abstract classes and interfaces best practices in java

Question

So you\'ve got an interface and an abstract class that implements a subset of the methods in the interface. You\'ve also got some classes that inherit the abstract class and giv

Answer 1

Interface methods are implicitly abstract, so if an abstract class implements an interface, there is no need to satisfy the interface contract in the abstract class. An abstract interface is technically legal, but also redundant. It is fine for an abstract class to implement an interface as long as all subclasses would otherwise implement the interface, but the abstract class should not contain the implemented methods.

Interface methods are implementation specific (unique to the implementing class), whereas abstract states and behaviours are meant to be common, or shared amongst implementations. It would be an oxymoron to implement interface methods as common functionality.

The key question is what do you plan on doing with the interface implementations? If the base interface isn't used for anything besides defining an additional contract, I would say simply add abstract methods to the abstract class, otherwise the interface is redundant overhead. If you find a use case where you would want access to the interface methods, but not necessarily the common functionality, then maybe an interface is worthwhile.