The only reason to extend the interface or implement another interface is if you need to provide additional methods.
I dont see any problem to have a classe with moore than 500 lines of code.
could you edit your first post and place the line you copied from the instructions.html between quote-tags (instead of code-tags), it will increase readibility of that line (and this thread).
Secondly you could create (if you want) a DBAccess class which accesses your database file and reads/writes records. Your Data class will have an instance of this class and the read (and other) method(s) of your Data class are just delegated to the DBAccess instance. As long as your Data class implements the given interface (directly or via an own created interface), it will be
Your data access class must be called "Data.java", must be in a package called "suncertify.db", and must implement the following interface:
For instance, do you know what your GUIs are going to look like?
What user functions are you going to support?
Are you going to use RMI or Sockets?