WEBVTT

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Hello, everyone.

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In this lecture we'll present
class and method definitions

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in Pharo.

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The specific thing about Pharo is that

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there's no dedicated syntax
for defining methods and classes.

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They're defined within tools.

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The tools give the context
to the creation of a class or method.

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Here's the main code browser in Pharo:

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Nautilus.

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You can see in the bottom part

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the definition of the class Point.

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You can see that the class Point
has the class Object as superclass

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and two instance variables, x and y.

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The superclass and
the two instance variables, x and y.

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And we see at the bottom
the name of the package

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in which this class is added.

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In the slides we have code like this
for creating a new subclass.

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We can see that
to create a class in Pharo

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we send a dedicated message
to the superclass

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with the name of the subclass
as parameter.

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The message
subclass: instanceVariableNames:

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classVariableNames: package:

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is a message sent to the class Object

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with the name of the subclass
as parameter,

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the list of the instance variable
names as parameter

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and the name of the package
as parameter.

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As for the method definition,

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we have, still in the bottom part...

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the name of the method,

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a comment

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and the method's code.

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And the method belongs to a class.

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In the syntax
to define the method

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we have no information about the class
to which the method should belong.

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In fact, it's the selection
of the class in the code browser

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which tells us the class
to which the method belongs.

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In the slides
we also need this information.

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So we can't just copy the content
of the definition of a method.

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We have to add a prefix
to the method name

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to indicate the class
to which this method belongs.

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So we add...

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Before the name of the method
we add the name of the class

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to which we're going to add
the method we're defining.

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So, in the code browser,

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this class only appears as a selection

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in the browser.

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Here we define the method factorial
of the class Integer.

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A little reminder about the messages.

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Here we're defining
the method factorial.

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We have all these binary messages...

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equals, superior to,
multiply by and minus...

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Binary messages.

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We have a recursive call
to the method factorial.

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Factorial, here, is a unary message

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sent to an object of a number type.

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And we have two keyword messages...

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IfTrue:, twice, and error:.

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And finally we have the caret,

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used for returning a value

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and leaving the method.

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Regarding the return,

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by default in Pharo

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all methods return self,

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the current object.

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The code above

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is exactly the same
as the code below.

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So the return self part
at the end of the method is optional.

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Class methods let you send messages
to classes.

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Classes are objects too,

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so you can send messages to classes.

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And to define a class method

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you press the class button
in Nautilus...

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and define the class method.

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In the slides we systematically add
the word class,

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to distinguish a method
that'll be executed on a class

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rather than on an instance
of the class.

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So here the method x: y:
will be executed on the class Point,

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not on an instance of the class Point.

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What you should remember

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from this lecture...

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A class isn't defined
by a particular syntax

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but by sending a message
to its superclass.

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Classes are defined inside packages,

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the names of which are passed
as parameters of the message.

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All methods are public.

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There's no keyword to indicate that
a method is public. They all are.

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There's no notion of it being
protected or private.

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By default a method returns
the receiver, which is self.

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You can use the caret to return
something other than self.

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And class methods
are methods like any other.

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To add them,
press the button class

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before creating the method.

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This concludes
this first week of lectures.

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In the coming weeks

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we'll look in more detail
at the syntax of Pharo,

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blocks and a tool
that is an object inspector.