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[MUSIC]

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Let's quickly recap what we know so
far about object oriented programming.

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With inheritance,
we can create a subclass of a class.

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In our project,
we've created many subclasses of Invader.

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We have FastInvader StrongInvader and
ShieldedInvader.

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Objects of type
FastInvader StrongInvader and

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shieldedInvader are also
objects of type invader.

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We also learned about interfaces.

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The public methods and properties of
a class are known as its interface.

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This is how other classes
interact with this class.

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In the case of an Invader, other external
classes can get the Invader's location.

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See if it HasScored.

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Check its health.

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See if it has been neutralized.

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Check if it's still active.

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Ask it to move down the path and
decrease its health.

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All of these methods and properties
are defined in the Invader class.

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And because these other types of Invaders
inherit from this base invader type,

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they also have the same interface.

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We could also add additional public
methods to these subclasses, but

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that would just be adding more
members to the public interface for

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that specific subclass.

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So in object oriented programming,

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we expose a public interface which other
classes use to interact with the class.

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We can override how each
of these properties and

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methods behave in subclasses
using polymorphism.

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That's the purpose of the virtual and
override keywords in C#.

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So long as these properties and

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methods behave as expected,
we can implement them however we want.

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That's the idea behind encapsulation.

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We don't usually need to know what other
private members there are in a class or

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exactly what the code for
the public members looks like.

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We only need to know that it takes our
input and gives us the expected output.

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That's why all the code that already
works for Invader will also work for

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FastInvader, StrongInvader and
ShieldedInvader as well.

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Thinking about software in terms
of objects and their interfaces

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greatly simplifies the way we think about
architecting a software application.

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It also makes it easy to
extend an application.

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Our new types of objects only need to
be sure to implement the interface that

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other parts of the software
application expect to use.

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Thinking about an object in
terms of its public interface

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is also known as abstraction.

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Abstraction is the fourth and final core
principle of object-oriented programming.

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When architecting a software application,
we don't need to think so

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much about the concrete
implementations of the class.

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We only need to think about
them in abstract terms.

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For example, we don't need to know what
the initial health of an Invader is or

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even that there's a StrongInvader that
has more initial health than other types

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of invaders.

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We just need to know that we can ask
an Invader how much health it has.

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When deciding about what
an objects interface should be,

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we need to think about
it in abstract terms.

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For example, what is the bare
minimum set of methods and

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properties that makes an invader,
an invader?

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This gives us the most freedom when
it comes time to create specific

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types of invaders.

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This all seems a bit
abstract right now and

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it's hard to understand what all this
means without seeing some real examples.

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Let's see what this means by
refactoring our Invader class a bit.