How to do black-box and white-box testing for java application?

How to do black-box and white-box testings for a java application which reads multiple text files(say:lecturer.txt, studentsdetails.txt, attendancelog.txt) and then print a couple of attendance reports to the console?

I do know how to do a black-box testing for an input validation but not for these kinds of scenarios.

Can someone please explain the format to carry out these tests? Thank you in advance.