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Abstract classes are classes that can't be instantiated - the can only be subclassed.
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So far we have four types of invaders.
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We have a basic invader
which we just call invader.
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We also have fast invader,
strong invader, and shielded invader,
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these last three types of invaders
are subclasses of our basic invader.
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This is sort of odd,
it makes more sense for
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our basic invader to just
be another type of invader.
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We can define a base class that defines
what it means to be an invader, but
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isn't actually a type of
invader that can be created.
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This is called an abstract base class.
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We can create an abstract base class
by adding the key word abstract here,
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in front of the word, class.
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Now if we try to compile or program,
we'll get a compilation error.
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It says it cannot create an instance
of the abstract class or
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interface TreehouseDefense.Invade.
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By adding the keyword abstract
to the class definition,
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we've said that this class is no longer
a concrete implementation of an invader.
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It only provides an abstract definition
of what it means to be an invader.
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We can no longer instantiate an invader,
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we can only subclass it to create
concrete implementations of it.
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Fast invader, strong invader and
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shielded invader are concrete
implementations of the invader type.
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We still want the ability to
create a basic invader, so
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we'll need to define a concrete
implementation of one.
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Let's create a new file
named BasicInvader, and
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copy the definition from
FastInvader into it.
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We'll change where it says
FastInvader here to BasicInvader.
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And we'll delete this
StepSize property for now.
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Now we have a concrete implementation
of our BasicInvader type again.
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We've named it BasicInvader
because the name Invader
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is already being used by
the abstract base class.
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Oftentimes, developers
will append the word
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base to the end of
the abstract base class name.
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So in this case we could rename
our class to InvaderBase, this is
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just a matter of personal preference, so
I'm going to leave it as Invader for now.
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Over in Game.cs, we're still
trying to instantiate an Invader.
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This is what is causing our
compilation error here,
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we need to change where it says
new Invader to new BasicInvader.
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By adding the abstract keyword to
the invader class and creating the basic
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invader class, we haven't actually
changed anything about the game.
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This is just a refactoring of the code,
we've refactored it to communicate
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the purpose of the invader
base class more explicitly.
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By making the invader class abstract,
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we're telling other
programmers that our code for
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this game deals with the BaseInvader type
and they can create their own invaders,
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so long as they subclass the BaseInvader
and implement their own invader types.
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Their new types of invaders
will work in the program and
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existing code for the game.
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Now if we compile we have
a working program again.
3:07
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