WEBVTT

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Hello, everybody.
In today's class, we are going to see

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if you have understood
our previous classes,

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namely as regards super.

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The idea is to make you think hard.

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I have a test to see
if you've understood.

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What will you learn?

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You'll think about what super is,

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revisit message lookup,

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and think about class methods.

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It's a lot of material for one class,
but it's fun.

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Imagine that I defined a method
in the dice class,

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and redefined
the "new" method as follows:

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Here is "new" and here is the code.

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inst := super new.

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inst initialize. Return inst.

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Now I execute
the expression dice new.

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Now my question for you is:

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In this expression, what is inst?

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What is super?

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And what is super new?

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I'll give you a few seconds
to think it over.

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I'll give you a few clues.

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They're based on my teaching experience.

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No, super is not the superclass.

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Get that out of your heads.

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No, inst is not
an instance of the superclass.

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If it was, we could never
write instances for the classes below.

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So what is it?

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Let's take a look.

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We said in the video on super

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that super was the receiver
of the message.

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Like self, super points out
the receiver of the message.

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This is not specific to Pharo.

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It's true in all object-oriented
languages such as Java, C#,

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and Smalltalk.

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Here the message is dice new.

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So what's the receiver?
Syntactically speaking, it's dice.

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The class is dice.

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With these clues,
you should understand better.

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Let's see.

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You remember how message lookup works?

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I send a message to an instance,

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for example,
ColoredRectangle.

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I look inside
the ColoredRectangle class

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to see if the method in question
is defined. It's not.

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"Area" is not defined.
I find it up here.

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I apply the definition I found.

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On what? On the receiver.

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Now you should understand
what this method does.

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I look up "new" in the dice class.

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I look it up and apply it on dice.

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Let's take a closer look.

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As I explained previously,

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in Pharo, only one message is sent
with one method lookup.

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So when I send a message
to any object whatsoever,

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I always look in the class.
I follow this link.

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Then this one here.

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It's always these two steps.

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What does this say about our system?

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I told you, super is the receiver.

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The receiver was dice.

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So I look for new
in the superclass of the dice class.

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The key here is that it's dice class
and not dice.

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We'll see it graphically afterwards.

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Once I've found the new "new,"

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I apply it to the receiver,
which is dice.

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What does this do?

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I apply new to dice,

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which will create and initialize
a new instance of dice.

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Inst will be my new instance.

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One dice.

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And I will initialize and return it.

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Let's take a look at a diagram.

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When I send new to dice,

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where do I look?
Which class do I look in?

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I look in dice class.

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This is where I use my famous method.

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What do I do now?
What did we say about super?

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Super is the dice class.
It is the receiver.

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I sent the message to this object.

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Super tells me to look inside

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the superclass
of the class containing the expression.

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So I look in the superclass
of the dice class.

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I look up here.
And somewhere up here,

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we'll see later on,

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new is defined.

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What about lookup?

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We said that the method
is defined here,

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and we apply it on the receiver.
I apply it on dice.

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And this gives me a new instance.
I'll draw it in.

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It returns an instance: A dice.

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And inst is pointed at
this new instance.

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So inst is a new dice.

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I send the initialize message to inst,
and then I return it.

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You often struggle with this example
because it's very rhetorical.

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I do it on purpose
to see if you've understood.

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It mixes two things.

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One, super.

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I look in the superclass of the class
that contains the expression super,

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knowing that super is the receiver.

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And two, new.
It's a method that creates objects.

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This is where the two meet,

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and you have
a hard time understanding this.

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I suggest that you really review
this lesson,

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because it's very important.

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I'll repeat it.

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Super is the receiver of the message,

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which in this case is dice.

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I look in the superclass
of the dice class for the new method

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that I will apply to dice, the receiver,
to find my terminal instance.

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Okay?

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In sum: Sending a message is looking up
a method in the receiver class.

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That's what we do each time.

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That's all you can do in Pharo,
so it's easy.

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By now you know that super means looking
in the superclass of the class

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containing the expression super.

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And that super is the receiver,
but it can be a class too.

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We're not done yet.
I have another problem for you.

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If you're studying Pharo,
you must like challenges.

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Here we go. Imagine that I have
the following problem:

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The method says:
"Make super-space-class

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"equal equal self class."

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What does "equal equal" mean?
It's the pointer's identity.

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In fact, I'm asking:
Is the object obtained

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with super-space-class the same

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as the object obtained
with self-equal-class?

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I defined this method,
which will return true or false.

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My question for you is:

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What is the result of A new foo?

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I create a new instance of A

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and I send the message foo,
which will edit this message.

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What is the result and why?

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You'll have to think
because I won't give you this answer.