How to send unsigned bytes through serial from arduino
It's up to the receiving program to interpret the data as an 8-bit fraction.

On the wire, the data is just eight bits; there is no inherent signed-ness or implied binary-point (or any other interpretation).  Your java program will have to interpret the byte as an unsigned fraction, if that's what you need, something like [pseudo-code]: frac = (float)(unsigned)b/256.;.