Interface Vs Abstract Class (Wiki forum at Coderanch) (original) (raw)

Note: This page should be updated to reflect Java 8's interface default methods.

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.
• use an interface if you model a system that needs not implementation inheritance but behavioral inheritance (inherit behaviour i.e. methods headers but not implementation/code as can be in abstract classes)

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:

• http://mindprod.com/jgloss/interfacevsabstract.html
• http://www.javaworld.com/javaworld/javaqa/2001-04/03-qa-0420-abstract.html
• http://www.javaworld.com/javaworld/javaqa/2001-08/03-qa-0831-interface.html