Alternatively, you might have a diverse set of classes that you want to work with -- creating the right inheritance tree for all of those classes could be difficult. Interfaces are the better answer.

Most good designers avoid implementation inheritance (the extends relationship) like the plague. As much as 80 percent of your code should be written entirely in terms of interfaces, not concrete ...

PremiumCustomer and DeadBeatCustomer objects can be accessed with a variable declared as Customer if both classes inherit from Customer. Interfaces provide another, more flexible way for different ...

Emphasis is put on the use of abstract base classes, rather than on inheritance from a concrete base class. For a C#, VB.NET, or Java developer, this means interface inheritance.