Here is the complete source code for the GenSig.java program, with some comments added. Compile and run it. Remember, you need to specify the name of a file to be signed, as in

java GenSig data

You can download and use this sample file named data or any other file you like. The file will not be modified. It will be read so that a signature can be generated for it.

After executing the program, you should see the saved suepk (public key) and sig (signature) files.