Java Abstract Class vs Interface Pros and Con's

This a quick reminder about pros and con's of Java abstract classes and interfaces:

Interface

• A class can implement multiple interfaces
• An interface cannot provide any code at all
• An interface can only define public static final constants
• An interface cannot define instance variables
• Adding a new method has ripple effects on implementing classes (design maintenance)
• JAXB cannot deal with interfaces
• An interface cannot extends or implement an abstract class
• All interface methods are public

In general, interfaces should be used to define contracts (what is to be achieved, not how to achieve it).

Abstract Class

• A class can extend at most one abstract class
• An abstract class can contain code
• An abstract class can define both static and instance constants (final)
• An abstract class can define instance variables
• Modification of existing abstract class code has ripple effects on extending classes (implementation maintenance)
• Adding a new method to an abstract class has no ripple effect on extending classes
• An abstract class can implement an interface
• Abstract classes can implement private and protected methods

Abstract classes should be used for (partial) implementation. They can be a mean to restrain the way API contracts should be implemented.