TechFAQ

* interface contains methods that must be abstract; abstract class may contain concrete methods.
* interface contains variables that must be static and final; abstract class may contain non-final and final variables.
* members in an interface are public by default, abstract class may contain non-public members.
* interface is used to "implements"; whereas abstract class is used to "extends".
* interface can be used to achieve multiple inheritance; abstract class can be used as a single inheritance.
* interface can "extends" another interface, abstract class can "extends" another class and "implements" multiple interfaces.
* interface is absolutely abstract; abstract class can be invoked if a main() exists.
* interface is more flexible than abstract class because one class can only "extends" one super class, but "implements" multiple interfaces.
* If given a choice, use interface instead of abstract class.