This is a very clear article on the difference between an abstract class and an interface. The article also provides examples of implementations for both.
Comparisons at a glance:
Feature | Interface | Abstract class |
Multiple inheritance | A class may inherit several interfaces. | A class may inherit only one abstract class. |
Default implementation | An interface cannot provide any code, just the signature. | An abstract class can provide complete, default code and/or just the details that have to be overridden. |
| Access Modfiers | An interface cannot have access modifiers for the subs, functions, properties etc everything is assumed as public | An abstract class can contain access modifiers for the subs, functions, properties |
Homogeneity | If various implementations only share method signatures then it is better to use Interfaces. | If various implementations are of the same kind and use common behaviour or status then abstract class is better to use. |
Speed | Requires more time to find the actual method in the corresponding classes. | Fast |
Adding functionality (Versioning) | If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method. | If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly. |
| Fields and Constants | No fields can be defined in interfaces | An abstract class can have fields and constrants defined |
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