There's currently some buzz around the size of certain Java objects. Here is some very good description of what you have to take into account to compute the (shallow) size of an java object.
I repost here the rules from my old blog at SDN, because I think they are a more compact description of the memory usage for java objects.

The general rules for computing the size of an object on the SUN/SAP VM are :

32 bit

Arrays of boolean, byte, char, short, int: 2 * 4 (Object header) + 4 (length-field) + sizeof(primitiveType) * length -> align result up to a multiple of 8

Arrays of objects: 2 * 4 (Object header) + 4 (length-field) + 4 * length -> align result up to a multiple of 8

Arrays of longs and doubles: 2 * 4 (Object header) + 4 (length-field) + 4 (dead space due to alignment restrictions) + 8 * length

java.lang.Object: 2 * 4 (Object header)

other objects: sizeofSuperClass + 8 * nrOfLongAndDoubleFields + 4 * nrOfIntFloatAndObjectFields + 2 * nrOfShortAndCharFields + 1 * nrOfByteAndBooleanFields -> align result up to a multiple of 8

64 bit

Arrays of boolean, byte, char, short, int: 2 * 8 (Object header) + 4 (length-field) + sizeof(primitiveType) * length -> align result up to a multiple of 8

Arrays of objects: 2 * 8 (Object header) + 4 (length-field) + 4 (dead space due to alignment restrictions) + 8 * length

Arrays of longs and doubles: 2 * 8 (Object header) + 4 (length-field) + 4 (dead space due to alignment restrictions) + 8 * length

java.lang.Object: 2 * 8 (Object header)

other objects: sizeofSuperClass + 8 * nrOfLongDoubleAndObjectFields + 4 + nrOfntAndFloatFields + 2 * nrOfShortAndCharFields + 1 * nrOfByteAndBooleanFields -> align result up to a multiple of 8

Note that an object might have unused space due to alignment at every inheritance level (e.g. imagine a class A with just a byte field and class B has A as it's superclass and declares a byte field itself -> 14 bytes 'wasted on 64 bit system).

Note that this is only the flat (=shallow) size of an object which is in practice pretty useless.

I will explain in my next post, how you can better measure how much memory your Java objects consume.