First, you should use equals() rather than == when comparing strings. To get the submitted value, you retrieve the values using form submission (either through GET or POST which passes key/value pairs)....depending upon the context of the server this is done through the ServletRequest object, using the getParameter method.Lots of tutorials about this online that can do a much better job than I at explaining it. You can use standard I/O, however multiple requests might cause a concurrency issue with the file. Stick the info into a database and this issue is dealt with appropriately.
PS Had to look up the project...quite interesting.