Saturday, 11 August 2012

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.