This quick reminder about pros and cons 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.