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Designing Interfaces in PHP

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Abstract Methods and Classes

3:48 with Alena Holligan

Creating 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.

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    Creating and using abstract classes can feel a little abstract when you're first 0:00
    learning about them. 0:05
    The following code may be a bit confusing at first, but 0:06
    work through the example with me. 0:10
    And then if you still have questions when we're done, try watching this video again. 0:12
    Or reach out for help from the Treehouse community. 0:18
    Let's start in the collection class. 0:21
    To make any class an abstract class, you simply add the word abstract in front. 0:23
    Now, in index.php, let's try to instantiate and object from this class. 0:30
    New Collection. 0:40
    By adding the keyword abstract, 0:48
    you prevent the object from instantiating this class. 0:50
    That's good for our collection, because our collection class uses the entity 0:54
    property which is not set in the collection class. 0:58
    Let's remove this line. 1:03
    Now, let's go into post. 1:05
    This class extends the collection class, and 1:07
    overrides the constructor to set the entity. 1:10
    There's no easy way to require that the property is set. 1:13
    Instead, we can require that a method is defined using abstract method. 1:16
    Instead of relying on a child class to override the constructor, 1:22
    we can have the constructor call the set entity method. 1:26
    Right after we set the repo, we'll call $this->setEntity();. 1:31
    Like with an interface, we can define abstract 1:38
    methods to guarantee that these methods be defined in the child. 1:41
    Let's preview in the browser. 1:52
    Non-abstract method setEntity must contain a body. 1:55
    Because all interface methods are abstract, 2:00
    we don't have to use an abstract keyword when defining our method. 2:03
    Within an abstract class, 2:08
    all abstract methods must start with a keyword abstract. 2:09
    Although interfaces are only allowed to specify public methods. 2:15
    Abstract classes are allowed to specify both public and 2:20
    protected methods, but not private methods. 2:24
    If we try to define an abstract method as private, we'll get another error. 2:28
    Both private and protected methods are restricted to internal access only. 2:34
    However, private methods cannot be overridden by a child class. 2:40
    Because abstract methods must be defined in a child, this is a forced override. 2:44
    We cannot use private access because it cannot be overridden. 2:50
    Instead, we can use protected. 2:56
    Protected methods can be overridden by a child class, 2:59
    but they are restricted to internal access only. 3:03
    This is exactly what we want for our internal setEntity method. 3:07
    Were ready to update our post class. 3:12
    Extending an abstract class works exactly the same way as extending any other class. 3:14
    So we don't need to change anything there. 3:19
    However, since we have added another abstract method, we 3:22
    do need to make sure that our post class implements this new setEntity method. 3:26
    Instead of extending the constructor, we make this the setEntity method. 3:31
    Let's preview our site again. 3:36
    Great, we've cleaned up our errors and everything is working again. 3:38
    Now, let's take a closer look at combining interfaces and abstract classes. 3:42

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