code: protected DateTimeField getField(int index, Chronology chrono) {
          switch (index) {
          case MONTH_OF_YEAR:
              return chrono.monthOfYear();
          case DAY_OF_MONTH:
              return chrono.dayOfMonth();
          default:
              throw new IndexOutOfBoundsException("Invalid index: " + index);
          }
      }
docs: Gets the field for a specific index in the chronology specified.
  <p>
  This method must not use any instance variables.
  
  @param index  the index to retrieve
  @param chrono  the chronology to use
  @return the field, never null