WEBVTT

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Hello. Today we are going to examine
a very interesting subtlety

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that you can apply in your programs.

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To start with, we'll quickly review
the dice program.

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If you recall, in this exercise
we defined the dice class in order

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to define instances of this class
and represent various dice faces.

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We defined a class called DiceHandle.

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Then we implemented
the "plus" method

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on the DiceHandle class,

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to add DiceHandles
and construct a new DiceHandle.

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This is the code for this class.

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We have DiceHandle new.

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We create a new DiceHandle,

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to which we add the dice
of the receiver class

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and the setting,

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and we return this handle
at the end of the method. Okay?

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In this sequence we will see
the difference,

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in this "+" method,
between writing DiceHandle new

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and self class new.

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And we'll see why
one technique is better.

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Here is the problem.

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If I create a DiceHandle subclass
called MemoDiceHandle,

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and I want to add MemoDiceHandles,

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with the "+" method,
which is inherited from DiceHandle,

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the result is an instance
of the DiceHandle class.

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Not of the MemoDiceHandle class.

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This is a problem
because when we add elements,

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we want to get
the same type of element back.

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How do we do this?

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Solution No. 1.

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I change the implementation

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of the "+" method
on the DiceHandle class.

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The "+" method, okay?

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And I simply add this method:
self handleClass.

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Instead of defining
the instance variable I wish to obtain,

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I use a method
that I call handleClass.

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In DiceHandle,
the DiceHandle class is returned,

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and in the MemoDiceHandle subclass,
MemoDiceHandle is returned.

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So by simply
redefining the handleClass method,

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we obtain the correct instance
when we add elements.

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If we execute the same code,

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using MemoDiceHandle instead,
we get an instance of MemoDiceHandle,

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because I redefined
the handleClass method

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in the MemoDiceHandle class.
We got an instance of the subclass.

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So we attained our objective.

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The problem is that

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each time we add a subclass
to DiceHandle,

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we must redefine
the handleClass method.

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If you don't, the "+" method will never
return the correct class instance.

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This is tedious.
You must constantly redefine the method.

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There are better ways.

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Here's Solution No. 2.

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Using the "+" method in DiceHandle,
this time we will write:

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self class new.

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I'll ask the current receiver
for its class,

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in order to get an instance.

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In fact, self class always returns
the class of the receiver.

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If it's the instance of a subclass,

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it will return the correct class.
For example, MemoDiceHandle.

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We get instances of the same kind
as that of the receiver.

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As a conclusion,

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as you saw,

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when you send the message "+"
to a DiceHandle,

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if you set the name of the class
you want an instance for,

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in a sense you prevent yourself
from creating subclasses.

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But with self class new, "+" returns
an instance of the receiver.

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Of the potential subclass.

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So it's much better to use
self class new,

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to directly get

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an instance of the class
according to the type of receiver.