Make my java - maven program thread safe. (Bug fix)

I have a bug in my program when i put it multithread. It create some files, but in multithreading, name are overwriten, so sometimes are files missing. Because if the program process one file at time, it doesn't get any error, i don't know where the error is and how to fix it. The program is written without frameworks, entirely pure java, and maven libraries. I developed entirely the program, so I can guide you until the point i think the problem is. Can u help me to solve it in next 4-8 hrs? It urged me.