WEBVTT

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Hello. In this session,
we will talk about class methods.

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In Pharo, everything is an object,

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and you can send messages to objects.

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So if you take these two pieces
of information,

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classes are objects too,

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and you can send information,
hence messages to classes.

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I'll give you two examples.

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Time now: you send the message now
to the object Time which is a class.

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And it returns the current time.

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So I send the message now

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to the object Time
which is a class name.

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It sends me back the current time.

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Likewise, I send the message today
to the object Date which is a class;

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it returns the date.

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I'll give three more examples.

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I send the message workingDirectory
to the class FileLocator;

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it returns the current path,

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the file where the execution
takes place.

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In the second example,

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I send the message getPng
to the object ZnEasy

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with the URL of a png file
as the argument.

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It returns the png file
which was previously downloaded.

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In the third example,

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I send the message startDefaultOn
to ZnServer which is a class again,

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with a port number.

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This will start the HTTP server.

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So in the three cases,
you send a message to one class.

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How do you implement a class method?

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You select the class

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and click on the button Class
to say you want to implement a method

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on the class side
and not on the instance side.

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And you implement a method like usual.

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So the method today is implemented
on the class side,

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in the class Date.

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On this slide,
you can see a common mistake.

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Here, you wish to send
the message withValue

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to the class Counter;

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to return a new Counter
with a value set as a parameter.

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So you want

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Counter withValue: 10 to return a new
Counter which starts with value 10.

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If I evaluate this expression,
I get the value Counter,

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not a new counter.

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In fact, I get the class,
not a new instance of this class.

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Why this mistake? What's the problem?

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The problem is that by default,
all methods return self.

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So if I don't specify the value
to return, the method will return self.

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So the code up here is equivalent
to the code down there.

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In this case, self is the class Counter.

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So the method returns the class itself,
not the instance created just above.

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To correct this problem,

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simply add a caret

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in front of self new.

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To summarize, classes are objects.

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You can send messages to any objects,
so you can send them to any classes too.

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To implement the corresponding methods,

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you need to press the button Class
after selecting a class.

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Most class methods create
not only new instances

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but also other things.

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You will see lots of other uses
of class methods in Pharo.

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Class methods are just
like other methods;

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there are no particular rules.

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The lookup works
exactly in the same way.

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We will learn about it next week.