This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
it means that either the description of the file format as stated in the documentation is incorrect or you (and everybody else) have been supplied with a corrupt file.
The general consensus is (and I believe there's an old post confirming this by someone who asked Sun about it and actually got a reply) that the description in the assignment documentation is incorrect and in fact the data is padded with spaces instead of null values.
I've just mentioned that in my choices.txt, using a realworld situation in which something similar happened (file format specs as supplied by customer didn't match supplied test data, customer wanted us to go from the supplied data rather than the documentation) as justification.