I know only two paths into Excel. First, it can act as an ODBC data source. You reference the file as the database, a named range as a table and rows & columns within the range as data. You can get to it from Java with JDBC-ODBC bridge. The other path is COM which we old folks used to call OLE. Google for "java com bridge". There are a bunch of products out there. The last time I surveyed, the commercial ones were huge and expensive. One free one is Jacob at DanAdler.com. SourceForge has one but I can never remember the name.
(OK, I left out DDE. It's been uncool since it was buggy in Windows 3.1 but I've built some very slick interfaces with it over the years. Again, you'd need a Java dde bridge, and there are some out there.)
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi