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[MUSIC]

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In the first part of this course we
wrote a class that models a point.

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It has x and y fields and

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even has a method that calculates
how far it is from another point.

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The Treehouse defense game uses this
class to identify locations on the map.

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But it could be used for
a lot more than that.

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Lots of software applications use x and
y coordinates to identify a location.

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Perhaps the next game we write
will also need a point class.

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Software that displays charts and
graphs also needs points.

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If we keep our implementation of the point
class as general purpose as possible,

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we can reuse it in all those
different applications.

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What if we need to add something
to the point class that's specific

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to Treehouse defense?

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For example what if we wanted to make
it impossible to create a point that

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isn't on the map?

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Making a change like that to the point
class would make it unusable to

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all other applications
that don't have maps.

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We don't want to add code to
a general purpose class that we're

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fairly certain won't be useful
to other users of the class.

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We also don't want to create a new class
that's specific to Treehouse defense and

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rewrite everything we wrote for
the point class.

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What we want is to create a new class
that has our new functionality, but

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also reuses the point class.

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The principles of
object-oriented programming

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provide lots of ways to reuse and
extend existing classes.

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One of them is called inheritance.

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There are four core principles
of object-oriented programming.

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Encapsulation, inheritance,
polymorphism and abstraction.

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By saying that C# is an object-oriented
programming language, we're really saying

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that C# has features built into the
language to support these four principles.

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As you can see, inheritance is
one of the four core principles.

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Don't worry about these other
three principles right now.

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We'll discuss them later.

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The principle we want to focus
on right now is inheritance.

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You see,

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many objects in the real world often share
many of the same attributes and behaviors.

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A classic example is an animal,
all animals have similar characteristics.

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For example, a defining characteristic
of all animals is the ability to move.

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There are two types of animals,
vertebrates and invertebrates.

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Vertebrates have backbones and
invertebrates don't.

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But they're both still animals,
so they can both move.

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Mammals and
birds are types of vertebrates.

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By saying that a creature
is a mammal we're

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also saying that it has a backbone and
it can move.

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You might say,

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that it inherits these characteristics
from the larger categories,

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or classifications, of vertebrates and
animals of which it's a member.

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You can see now that the C# key word
class is short for classification.

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C# classes define what it means to
be in a particular classification.

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As we saw with the animal example,
classes of animals

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can have more refined classifications
within them, we call these subclasses.

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Subclasses inherit the attributes and
behaviors of the more general classes.

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So if we wanted a new type
of point that inherited

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all of the features of the point class,
but

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was more specific to our application, we
just need to create a subclass of point.

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Let's look at how we do that in code.