Processing Large Dataset - Class Memory Overhead

I've been tasked with processing a large dataset as part of a class assignment. One of the fields is a 24-digit unsigned hex number. I realized that, rather than storing the field verbatim in a char array of length 24, I could store the actual value of the hex number in an array only 6 chars long (I chose char over int because chars are unsigned). To do this, I wrote the following simple class to accept the hex string and convert it so that it can be stored in that manner:

private class Hex24
{
private final char[] hexAsInt = new char[6];
public Hex24(String h)
{
for(int index = 0; index < 6; ++index)
hexAsInt[index] = (char)Integer.parseUnsignedInt(h.substring(index * 4, (index + 1) * 4));

[Code] ....

Assuming all this works (haven't tested it, but I think it should), what I'm wondering is how much memory this will save me (if any) compared to just throwing everything into a char[24] array. The underlying char[6] is obviously quite a bit smaller, but objects must take up more space than just their fields since Java needs to know what kind of object it is so it can know what methods it has, etc..How to accurately compare the size of a Hex24 object to the size of a 24 character array.

I guess another option/thing I might want to compare size efficiency for is putting the char[6] along with the conversion/comparison logic directly into the classes (as fields/member methods) where I'm currently using Hex24 fields. I'd guess this is the most memory-efficient option I've come up with, but it would lead to a lot of code duplication.

Another thing I'd like to compare is the size of a String vs. the size of the equivalent char array for shortish text fields. If this difference is big enough it might be worth storing those fields as arrays rather than Strings.