Have you seen this? http://code.google.com/p/memory-measurer/wiki/ElementCostInDataStructures
It shows the memory cost per each entry for various common and not-so-common Java data structures. For example, have you wondered when you put() an entry to a HashMap, how much memory footprint you create, apart from the space occupied by your key and your value (i.e. only by internal artifacts introduced by HashMap itself)? There are the answers to such questions.
It also exemplifies the utility of my memory measuring tool, which I recently redesigned to decouple object exploration from measuring the memory usage (bytes) of objects, so I can use the same exploration to derive more qualitative statistics, like number of objects in a object graph, number of references, and number of primitives (which would directly reflect memory usage if it weren't for effects of alignment).
If you are designing data structures in Java, it would be silly not to use this tool. Seriously. (Profilers can be used to somewhat the same effect, but why fire it up and click buttons till you get where you want and take numbers and calculate by yourself, when you can do it more precisely with very simple code).
Future work: do the same for the immutable Guava collections.