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-In this lecture,
I will teach you what an object is.

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In fact, an object is a small entity
that has a state

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and reacts to messages
by executing methods.

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So, what is an object?

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An object is a small computer entity
characterized by three properties.

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It has a state,
a behavior and an identity,

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and it will collaborate
with other objects,

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because it does not live in isolation,
to perform more complex tasks.

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I will start with a few examples.

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Imagine a really simple object,
such as a die.

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The state of a die
is its number of faces.

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Its behavior is:
"I want to roll the die to get a result."

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And this die will probably live
in "aDieHandle",

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so that we can roll
several dice together.

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Now, if we take
the example of a rectangle...

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A rectangle is characterized
by an X and a Y,

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and an extent,
or a width and a height.

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That is the state of the rectangle.

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Then, the rectangle knows
how to clip itself,

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what its area is,
how to expand, shrink,

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and all these kinds of behavior.

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In fact, a rectangle also interacts
with other objects.

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For example, the canvas on your computer

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will have rectangles to minimize
the redisplay of your window.

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So, when you move your window,
the display will generate rectangles

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and compute new rectangles
to redisplay what should be redisplayed.

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Now, here is the example of a contact.

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In your iPhone,
you have a friend as a contact.

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So, this is an object that will represent
his or her e-mail address, name,

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address, phone number and picture.

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The behavior will be
that you can display this object

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or query it.

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And also, this contact will work
in a contact manager.

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So, the contact manager will be able
to manage some of these objects.

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Similarly, if you collect games,
for example,

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and you want to know
what games you have,

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you will have a game item
which has a name.

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Is it on PS2 or PS3?

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How much did it cost?

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Did you finish it or not?

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And then, you will be able
to serialize this object on a disk,

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and it will collaborate
with an item-collector system

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that will help you collect your games.

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So, what you see is that
an object has a state,

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a behavior and an identity,
and will collaborate with other objects

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to perform more complex tasks.

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I mentioned the identity of the object.

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Often, you confuse identity and equality.

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Here is a trick to avoid confusion.

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You enter a pizzeria
and want to order a pizza.

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You want to order
the same pizza as a guy.

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In the case of identity,
you would eat his pizza.

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In the case of equality,
you only want the same.

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So, identity is the real object
that you point with your finger,

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whereas equality means wanting the same.

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Are those objects the same?

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In fact, an object has a responsibility,

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and it will expose
this responsibility to you

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via a behavior,
by executing a behavior.

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For example,
a turtle has a responsibility

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to draw a line when it moves.

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This is the responsibility
of this object.

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A file, for example, has a responsibility
to have a content and to open itself.

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It is really important to understand
that objects have responsibilities.

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And these responsibilities
are embedded and realized

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by the behavior that is given
when it is executed.

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An object will also encapsulate data.

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So, what you can see on the slide

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is that we can think
that data is not exposed to the client.

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So, data is something that only
the method of the object can access.

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So, we should consider data
as something private.

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The methods are exposed,
and you, as a client,

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or other objects, as clients,
can only send messages to the object

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to execute the methods
that will modify or not,

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or use the private data of the object.

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That is why, on this slide,
I have this larger border

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between messages and methods.

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An object will react to messages.

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For example,
I send the message "go" to the turtle

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and as a reaction,
the turtle will execute a method

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that will move the turtle on the screen
and draw a line on the screen.

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Now, what you should see
is that different objects

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can understand the same message.

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In fact, I can ask a window to open
and get a new window on my desktop,

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or I can ask a window to open a file,
and I will look at its content,

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or if I open a Zip archive,
I want to see the Zip item inside.

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And what you see
is that it is the same message,

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but I have three different behaviors
that have been executed.

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Now, a method specifies a behavior.

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So, you have the class "rectangle"
that defines the height of a rectangle.

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To do so, it will compute, for example,

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the difference between the bottom
and the top point of the rectangle.

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Now, a rectangle
can also define a method "area"

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to compute its area
by multiplying the width by the height.

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The methods specify
the behavior that will be executed

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in reaction to the message.

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Now, a method can also send
other messages to itself.

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For example, the message "topCenter"
will send the message "center"

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and the message "top".

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The message "center" will also send
the message "topLeft" and "bottomRight"

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to the rectangle.

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This means that we will have
a cascade of messages

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in reaction to the execution of a method.

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And there is no problem.

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What I show in this example
is that all the methods

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were methods of the class "rectangle".

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We will see what a class is later.

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But in fact,
it does not have to be this way.

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This means that an object
can also delegate to other objects.

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So, an object can say:
"To perform this task,

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I will send messages to other objects
and ask them to perform subtasks.

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And then, I will take the result
and recombine it

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to build something larger."

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Here is a typical example of delegation.

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An item collector can ask its items
to save themselves on the disk.

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Or it can say:
"I will ask another object

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that is responsible
for serializing objects

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to serialize myself."

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It depends. This is a design choice
to know how we distribute

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the responsibility in the system.

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What is important to realize
is that you, as an object,

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can delegate a task to somebody else.

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What you should know is that an object
is characterized by a state,

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a behavior and an identity,

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that an object collaborates
with other objects to realize tasks,

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that an object answers to messages
and executes methods by reaction,

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and that an object may delegate
a computation to another object,

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or to several other objects,

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and combine them to perform a task.