There's no difference. The abstract keyword may be allowed in an interface declaration, but it doesn't have any effect because all interfaces are abstract anyway. Recommendation is never to use abstract in an interface declaration.
Betty Rubble? Well, I would go with Betty... but I'd be thinking of Wilma.
Hi, Interfaces are 100% abstract, that is they can NOT have implementation code, only abstract method declarations. They may define instance variables, however these must be public final and static, and i believe they are implicitly so. While and abstract class may define both abstraact method declarations and may have implemented methods.
As was pointed out, an Interface can not implement another interface, by default an interface is abstract, this is implicit so there is no need to be stated, and if i remember the book i am studying rightly, methods in an interface are implicitly public and abstract.