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.
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.