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Abstract Methods and Classes
3:48Creating and using abstract classes can feel a little abstract when you're first learning about them. We'll step through defining an abstract class with it's requirements and the errors you may encounter.
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.
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