Inheritance and Other Advanced OOP Features - Abstract Classes
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An abstract class is a class that really isn’t supposed to ever be instantiated, but instead serves as a base class to be inherited by other classes. For example, consider a class titled Media, intended to embody the common characteristics of various types of published materials, such as newspapers, books, and CDs. Because the Media class doesn’t represent a real-life entity, but is instead a generalized representation of a range of similar entities, you’d never want to instantiate it directly. To ensure that this doesn’t happen, the class is deemed abstract. The various derived Media classes then inherit this abstract class, ensuring conformity among the child classes, because all methods defined in that abstract class must be implemented within the subclass.
A class is declared abstract by prefacing the definition with the wordabstract, like so:
abstract class classname { // insert attribute definitions here // insert method definitions here }
Attempting to instantiate an abstract class results in the following error message:
Fatal error: Cannot instantiate abstract class staff in /www/book/chapter06/class.inc.php.
Abstract classes ensure conformity because any classes derived from them must implement all abstract methods derived within the class. Attempting to forego implementation of any abstract method defined in the class results in a fatal error.
ABSTRACT CLASS OR INTERFACE?
When should you use an interface instead of an abstract class, and vice versa? This can be quite confusing and is often a matter of considerable debate. However, there are a few factors that can help you formulate a decision in this regard:
If you intend to create a model that will be assumed by a number of closely related obects, use an abstract class. If you intend to create functionality that will subsequently be embraced by a number of unrelated objects, use an interface.
If your object must inherit behavior from a number of sources, use an interface. PHP classes can inherit multiple interfaces but cannot extend multiple abstract classes.
If you know that all classes will share a common behavior implementation, use an abstract class and implement the behavior there. You cannot implement behavior in an interface.
Please check back next week for the conclusion to this series.
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