WEBVTT

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Hello, everyone.
This second sequence on inheritance

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will teach you more
about behavior inheritance

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and what happens when you
send a message to an object.

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The goal of this sequence

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is to understand
the message-sending algorithm

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that makes it possible
to convert a message

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sent by a receiver to a method.

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That is, what method will be run,
in relation to the message sent,

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and 'self" semantics,
or what self really means.

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As we saw in sequence 1,

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inheritance is static, for state.

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That is, when the subclass is defined,

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its state is known.

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But inheritance of behavior is dynamic.

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When the program is running,

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when you send a message to an object

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you look for methods
for class and superclass.

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Message-sending is a two-step process.

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First, you look up the method
that matches the message.

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Step two, you execute the method
on the receiver.

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So here, we have the Rectangle class,

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the ColoredRectangle class,

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and an instance of ColoredRectangle
called aColoredRectangle,

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We send the message "area"
to this object.

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Our lookup algorithm looks for a method
matching the "area" message.

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It finds the method

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and carries it out

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in relation to the object
aColoredRectangle.

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The lookup algorithm

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identifies the method to be executed

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when a message is sent to an object.

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There are clearly two steps:
message sending, and method execution.

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They are fundamentally different,
in object programming.

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Even if you use another language,

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it is important to understand
the difference between the two.

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If you send the "area" message
to the aColoredRectangle object,

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the first thing the lookup algorithm
will do

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is search for the class of the receiver.

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The receiver of the message "area"

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is aColoredRectangle.

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The lookup algorithm
starts searching for a method

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in the object's class:
ColoredRectangle.

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It looks for an "area" method
in that class.

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The lookup algorithm

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will not find the method "area"
in that subclass.

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So it will look elsewhere.

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It looks in the superclass Rectangle.

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It finds the method "area" there,

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and returns it.

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The lookup algorithm is done
when it finds a method.

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Let's study two cases, and review
the algorithm I just listed on them.

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We send the message "color"

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to aColoredRectangle.

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The algorithm searches
the receiver class

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for a method called color.

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It finds it, and returns it.

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If we now send the message "area"
to aColoredRectangle,

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the algorithm searches
the receiver's class

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for the method "area."

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Not finding it, it searches the superclass.

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It finds it, and returns the method.

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"Self" always represents the receiver.

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The following examples
will make it clear to you

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that the lookup algorithm always
applies to the receiver self.

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Here we have an instance of class A.

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We send the message "foo."

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The lookup algorithm searches for foo,

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finds it, and executes it.

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For B new, we start with aB,

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the same thing as B new.

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We search for a method, "foo."

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The lookup algorithm finds it,

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executes it, and returns 50.

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So, here we have 10,

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and 50.

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It's important for you to take the time
to think about what "self" means.

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It is important to describe two points.

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What does "self" represent?

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How does message sending behave

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when a message is sent to "self"?

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"self" and "this" -
"this" being the term in Java -

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are the same thing.

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They always represent the receiver
of the message.

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So, we saw this example

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with A new foo and B new foo.

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Now we'll send the message "bar."

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Bar is sent to an instance.

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Bar is found here, and executed.

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and we send the message foo
to object self.

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"Self" is this object,

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A new.

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So we search this object
for a method "foo."

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We search the class for the method.

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We find it, execute it, and return 10.

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The other case is exactly identical.

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But pay attention, because many
beginning object programmers

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get it wrong.

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But the algorithm is identical.

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We send "bar" to this object.

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bar is not found here.

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It is found here.
So this is the bar that is executed.

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At that point, the message "foo"
is sent to self, which is...

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aB, thus B new.

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We send the message foo,

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so the algorithm looks for the method
in the object class.

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It finds it.

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50 is returned.

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This is how the algorithm unfolds.

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Look at each step in the process.
It is what I just told you.

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This example is also identical

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to the preceding one.

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Class B, here, is now empty.

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A new class, C, is defined
with the method foo return of 50.

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You go through the algorithm process
I just presented.

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Here, you will get 10.

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This is the foo that is executed,
in the end.

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Here, you get 50.

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To conclude:

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"Self," like "this" in Java,
always represents the receiver.

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Message sending always involves
two steps.

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First, the lookup algorithm looks
for a match between the method

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and the message sent.

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Second step: the method found
is executed on the receiver.

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The lookup algorithm travels up
from the receiver's class.

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The goal of the algorithm lookup

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is to find a method that matches
the message sent to a receiver.