This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Trial and error is one way to find out the answer to these questions, but it can lead to misunderstandings and bad assumptions.
The best way to find out what is allowed in the language is to read the Java Language Specification, which is available on-line for free. For instance, here is the section on interfaces, which tells you what is and is not legal.
By the way, while what you called a "pure abstract class" (a class with no concrete methods) is legal in Java, it is rarely useful. Why would you not want to use an interface? Perhaps you have come from C++, where interfaces are not a separate kind of thing and are instead represented as "pure-virtual classes". [ July 31, 2007: Message edited by: Peter Chase ]
Betty Rubble? Well, I would go with Betty... but I'd be thinking of Wilma.