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.
The common consensus is that you should not modify the provided interface. An interface is a contract between you and the users of your classes. If you modify the interface, you are changing the contract. If the user has written code based on your expected interface then their programs will not work. (To put that in more concrete terms, you possibly have a statement in your instructions telling you that when your assignment is being marked, "software checks that overall structure and nomenclature conform to specification" - this automated testing could fail if you change the interface.)