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Creating and using abstract classes can
feel a little abstract when you're first

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learning about them.

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The following code may be
a bit confusing at first, but

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work through the example with me.

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And then if you still have questions when
we're done, try watching this video again.

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Or reach out for
help from the Treehouse community.

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Let's start in the collection class.

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To make any class an abstract class,
you simply add the word abstract in front.

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Now, in index.php, let's try to
instantiate and object from this class.

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New Collection.

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By adding the keyword abstract,

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you prevent the object from
instantiating this class.

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That's good for our collection, because
our collection class uses the entity

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property which is not set
in the collection class.

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Let's remove this line.

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Now, let's go into post.

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This class extends the collection class,
and

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overrides the constructor
to set the entity.

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There's no easy way to require
that the property is set.

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Instead, we can require that a method
is defined using abstract method.

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Instead of relying on a child
class to override the constructor,

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we can have the constructor
call the set entity method.

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Right after we set the repo,
we'll call $this->setEntity();.

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Like with an interface,
we can define abstract

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methods to guarantee that these
methods be defined in the child.

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Let's preview in the browser.

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Non-abstract method setEntity
must contain a body.

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Because all interface
methods are abstract,

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we don't have to use an abstract
keyword when defining our method.

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Within an abstract class,

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all abstract methods must
start with a keyword abstract.

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Although interfaces are only
allowed to specify public methods.

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Abstract classes are allowed
to specify both public and

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protected methods, but
not private methods.

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If we try to define an abstract method
as private, we'll get another error.

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Both private and protected methods
are restricted to internal access only.

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However, private methods cannot
be overridden by a child class.

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Because abstract methods must be defined
in a child, this is a forced override.

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We cannot use private access
because it cannot be overridden.

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Instead, we can use protected.

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Protected methods can be
overridden by a child class,

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but they are restricted
to internal access only.

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This is exactly what we want for
our internal setEntity method.

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Were ready to update our post class.

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Extending an abstract class works exactly
the same way as extending any other class.

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So we don't need to change anything there.

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However, since we have added
another abstract method, we

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do need to make sure that our post class
implements this new setEntity method.

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Instead of extending the constructor,
we make this the setEntity method.

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Let's preview our site again.

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Great, we've cleaned up our errors and
everything is working again.

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Now, let's take a closer look at combining
interfaces and abstract classes.