WEBVTT

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Hello. In this course

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we'll take the time to really
understand the class methods.

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You should have done the exercise
with the counter,

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you have done a class method,
it worked very well.

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Now we'll take a good look,
so you understand for once and all.

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You'll learn in this course
that there's no difference in Pharo

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between class and instance methods
on a search algorithm level.

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There's only one search algorithm

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alluded to and used,
that's defined in the virtual machine,

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and you'll see that, while we could
have said that the class methods,

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at first glance, are like static
Java methods, they're not.

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In fact, they are dynamically linked.

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There's a search at execution,

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while in Java, typically,
static methods aren't dynamically sought.

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Let's see how it works.

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In fact, what you've seen

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is that there's only one lookup,
which starts in the receiver's class.

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If the method is defined in the class,

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it's returned, otherwise
it continues in the superclass.

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What this means, graphically,

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is that I've my example
of the Counter class.

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When I send the increment message,
what will happen?

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Step 1, I look in the Counter class,
and I'll go back up...

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If I've "increment" it's defined here,

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if it's a method defined
higher up in the hierarchy

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it will be sought higher up.

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In fact, this mechanism is used
in Pharo, there's only one,

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it's used for both instances and classes.

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When you did the exercise with
the class counter and sent the message

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"with value" to the class counter

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to create an instance, what happened?

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The class counter is instance
of a class called "Counter class"

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and we looked, we used
the same method as before.

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We looked in the receiver class,
the receiver was "Counter",

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so we looked in the class
"Counter class" and found the method

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"with value" which we applied
to the receiver

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and it gave us a new instance.

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So we used exactly the same process
as for the messages we sent

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to an instance of the Counter class.

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So what is a "class" in Pharo?
A class is an instance.

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It's an object like any object.
In Pharo there are only objects.

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A class is an instance
of another class, called a metaclass.

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It's just to distinguish them.
A metaclass is just a class

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whose instances are classes,

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to differentiate between
terminal instances and classes.

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The next thing is that
the Counter class, in this line,

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the Counter class is the only instance
of the class Counter class.

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This Counter class is created
automatically, without you knowing it.

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When you defined the Counter class,
the system automatically created

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the class Counter class,
and it created the Counter class.

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You thought you were creating 1 class,
but in fact you were creating 2.

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What you have to know in Pharo
is that all the classes

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in a class called XXX
are called XXX class.

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So if I have Counter,
the Counter class will be

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Counter class, systematically.

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So, in fact, what you do in reality,

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we're just showing you how it works.
We needn't have explained it,

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but it means that when
the browser shows you

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the Counter class, it shows you

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the code that will be executed
on the instances of this class.

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That means, "increment"

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is the code that will be executed
on this instance here.

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When you click on the "class" button
to define a class method,

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it shows you the code that will be
executed on this object here.

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And in fact, if we look,
when I send the message "increment"

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to the counter instance,
I'll look in the Counter class.

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Look, that's the method executed.

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When I send the message
"with value" to the Counter class,

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where will I look? In its class.
I follow this link,

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and it's this method here.

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So what should you know?

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A class is an object in Pharo.

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A class can receive messages.

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I told you when we did the syntax,

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I told you date, today...

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What's that? It's a message
that we sent to a class.

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There are no elements
syntaxed differently.

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It's the same as sending a message
syntactically to an instance.

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What's more, this course shows you,
there's only one method lookup.

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The class methods
are looked up dynamically

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like the instance methods.
Where are they looked up?

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In the class of the receiver.

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When it's an instance,
it's looked up in the class,

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when it's an instance class
like the Counter class

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it's looked up in the metaclass,
in the class called Counter class.

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So a class is an instance
of another class, called a metaclass,

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and only one look up works.

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So we'll return to this notion
in the final session,

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in the lecture
Understanding Metaclasses

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and we're going to revisit,
we're going to lift the lid on Pharo

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and show you how it works,
and it's completely uniform.