code: public String getSharedString(int i)
      {
          String ret = null;
          CTRst string = strings.getSi().get(i);
          if(string != null && string.getT() != null)
              ret = string.getT().getValue();
          if(ret == null) // cell has multiple formats or fonts
          {
              List<CTRElt> list = string.getR();
              if(list.size() > 0)
              {
                  for(CTRElt lt : list)
                  {
                      String str = lt.getT().getValue();
                      if(str != null)
                      {
                          if(ret == null)
                              ret = "";
                          ret += str;
                      }
                  }
              }
          }
          return ret;
      }
docs: Returns the string at the given index in SharedStrings.xml.
  @param i The index of the string
  @return The string at the given index in SharedStrings.xml