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There are three differences between an interface and an abstract class:

  • you can implement multiple interfaces at the same time, but only extend one class,
  • an abstract class is allowed to contain implementation (non-abstract methods, constructors, instance initializers and instance variables) and non-public members, and
  • abstract classes may be a tiny bit faster (or they may not.)

  • Actually the first point is the reason for the existence of interfaces in Java: to provide a form of multiple inheritance. In languages with multiple implementation inheritance, an interface would be equivalent to a fully abstract class (a class with only public abstract members).

    The above differentiation suggests when to use an abstract class and when to use an interface:

  • use an abstract class, if you want to provide common implementation to subclasses,
  • use an abstract class, if you want to declare non-public members,
  • use an abstract class, if you want to be free to add new public methods in the future,
  • use an interface if you're sure the API is stable for the long run
  • use an interface if you want to provide the implementing classes the opportunity to inherit from other sources at the same time.

  • In general, prefer interfaces if you don't need to use an abstract class, because they provide more design flexibility.

    11/29/07 Stan James: Added two bullets about the ability to add methods to an abstract class without breaking any clients. If you add a method to an interface, you break all implementations. This puts a different spin on "flexibility" in the paragraph above. This distinction did not occur to me on my own; I just ran across it in Kent Becks Implementation Patterns.

    See InterfaceVsAbstractClassDiscussion

    Other resources:


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