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.
There is no simple method in Java for finding that out. There are good reasons why you don't want to know. Java is a high-level language and the sizeof operator which is found in C was excluded. That is because its only use is for programmed memory allocation and deallocation, both potent sources of subtle and dangerous errors.