from each instance,
or from the class name.
There isn't information duplication anymore,
and it's easy to do modifications.
Here we don't have to look for each instance of every employee anymore,
just to modify the retirement age and change it to 67 years old.
Voilà! I hope that this small example will have convinced you
of the benefits of using static variable well.
Also note that for constants
that are common to every instance of a class,
it is unnecessary to store the value for each object of the class.
You must in fact declare them as "final static".
For example, for a "Planete" (= Planet) class,
we need to use G, the gravitational constant.
This constant is the same for each planet,
and in this case, it is better to declare
the constant in question as "final static",
whether than only "final",
to avoid duplicating this constant for each instance of Planete.
To conclude on this chapter of static variables,
let's come back to an often-used instruction line.
We are now in position to understand
what this instruction line does,
which we had used up till now as a magical incantation.
What can we use directly through a class name,
well that's nothing else than a static variable.
So 'out' is a static variable,
it must therefore most likely contain a reference to an object,
as we apply a method on this 'out' variable,
obviously the 'println' method.
'out' is indeed a static variable of the 'System' class,
a variable of type 'PrintStream',
so it's an instance of the 'PrintStream' class,
and this 'PrintStream' class contains a 'println' method,
which is invoked like this.
This concludes our episode on static variables.
In a following episode,
you'll have the opportunity of learning about static methods.
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