Code Reuse By Inheritance

To me, inheritance has two properties:

• Defining an is-a relationship


• Making it possible to share code between classes by extending from a common base class

The is-a relationship between classes is one thing I like to use inheritance for. So, to me, a concrete Cache - for example a MemcacheCache implementation - extending from an abstract Cache base class is a very sane thing to do. Implementing an interface means, in my opinion, adding an additional usage aspect / feature to a class, while the base class defines the type of a class. Mind: The base class, most likely, only defines abstract methods and does not provide any implementation in this case. I discussed this in more detail in a dedicated blog post, which is why I skip a detailed explanation now.The other thing you can use inheritance for is to share code between multiple classes. A prime example for this is Active Record, where a concrete record class (for example a BlogPost) extends from a base class which adds the database access logic.I personally think that the latter is one of the worst things you can do in object oriented design. Bear with me for a moment and let me try to explain why I think that.

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