I'm assuming this is a legacy database? You would not normally use LONG data types these days, you would us a BLOB or CLOB as appropriate.
You need to treat LONGs as binary data, so you will need to use ResultSet.getBinaryStream() to read them. There used to be all sorts of gotchas in the Oracle driver handling LONG data so if you can use a C/BLOB it will save a lot of headaches.