WEBVTT

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Polymorphism and

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virtual overridable methods can be
difficult to understand at first.

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So let's work through another example.

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This time,

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we'll make a couple classes using
the animal example we discussed earlier.

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We'll do this in the C# repel.

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To start the repel,
just type csharp in the console.

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Let's have a Bird class that
contains a Move method.

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This method will simply print Birds fly.

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We'll make it virtual so
that we can overwrite it in subclasses.

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We'll make a subclass of
the bird class named Penguin.

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In the Penguins Move method,
we'll print, Penguins waddle.

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We need to remember to type override
here cuz this is the subclass.

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Now let's instantiate a Bird and
call its Move method.

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As expected, it says Birds fly.

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Let's instantiate a Penguin and
call its Move method.

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Again, as expected,
it prints Penguins waddle.

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Now let's instantiate a penguin but
assign it to a bird variable.

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Remember we can do this
because penguins are birds.

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Even though the variable is of type bird,

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the object it stores is
really of type penguin.

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Now let's see what happens when
we call Move on this variable.

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It prints Penguins waddle.

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Even though we're calling
Move on a bird variable,

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the Move method in the bird
class isn't being executed.

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Instead, it's the Move method in
the penguin class that was executed.

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The Move method in the bird class
was overridden by the Move method in

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the penguin class.

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This is a very powerful
feature of C# because

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we can write code that only
deals with bird variables.

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Later, as we add new types of birds,

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the code still works as expected
without making any changes.

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This is why we were able to add a new type
of invader without changing the logic

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of the game as it's written in
the play and fire on invader methods.

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The changes are contained
inside ShieldedInvader and

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the rest of the game treats it
like any other type of invader.

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We'll get lots of practice
using polymorphism to

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create new types in our game.

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Keep note as we go along that
the core logic of the game,

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as it's coded in the play method,
never changes.

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This is the power of polymorphism and
virtual methods.