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.
I think it contains only questions about it's more performant (but not thread-safe) brother: StringBuilder. So you need to know the most important methods of StringBuilder (StringBuffer has exactly the same methods, although each method is marked synchronized)