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.
The actual amount of memory that an object uses depends on the Java implementation -- i.e., it's not specified by the Java VM specification or the Java Language specification. The number is never directly useful in any program -- no Java language function or construct depends on the memory image size of an object. The precise answer may well vary by JVM vendor, by platform, and even by version.
That said, for Sun's JVMs a good rule of thumb is 16 bytes for an object, plus four bytes for each member -- except doubles and longs, which are 8 bytes each.