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You have completed C# Objects!
You have completed C# Objects!
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Let's use inheritance to make a subclass of the Point class.
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Let's create a subclass of the point class
to do that we need to create a new class,
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let's call it map location.
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So we'll create a new file
called MapLocation.cs in here
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we'll create a new class
called map location.
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We specify that the map location class
is a subclass of the point class by
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typing : Point after the name of
the class here, and there you go.
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We've just subclassed the point class.
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We need to do one more thing
to make this class usable.
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When a map location object is created,
we're creating a point object too.
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They aren't two distinct objects, it's
just that a map location is also a point.
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In the same way that when a mammal
is born, a vertebrate and
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an animal are also born because a mammal
is both a vertebrate and an animal.
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Remember that the point object
requires that we pass in both x and
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y coordinates to the constructor
in order for it to be created.
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So we still have to
satisfy those requirements
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because a map location is
just another type of point.
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When a map location object is created
it first needs to call the point class
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constructor and pass it what it needs.
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So the map location class will also
need a constructor that takes the x and
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y coordinates, so that it can pass
them to the point class constructor.
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Again, we name the constructor
the same as the class name.
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In this case,
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it will take the same parameters as
the constructor of the point class.
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Now that we have a map
location constructor,
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we need to tell it to call the point class
constructor and pass these parameters.
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We do that by typing : base here and
list the parameters.
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We use the base keyword here because the
point class is MapLocation's base class.
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You'll hear people refer to the base class
as the parent class or the super class and
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you'll hear people refer to the sub class
as the child class or derived class.
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We're calling the base classes constructor
here and passing it the values it needs.
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So now, any map location objects we
create will also be point objects,
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let's go to main to see what this means.
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Instead of using the point class here,
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let's change it to use
the map location class.
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To avoid any confusion I'll
rename this variable to x.
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Even though x is now a map location,
this line here will still work.
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The map location class doesn't have
a distance to method directly but
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it inherits it from the point class or
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you could say that x is both a point and
a map location.
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So it has all of the methods
of both classes.
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We can even assign map
location to a point.
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We can also assign it to a point
at the same time we create it.
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We can even pass MapLocation objects
into methods that are expecting a point.
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Remember the maps OnMapMethod,
that takes a point as a parameter?
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Let's pass it a MapLocation instead.
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Let's compile just to
make sure this works.
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See, no compiler errors.
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These all works because map
location is a subclass of point.
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We can say that the point and map location
classes have an is a relationship,
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in that map location is a point.
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There's even a way in C# to see
if an object is a certain type.
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The is operator returns true if
a variable is of a given type
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this is called a type check.
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Let's print out the result
of a few type checks.
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Let's first check to see
if x is a map location.
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Let's also check to see if x is a point.
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Finally let's create a new point object
and see if it's of type map location.
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What do you think the result
of this expression will be?
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Let's run it and see.
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As you can see,
x is both the map location and a point but
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point is not a map location.
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That would be like saying that
an animal is a vertebrate,
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when the only thing we know
is that it's an animal.
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