Problema em laço do/while usando JExcelAPI

package controleweb;

import java.io.File;
import java.io.IOException;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.SQLException;
import java.util.Date;
import jxl.Cell;
import jxl.DateCell;
import jxl.NumberCell;
import jxl.Sheet;
import jxl.Workbook;
import jxl.read.biff.BiffException;



/**
 *
 * @author petter
 */
public class Main {
    
    private static Connection conn;
    private static int i = 1;
    private static Date datea1;
    private static String stringb2;
    private static double intc3;
    private static String stringd4;
    private static String stringe5;
    private static String stringf6;
    private static String stringg7;
    private static String stringh8;
    private static String stringi9;
    private static String stringj10;
    
    /** Creates a new instance of Main */
    public static void main(String[] args ) throws IOException, BiffException, 
                                                   ClassNotFoundException, SQLException {
        
      /* pega o arquiivo do Excel */
      Workbook workbook = Workbook.getWorkbook(new File("enviados.xls"));  
      
      /* pega a primeira planilha dentro do arquivo XLS */
      Sheet sheet = workbook.getSheet(0); 
     
      do{
      /* pega os valores das células como se numa matriz */
      Cell a1 = sheet.getCell(0,i);
      Cell b2 = sheet.getCell(1,i);
      Cell c3 = sheet.getCell(2,i);
      Cell d4 = sheet.getCell(3,i);
      Cell e5 = sheet.getCell(4,i);
      Cell f6 = sheet.getCell(5,i);
      Cell g7 = sheet.getCell(6,i);
      Cell h8 = sheet.getCell(7,i);
      Cell i9 = sheet.getCell(8,i);
      Cell j10 = sheet.getCell(9,i);

      /* pega os conteúdos das células */
      DateCell dc = (DateCell) a1;
      datea1 = dc.getDate();
      
      stringb2 = b2.getContents();
      
      NumberCell nc = (NumberCell) c3;
      intc3 = nc.getValue();
      
      stringd4 = d4.getContents();
      stringe5 = e5.getContents();
      stringf6 = f6.getContents();
      stringg7 = g7.getContents();
      stringh8 = h8.getContents();
      stringi9 = i9.getContents();
      stringj10 = j10.getContents();
      
      //Auto incremento para pular de linhas na planilha do Excell
      i++;
      }while(stringg7 != null);
      
     /*Executa o insert para inserir os dados no banco de testes MySQL*/
     Class.forName("com.mysql.jdbc.Driver");
     
     Connection conn = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/jar",
                                                                "user",
                                                                "senha");
     
     PreparedStatement ps = conn.prepareStatement("INSERT INTO Teste(data,hora,tamanho,ip," +
             "                                                       string1,string2,de," +
             "                                                       para,cc,titulo) "+
                       "VALUES('"+datea1+"','"+stringb2+"','"+intc3+"','"+stringd4+"','"+stringe5+"'" +
             "                ,'"+stringf6+"','"+stringg7+"','"+stringh8+"','"+stringi9+"','"+stringj10+"')");
         
     ps.executeUpdate(); 
     ps.close();
     
     workbook.close();
    }    
}