WEBVTT

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Hello, everyone.

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Welcome to Sequence 1 of Week 4.

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This week, we'll be covering
Pharo inheritance basics,

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and look-up
and web-development mechanisms.

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In the first sequence,

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we'll introduce inheritance basics.

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These concepts are quite similar

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to inheritance
in other languages with classes,

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like Java.

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Even if you are familiar
with these basics, please watch.

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We'll introduce some vocabulary,

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and a graphic representation
you'll be seeing again

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throughout the rest of the course.

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This sequence will cover
what inheritance is,

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and how to use it.

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Two classes are represented
on the right:

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the Rectangle class,
with its name, first of all,

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its instance variables,

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and its methods.

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Under it, you see the same thing.
The class name, ColoredRectangle,

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its instance variables, and its methods.

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The vertical arrow here, with
its point represented as a blank triangle,

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indicates an inheritance relationship.

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The ColoredRectangle subclass
inherits from the Rectangle class.

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Generally, subclasses like
ColoredRectangle

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refine the behavior and state
of their superclass.

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A subclass like ColoredRectangle
can add state and behavior

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to its superclass.

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Here, the ColoredRectangle class
adds state

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in the form of two instance variables,
color and border color.

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It also adds a method: color.

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At some point, we could say
that every class

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inherits from the Object class.

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But that's not quite true.

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It is true that every class
inherits from ProtoObject class.

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The ProtoObject superclass
is used for very special cases.

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For the purposes of this course
in Pharo,

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we'll say that all classes inherit
from Object.

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That will suffice, for our needs.

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The ProtoObject class is reserved
only for certain specific cases.

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You're not likely to need it
to learn the basics of Pharo.

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Inheritance behaves
in two different ways,

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depending on whether state
or behavior is being inherited.

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Inheritance of state is static.

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That means that
when you create the subclass,

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you know exactly what
all the instance variables are.

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However, inheritance of behavior
is dynamic.

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During the program's execution,
you look at the methods

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defined in sub and superclasses.

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Concerning instance variables,

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inheritance happens
during class definition.

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When you define the subclass,

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you take the instance variables
defined in the subclass -

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here, they are color and border color,

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and you merge them
with all the instance variables

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defined in the superclass
and the line of the superclasses,

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all the way up to the Object.

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Here, the instance variables
of ColoredRectangle

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will be with a color and border color.

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For behavior,
inheritance mechanism changes.

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Watch the following sequences

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to see exactly how it occurs.

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So, on the whole,

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behavior inheritance happens at runtime.

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In other words, when you send
a message to an object,

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a method match is searched for
in the class hierarchy.

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Here, if I send a message
to ColoredRectangle...

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The "area" message, for example...

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it will be searched in the class.

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The method will be searched
in the ColoredRectangle class.

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The "area" method is missing.

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So we go up to the superclass.

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We find the "area" method.
It is selected and executed.

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Now, I'll sum up:

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For inheritance, the concepts
of subclass and superclass

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enables the subclass
to refine the behavior

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of its superclass, and extend it.

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The subclass can also extend, or add,

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state to its superclass.

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Pharo's simple inheritance model
stipulates one superclass per class.

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The concept of traits
enables you multiply inheritance,

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but we will not cover that.

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Object is at the top of the hierarchy.

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So is ProtoObject,
but we will not go into that detail.

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Inheritance of state is static.

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It happens when the subclass is defined.

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But inheritance of behavior is dynamic,

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happening at runtime,

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whenever you send a message
to an object.