When should I use an interface instead of an abstract class?
There are two primary axes of "inheritance" in object-oriented languages like Java. "Implementation" inheritance is where the sub-class inherits the actual code implementation from the parent. "Interface" inheritance is where the "sub-class" adheres to the public interface of the "parent".
Alas, Java actually mixes the two notions together a bit... Java interfaces are nice and clean -- when you "implement" an interface, you are stipulating that your class adheres to the "contract" of the interface that you specified. Java class inheritance isn't so clean -- when you sub-class in Java you are getting both the code inheritance but you are also stipulating that your sub-class adheres to the "contract" of the interface of the parent class.
So, it's generally considered good OO practice to specify the "contract" which you want to adhere to via Java interfaces. Then use normal Java class inheritance primarily for code reuse purposes. Use abstract Java classes when you want to provide some standard base code but want/need to force the user's of your class to complete the implementation (i.e., you create a skeleton implementation and the sub-classes must flesh it out).