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Hello, everyone.

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Welcome to sequence 3.

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Now that we have looked at inheritance,

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self, and lookup,
we'll examine "super."

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We'll start by focusing on
sending a message

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in cases where "super"
is the receiver.

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What is "super"?

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Take a few minutes to think of
a definition covering these two points:

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What does "super" represent?

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How is a method looked up

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when a message is sent to "super"?

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The following principle

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is the same in both Pharo and Java.

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The example you see here

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is similar to the ones we saw
in earlier sequences.

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"Super" is used in one method.

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Let's think about what happens.

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For A new bar,

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there's no problem.

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It's the same process as before.

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The method is run,

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and foo returns 10.

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In the case of B new bar,
let's look at each step.

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We send the message "bar"

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to the B new object.

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The lookup algorithm
seeks the method "bar"

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starting with the object class.

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It finds the method "bar."

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It is executed on the receiver.

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This "bar" method is the one
executed on the receiver.

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Now I have to compute
"super bar"

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and "self foo," and add them together.

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We can do "self foo,"

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"self foo" sends the message
"foo" to the object "self."

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"Self" is my B new.

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The only "foo" possible
is the one in class A.

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So "self foo" returns 10.

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In "super bar,"

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"super" is the receiver.

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But the lookup algorithm changes

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in the presence of the receiver "super."

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It will seek the method "bar"

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starting with the super class
containing the keyword "super."

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"Super" is located in the "bar"
method of class B.

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We'll be looking for method "bar"

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starting in B's superclass, which is A.

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So we find this method
and that is the one executed.

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Foo is sent to self.

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"Self" is always the receiving object.

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It is B new.

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So foo is sent to the initial object.

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This is the "foo" that is executed,
returning 10.

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10 + 10 = 20.

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In the case of C new bar,

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we note C new, here.

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It is an instance of class C.
We send it the message "bar."

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"Bar" is sought in class C new.

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Not found.
The class above is searched.

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It is there, in B.

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So the B new bar will be executed.

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This bar is the sum
of two message-sendings.

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We can start with the second.

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We send the message "foo"
to the object "self."

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Self is still my C New, here.
I send the message "foo."

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The method is found, and 50 is returned.

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I put the value 50 there.

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Now we need the first part
of the sum, "super bar."

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I send the message "bar"
to "super," always the receiver.

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But the lookup algorithm changes.

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It looks for "bar" in the superclass
for the class containing "super."

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That is, "bar" method in class B.

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The algorithm starts looking up "bar"
in B's superclass, which is A.

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Bar is located here.

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This "bar" is executed.

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"Self" is still C new. Still this one.

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Always the receiver.

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We send it the message "foo."

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The result is 50, and I have 50 here.

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50 + 50 = 100.

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The next slide

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reviews the details
of the lookup algorithm process,

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for further study.

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"Super" always refers to the receiver.

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It's exactly like "self,"

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or "this" in Java.

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Likewise, "super" in Java is like "this,"
and always refers to the receiver.

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However, when a message is sent
to "super," the lookup algorithm changes

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and begins searching for a method
in the superclass

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of the class containing the method
being executed.

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"Self" and "super" are therefore
fundamentally different.

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"Self" is dynamic,
while "super" is static.

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I'll explain that further.

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When "foo" is sent to "self,"

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the developer has no idea
which "foo" method will be executed.

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It could be the "foo"
located in the same class,

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or a "foo" in an existing subclass,

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or a "foo" in a subclass
created by another developer

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before the program is run.

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When the "bar" method developer
writes "self foo,"

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he has no idea which "foo"
will be executed.

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That's a convenient feature.

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It means developers
can create a new subclass

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to change the behavior of class A.

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Conversely, "super" is static.

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When the developer writes "super foo,"

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he knows which "foo" method
will be executed

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when the program is run.

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When he writes "super foo" here,
he knows he is referring

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to "foo" in superclass A.

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Invariably. Super is static.

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We know how the program
will be compiled.

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Unfortunately, certain books

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define "super" wrong.

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The definitions make no sense.

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Here is a definition we found in a book.

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It says that "super"
looks for the method...

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In other words,
"super" prompts the lookup algorithm

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to search for the method in
the superclass of the receiver's class.

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The superclass of the receiver's class.

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Actually, this is wrong,
as proved by the example shown.

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If you take aC as the receiver,
its class is C.

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The receiver's superclass
is therefore B.

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If I come down here
and execute the command "super foo,"

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if the definition given by the book
applied,

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we would send the message "foo"

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to "super," and the "foo"
method executed

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would be the one in the superclass
of the receiver's class.

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In other words, this one.

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It would send "foo" to "super"

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over and over again,
in an infinite loop.

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So the definition is wrong.

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In practice, this example functions
perfectly.

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It's just that this definition is wrong.

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Here's what to remember:

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"Self" always represents the receiver.

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So does "super."

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It's the same in Java,
with "this" and "super."

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However, "super" changes the lookup.

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It will start looking for a match

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in the superclass

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of the class containing the method
that contains the keyword "super."

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"Self" sends are dynamic.

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The developer can use them to extend
the behavior of an existing class,

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by creating a subclass
that redefines the method.