code: public void writeColumn(String value) throws IllegalArgumentException, IOException
      {
          if (value == null)
          {
              if (! this.nullableColumns[this.currentColumn])
              {
                  throw new IllegalArgumentException(
                      "Bulk load will fail as a null value cannot be assigned to column '" + this.columns[this.currentColumn] + "'"
                  );
              }
          }
          else
          {
              this.dataFileWriter.write(value);
          }
  
          if (this.currentColumn < (this.columns.length - 1))
          {
              this.dataFileWriter.write(FIELD_TERMINATOR);
          }
          this.currentColumn++;
      }
docs: write a single column. After all the columns on a single row have been written, call endRow()
  @param value
  @throws IllegalArgumentException if the row values contains a null value but the column isn't nullable.
  @throws IOException if there was a problem writing to the file.