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.
Language. It contains the classes that are essential in just about every Java program, like Object, String and Thread. (The latter is always used, even if you don't know it. There's always the main thread.)