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Hello. Welcome to the fourth sequence
on inheritance and lookup.

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We will now learn what happens when
the algorithm does not find a method.

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As you will see,

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the class codes
can add processing in this case,

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to handle the fact
that a lookup message

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was not understood.

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I'll review message-sending briefly:

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The "area" message is sent
to ColoredRectangle.

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First, the algorithm looks up
the method matching the message.

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It starts by searching for the method
in the receiver's class:

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If it finds "area" in ColoredRectangle,
it executes it.

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But "area" is not there,
so the algorithm searches the superclass

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In "Rectangle," it finds "area."

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The method is executed on the receiver.

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What happens in the total absence
of an "area" method?

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What if the lookup fails?

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Here is an example: a "coucou" message
is sent to Object Node 1.

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"Coucou" cannot be found
in the hierarchy.

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The "coucou" message is sent to Node 1.

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The algorithm fails to find "coucou"
in "Node."

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Next, the algorithm searches
"Object." No "coucou"

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At that point,

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the Pharo virtual machine
sends a "does not understand" message

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to the receiver - Node 1.

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DNU = Does Not Understand.

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It transfers the initial message
as a parameter.

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This is the parameter: "coucou stef."

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If I go through it again step by step,

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"Coucou" is sent to Node 1.
A match is sought in the class,

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then in the superclass.

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"Coucou" is not there, so the message
"does not understand" is resent

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to Object Node 1.

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The algorithm looks up the method
"does not understand."

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in Node 1 class.

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Not finding a match there, the algorithm
searches the superclass.

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It is located there.

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A "does not understand" method
is found in Object and executed.

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That means

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in all the subclasses of Object,

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"does not understand"
can be implemented

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to trigger a specific behavior

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when an object, instance, or class
does not understand a message.

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This slide shows the details,

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step by step.

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"Does not understand" is a message
like any other.

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Therefore, you can implement
a "DNU" method to be executed

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whenever an instance
does not understand a message.

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This mechanism is used

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in certain somewhat complicated cases

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like proxies and automatic delegation:

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when you want a certain object
to transmit all the messages it gets

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to another object.

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The "does not understand" mechanism
can be used for that.

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Default implementation
of "does not understand" is found

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in the Object class.

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It raises one exception.

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The exception

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is called "Message Not Understood."

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So if the "DNU" method
of the Object class

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is executed, an exception is raised.

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That means the code you are writing
can catch the exception,

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the way it would catch any exception.

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The exception opens a debugger
for messages that are not understood.

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In this example,

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we return to the class "Node."

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We have a "say hello" method in Node,
which sends a "coucou" message

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to the receiver or self.

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There's also a "welcome" method

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that sends a "say hello" message
to self.

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It provides special handling
in the context

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of a misunderstood message.

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If a message is not understood,
it will be handled.

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Here, "say hello"
sends a "coucou" message to self.

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"Coucou" is still unimplemented.

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Ultimately,
the "does not understand" method

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will be executed.

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The "message not understood"
exception will be raised.

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The code here, following on/do,

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is used to catch exceptions,
like "try/catch" in Java.

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A later sequence of the course
will go into these exceptions.

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The point here is to notice
that this code

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is the same as Java's "catch," and
it will be executed for every exception.

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Say we send the message "welcome"

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to Node 1.

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Knowing that "coucou"
is not implemented,

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the console will display "something
went wrong with the message."

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What you should know:

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When the lookup algorithm does not find
a method to match a message,

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a "does not understand" message is sent
to the initial receiver,

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with the first message as a parameter.

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Each class can implement this method.

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The default implementation in the Object
class will be executed.

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This default implementation
raises an exception,

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called "message not understood."

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The code you are writing can catch
the exception

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and subject it to debugging.