how to wrtie "null" for every blank cell while converting excel to csv in java.

language: Java

 i have one bank statement  in xls format , which i already convert into csv format in java .  and display in jsp  ,but how user can recognized the amount is debit or credit because it come in one column i.e  amount . and  after edit  form jsp it will save in database , where two different column for debit and credit .....

Problem:  In excel File : there is two column Debit Amt. and credit Amount , but the transaction will be debit or credit , but when i convert into csv file ,  it not write null on blank cell . i

 used missing policy ,
 case Cell.CELL_TYPE_BLANK:
                                                                                               
                                  data.append("***" + ",");
                                    break;

 but it work on total blank column , not a every blank cell .

Kindly guide .Thanks  in advance .