Abstract Methods and Classes

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