如何在java中使用POI替换excel中的空白单元格

FileInputStream file= new FileInputStream(new File(path)); //Read the spreadsheet that needs to be updated            
            HSSFWorkbook wb = new HSSFWorkbook(file); //Access the workbook            
            HSSFSheet worksheet = wb.getSheetAt(0); //Access the worksheet, so that we can update / modify it.            
            Cell cell = null; // declare a Cell object
/////////////////////////////////// PRENDRE LES LIGNE ET LES COLLOLNES A MODIFER             
            int nbrow=Integer.parseInt(tx1.getText());
            int nbClmn=Integer.parseInt(tx2.getText());
//            int nbClmn = worksheet.getRow(0).getPhysicalNumberOfCells();            
                  System.out.println(nbClmn);    
                  for(int j=0;j<nbrow;j++){
                  for(int i=0;i<nbClmn;i++){   
                  cell = worksheet.getRow(j).getCell(i);}
for(int i=1;i<=nbClmn;i++){  
                   for(int j=1;j<nbrow;j++){  
                  System.out.println(wb.getSheetAt(0).getRow(j).getCell(i));    
                  String r=wb.getSheetAt(0).getRow(j).getCell(i).toString();  
                if(r.equals("BT") || r.equals("CB") || r.equals("CX") || r.equals("EB") || r.equals("EC") || r.equals("EP") || r.equals("NA") || r.equals("PP")){
                cell = worksheet.getRow(j).getCell(i);
                     cell.setCellValue("BT");
                  }else{ cell = worksheet.getRow(j).getCell(i);

                  }                  
        if(r.equals("CM") || r.equals("EM") || r.equals("GC") || r.equals("HT") || r.equals("MT")){
                cell = worksheet.getRow(j).getCell(i);
                     cell.setCellValue("MT");
                  }else{ cell = worksheet.getRow(j).getCell(i);

                  }}}
file.close(); //Close the InputStream                 
                FileOutputStream output_file =new FileOutputStream(new File(export));  //Open FileOutputStream to write updates                  
                wb.write(output_file);

                output_file.close();
FileInputStream file= new FileInputStream(new F