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.
An empty class with no fields must still extend another class. Say it extends object, it will inherit the methods of the Object class so must have a non-zero size. There has been discussion on JavaWorld about object size such as Sizeof for Java. Playing around with the example program listed in that article suggests that an empty instance just like an object instance occupies eight bytes.