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.
Well, the safest thing I would suggest is not to touch provided file at all. Personally, I didn't change a single character in that file. For synchronization, I used locks in actual methods instead of making interface methods as synchronized.