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Abstracting classes

From: Object-Oriented Programming with PHP

Video: Abstracting classes

Visibility scope is not the only object-oriented way to restrict access. PHP5 also introduced the concept of abstract classes and methods. If a class is defined as abstract, it cannot be instantiated. If you define a method as abstract, then any class that extends that abstract class containing the method must also declare a method with the same name and arguments. Further, if a class has an abstract method, then the class itself must be abstract.

Abstracting classes

Visibility scope is not the only object-oriented way to restrict access. PHP5 also introduced the concept of abstract classes and methods. If a class is defined as abstract, it cannot be instantiated. If you define a method as abstract, then any class that extends that abstract class containing the method must also declare a method with the same name and arguments. Further, if a class has an abstract method, then the class itself must be abstract.

This solves the problem of the generic address class, and provides a mechanism to specify the behavior of any child classes. Open the address class, scroll to the top, and add the word "abstract" at the beginning of the class declaration. Save the address, then switch to your browser, and refresh the demo. At the bottom, the attempted instantiation of the generic address class has been disallowed, and caused a fatal error. This is a good thing, as it forces the developer to be specific. Edit the demo, and re-factor the second address instance to be a business address, including the variable name. AddressBusiness. address_business for the variable name as well. And then, add a var_ export at the end, so we can inspect the contents. Save the demo, go back to the browser, and refresh.

The business address is now displayed correctly. However, there is a data inconsistency; it's the correct class (AddressBusiness), but the _address_type_id is not set. Missing property values is a common development problem. When working with multiple similar objects, there can be a lot of easily forgotten details to keep track of. The solution is to initialize objects automatically, so you don't have to remember to do it, but what's the best way to initialize them? Your first reaction may be to make the construct magic method abstract. This would technically work, but you'd end up with a lot of copied and pasted code, as everything in construct would have to be duplicated.

This can easily lead to a fragile system, where re-factoring or adding new functionality becomes cumbersome. It's not a best practice, and not very object-oriented. Instead, I'm going to construct the object in the parent, then call an abstract initializer. This way, common behavior in the parent's construct method always executes upon instantiation. Then, custom behavior can be put in each child's initialization method. Neat! With these relationships, visibility is also a factor, and abstract method visibility can be a bit tricky.

Methods that implement an abstract method will also need to have the same scope, or something a little bit less restrictive. If an abstract method is declared as public, I won't be able to change it to private. However, the opposite is true. I can make an abstract private method public. It's a one-way change. You can relax the scope restriction, but not make it stricter. This prevents a situation when you're expecting to be able to do something, but for some strange reason, you are prevented by scope.

To practically apply this to the addresses, I'm going to require that extending classes set the address type id upon creation. Let's open the Address class. After the magic method toString, define a new abstract protected method called init, abstract protected function _init(). Force extending classes to implement init method, then add a call to the initialization method at the beginning of the constructor. $this->init. Save the address class, then go to your browser, and refresh.

You will see an immediate error. The AddressResidence class contains an abstract method that needs to be defined. So, open the AddressResidence class, and declare the private function initialized. protected function _init(). Set the address type id to the constant from the address class, using your validating method: $this-> setAddressTypeIDAddress::ADDRESS_TYPE_RESIDENCE. Add some documentation, and copy the contents of this method, and paste it into the business address. ADDRESS_TYPE_ BUSINESS. And, again for the Park. Save, then return to your browser, and refresh.

You'll see that the _address_type_id for the business is now set correctly. However, a redundancy has now been introduced with the potential for corruption. You can still set the _address_type_ id manually, and if you look at the demo, AddressResidence is doing just that. You can just remove the _address_type_id, but that treats the symptom, not the problem. Return to the address class, and navigate to the magic set method. Remove the special case for _address_ type_id. Save, then refresh the demo.

You will see a notice for undefined or unallowed property. Data corruption is now prevented. Edit the demo, and remove the now- broken attempt to set the _address_type_id. Then save, and refresh. The notice is now gone, and execution is error-free. In the next segment, I'm going to demonstrate how to implement a common interface across all these child address classes without having to know what kind of an address type it is.

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This video is part of

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Object-Oriented Programming with PHP

37 video lessons · 17736 viewers

Jon Peck
Author

 
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  1. 4m 10s
    1. Welcome
      39s
    2. Exercise files
      2m 16s
    3. What you should know
      1m 15s
  2. 7m 47s
    1. What is an object?
      2m 6s
    2. What is a class?
      2m 7s
    3. Why should you use object-oriented programming?
      2m 14s
    4. The history of object-oriented PHP
      1m 20s
  3. 16m 40s
    1. Defining a class
      2m 58s
    2. Defining class properties
      3m 26s
    3. Creating a method and exploring object context with $this
      2m 50s
    4. Instantiating an object and accessing its contents
      3m 19s
    5. Specifying the visibility scope
      4m 7s
  4. 15m 51s
    1. What is a magic method, and do I need one?
      2m 23s
    2. Overloading property access
      6m 37s
    3. Customizing object construction
      4m 34s
    4. Standardizing object rendering as a string
      2m 17s
  5. 20m 54s
    1. Using the static keyword
      3m 36s
    2. Leveraging scope resolution operators
      1m 10s
    3. Setting constant values
      2m 47s
    4. Implementing static methods
      5m 43s
    5. Creating a database class
      7m 38s
  6. 26m 19s
    1. Extending your class
      6m 8s
    2. Abstracting classes
      5m 57s
    3. Sharing interfaces using polymorphism
      4m 39s
    4. Overriding methods, properties, and constants
      3m 25s
    5. Cloning and comparing objects
      2m 51s
    6. Referencing objects
      3m 19s
  7. 14m 52s
    1. Leveraging standard class objects
      2m 42s
    2. Retrieving objects from the database
      5m 41s
    3. Error handling with exceptions
      2m 6s
    4. Customizing PHP exceptions
      4m 23s
  8. 8m 2s
    1. Identifying the singleton pattern
      1m 42s
    2. Using the factory method pattern
      1m 51s
    3. Implementing a strategy pattern
      4m 29s
  9. 2m 57s
    1. Looking forward to namespaces
      47s
    2. Next steps
      1m 6s
    3. Goodbye
      1m 4s

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