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"Learning Object-Oriented Programming
and Design with TDD"

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"First look at Objects,
Classes, Methods and messages"

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Stéphane Ducasse,
research director, Inria.

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-In this lecture, I will do a survey
of all the concepts

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that we will see in future lectures.

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So, we will look at what objects,
classes, methods and messages are.

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So, what is an object?

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In fact, an object
is a small living entity

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in the object-oriented world,

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and this entity
is characterized by three properties,

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a state, a behavior and an identity.

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So, let us look at an example.

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If we take a Logo turtle...

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A Logo turtle is a small entity,
an object.

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It has a state. For example, the pen,
if its pen is up or down,

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the size of the pen,
the color and the direction.

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And the turtle also has a behavior.

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This means that it knows
how to draw lines on the floor,

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jump or hide itself.

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But also, these turtles have an identity,

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and the idea is that,
when you look at the picture,

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you have two turtles,
one spider and one turtle,

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and they are different entities,
the spider is not the turtle.

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Now, in fact, in the object-oriented
programming paradigm,

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everything is an object,

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so what you will see is that even a file
that you have on your disk is an object.

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It is represented
by a path and a content.

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That is the state of the object.

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The behavior is...

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For example, you can copy your file,
or you can rename the file.

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And the identity is clearly that a file
is different from another one.

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Else, you would have only one
on your hard disk.

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Now, I would like to say something

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about the difference
between identity and equality.

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This can be confusing,
but here is something really simple.

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Imagine that you enter a pizzeria
and you want to order a pizza.

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There is a guy eating a pizza,
and you want the same.

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Identity would mean
that you go and eat his own pizza,

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whereas equality means
that you want to eat the same pizza.

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So, you see that identity
is the pointer to the object,

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whereas equality is something
that is similar to the object.

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Now, let us go back to the object.

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An object reacts to messages,
and it will perform an action.

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So, what I show on the slide
is that you have a turtle,

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and then,
you send a message to the turtle

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to go to a certain location.

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Then, you send
the message "go to: 100",

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and then, the turtle will go
and leave a trace on the floor.

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Now, what we should see

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is that different objects
can understand the same message.

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Each object
can perform a different reaction,

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but this is still the same object.

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When we have "aWindow open",
we will open a window.

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When we have "aFile open",
we will get the content of the file.

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And if you have "aZip open",
you will get the content of the Zip.

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Now, in the vocabulary,

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it is really important to understand
what the message receiver is.

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The message receiver

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identifies the object
that actually receives the message.

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For example, when I say
"aTurtle jumpTo: aLocation",

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I send the message "jumpTo" to a turtle,
and this is the receiver.

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And similarly, if I say "aWindow open",
the receiver is a window,

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and it is the same for "aFile open".

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Now, what is a method?
It is in fact a sequence of instructions,

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and we give it a name.

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So, let us take the example of crepes.

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To make crepes,

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we know that we need eggs,
milk, flour and some sugar.

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This is a recipe,
and a method is a recipe.

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It is a named sequence of instructions

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that the object will execute
in response to a message.

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So, what is the difference
between a message and a method?

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A message is what to do.
It is a kind of order.

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"stef makeCrepes",
"robert makeCrepes"...

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Those are orders,
and a method is how to do it.

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It is a list of steps.

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So, the message is what to do,
whereas the method is how to do it.

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It is important to understand
that we can have one message

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and multiple methods with the same name.

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When I have "Window >> open",

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I will have a specific sequence
to open a window.

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When I have "File >> open", 

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I will have a specific sequence
to open a file.

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And still, they have the same message,
they understand the same message.

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Now, let us look at classes,
because objects come from somewhere.

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Objects come from classes,
and a class is like a factory of objects.

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A class acts as a mold for objects.

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If you check the picture on the slide,

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this is a bathtub-making system.

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You put some steel and you get a bathtub.

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What is important is that the class
will create many instances,

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but each instance
is still a separate entity.

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This means
that you can destroy one bathtub,

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but you will not destroy the other ones.

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Each entity has its own identity.

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Still, they all are instances of a class.

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Now, if you look at it
from another perspective,

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a class will define
all the variables of its instances,

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and it will define the behavior
that the instances should have.

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So, how do we create an object?

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In fact, we send
the message "new" to a class.

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For example, here,
I have a class turtle and I ask it:

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"I want to have a new instance,
can you create one for me?"

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I will send the message "new",
and I will get a new turtle.

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And that is the same
for every object in the system.

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For a file, if I send the message "new",
I will get a file.

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What is important to understand
is that methods are dynamically looked up

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in the class of the receiver.

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So, here, I show three different kinds
of classes and objects.

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I have a dog,
instance of the class "dog",

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a cat, instance of the class "cat"

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and a fish, instance of the class "fish",
and they can be afraid.

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But the dog
will react differently from the cat.

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This means that when I send
the message "angry" to a dog,

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I will look in the class "dog"

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to find the correct methods
that will be executed.

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And this is the same.

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I have one message that is "angry",
and I will send it to multiple instances,

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and they will look respectively
in their class

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to find the correct behavior,
the correct methods to be executed.

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So, with that,
we have basically covered

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all the most important concepts
in object-oriented programming.

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An object is a unique entity
and an instance of a class.

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A class specifies
the shape and the behavior

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of all its instances.

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An object receives messages

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and executes corresponding methods.

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A message is an order

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and a method
is a sequence of instructions.

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And different objects
can understand the same message

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but react differently.

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This is what we will see
in future lectures.