code: public void set(DateTimeFieldType type, int value) {
          if (type == null) {
              throw new IllegalArgumentException("Field must not be null");
          }
          setMillis(type.getField(getChronology()).set(getMillis(), value));
      }
docs: Sets the value of one of the fields of the instant, such as hourOfDay.
  
  @param type  a field type, usually obtained from DateTimeFieldType, not null
  @param value  the value to set the field to
  @throws IllegalArgumentException if the value is null or invalid