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-Hi! In this video, we are going to look
at two extremely important features

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in the object-oriented language,
which are senders and implementors.

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In order to understand
what they mean,

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let us open the System Browser.

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Once again, go to Tools,
then System Browser.

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We will go to a package,
let us say in AST-Core.

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We will go to our RBComment,
and we will look at the size.

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If you right click
on the method Size,

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you will see that the Implementors
and Senders options show up.

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So, what are the implementors of size?

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Basically, the implementors of size,
if we look at them,

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will be all of the different classes,
and their associated packages,

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that implement a method called "size".

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We can see that we have
quite a lot of them, 109.

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We can look inside them.

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For example, LGitDiffFile,
Pointer Layout, RBComment.

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But if we look at it,

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we can see that we have the package
from which it is defined,

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so System-Sources,
the class SourceFile,

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and we can simply browse it
by clicking on Browse.

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It will open another System Browser
on the correct class method.

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So, the implementors
are all the different classes

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that have size method defined.

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Now, if we look at the senders...

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So, Senders...

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We can see that the senders of size

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are all the methods
that call "size" in their body.

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For example, here,
we have "at:put:" that shows its "size",

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we have "extendMaxAsciiTo:"...

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So, any method, basically,
that will call "size" in its body.

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This is really useful,
because implementors will show you

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all the classes that define
a method of the same name

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and how it can be used,
while senders will show you examples

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of how the method "size"
is used concretely.

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So, in order to have
a quicker access to them,

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you can see
that there are some shortcuts.

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So, basically, implementors...
This is the M, right there, that is used.

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So, Cmd+M will open the implementors.

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While the senders...
N will be the shortcut.

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So, Cmd+N.

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So, senders and implementors
will really help you

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understand the method
you do not understand.

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You will see how it is used,
and I really encourage you to learn

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both the shortcuts and how to use them,

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just to navigate and to understand
the structure and class better.