This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
Unless you use the BigDecimal class, there is no such thing as a "format" for a number. It is stored as 32 or 64 bits, which represent the value (or as close as possible) and this doesn't change. You can display the number in any format, probably best done with the % tags in the printf() Formatter#format() and String#format() methods. Start reading in the Java™ Tutorials.