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-Hello. In this lecture,
we will look at what classes are.

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Basically,
a class is a factory of objects.

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But more precisely,
a class is a mold of objects.

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What you see on the slide
is that we have this big machine

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that creates bathtubs.

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And what you see is that
this machine will create many bathtubs.

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Probably thousands
or hundreds of thousands of them.

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A class will also create
hundreds of thousands of objects,

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and all these objects
are still autonomous.

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This means that you can take an object,

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and this object
can have specific properties,

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a state that is not shared
by the other objects.

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But since they are all created
by the same class,

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all these objects
will share the same behavior.

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So, what we see is that in fact,
a class defines all the variables

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and the characteristics
that an object will have.

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And all its instances
will have the same characteristics.

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And a class defines the behavior,

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which will be shared
by all these objects.

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The idea is that the object
should not define all the same methods.

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These methods are shared
by all the instances.

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This way, we optimize space

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and we do not have to have thousands
of similar methods in the system.

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A class factorizes object properties.

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For example, all rectangles
will have a height and a width,

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but a given rectangle will have
a value for the height and the width

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different from another rectangle.

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We will define the behavior once,

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and all the objects will be able
to reuse this behavior.

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In fact, all the instances of the class
will reuse this behavior.

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This is what I show
with the definition of the method "top".

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All the rectangles will be able to answer

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and execute this method
in reaction to a message.

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What I show in this UML diagram
is that, for example,

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rectangles, here,
are represented by two points,

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the origin and the corner.

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And they offer multiple methods
to the system,

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like "area", "topCorner", etc.

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Now, what is really important

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is that each of the rectangles

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that are instances
of the class "rectangle"

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has different properties.

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You can have a rectangle that is flat,
one that looks like a square,

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or one that is really tall.

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They all have a width and a height,
and they will share the same behavior,

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but they have different values,

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so each instance
will have a specific value.

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This is what I said in this slide.

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The class defines the properties

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that all of its instances should hold,

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but not the actual value.

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Each instance
will have its own specific value.

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And then, the class defines the behavior

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that is shared
among all the instances of the class.

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If you look at the differences
between classes and instances...

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An object is an instance of a class.

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It has been created by a class.

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A class will specify the behavior

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that all its instances
will comply to or execute,

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and this behavior is shared
among all the instances of a class.

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And a class will specify
the structure of the instance,

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but not the value
that the instance should hold.

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So, what we should see
is that the instances of a class

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all share the same behavior.
How does it work?

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In fact, when I send a message
to one rectangle,

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I will look for the methods
in the class of the rectangle,

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and I will find a method
which will be applied on this object.

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That is why, if I send a message
to another rectangle,

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I will find the same method
and apply it to this other rectangle.

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This way, I share the behavior.

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So, all the instances of a class
will share the same behavior.

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What we should see is that
methods are looked up dynamically.

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This means that I will look for a method
in the class of the object.

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Here is an example
with dogs, cats and fish.

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When I send
the message "angry" to a dog,

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I will look in the class "dog",

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find the definition of the method
and execute it.

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And this behavior is different
from the one in the class "cat".

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But all the dogs
will execute the same method.

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And here again,
this is the same message

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that I will send to different instances
of different classes

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and that will execute different methods.

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So, when I send the message "angry"
to an instance of the class "cat",

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I will not execute the same method

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as when I send the message "angry"
to an instance of the class "dog".

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So, what you should know is that
any object is an instance of a class.

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A class is a factory of objects.

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It describes the state of an object,
of every of its instances.

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And it defines the behavior
that all its instances will share.