When creating an excel file using HSSF that contains more than 1000 rows , it can not be opened by excel application , the error message the excel application gives when openning the file is : "Too many different cell formats". So it means I can use HSSF to write excel files with less than 1000 rows. I need to deal with maximum rows as excel can handel (~65000 rows)
http://jakarta.apache.org/poi/faq.html#faq-13