Hard Determinism and the Java Random generator

My roommate and I are in another argument about our free will. His newest argument against my sweet sweet deterministic world view is that random number generators like the one in Java is unpredictable and nothing prior to running a random generator can determine the outcome of the number.

Anyway, my question is that when you use the built in Java random methods like Math.random and java.util.Random, is the number truly random or is it dependent on any predictable event? Totally need this for my code.