code: public String toHexString(){
          if(nWords ==0) {
              return "0";
          }
          StringBuilder sb = new StringBuilder((nWords +offset)*8);
          for(int i= nWords -1; i>=0; i--) {
              String subStr = Integer.toHexString(data[i]);
              for(int j = subStr.length(); j<8; j++) {
                  sb.append('0');
              }
              sb.append(subStr);
          }
          for(int i=offset; i>0; i--) {
              sb.append("00000000");
          }
          return sb.toString();
      }
docs: Converts this <code>FDBigInteger</code> to a hexadecimal string.
  
  @return The hexadecimal string representation.