@Override
  public int hashCode() {
    int result = minutes != null ? minutes.hashCode() : 0;
    result = 31 * result + (hours != null ? hours.hashCode() : 0);
    result = 31 * result + (daysOfMonth != null ? daysOfMonth.hashCode() : 0);
    result = 31 * result + (month != null ? month.hashCode() : 0);
    result = 31 * result + (dayOfWeek != null ? dayOfWeek.hashCode() : 0);
    result = 31 * result + (year != null ? year.hashCode() : 0);
    result = 31 * result + (startTime != null ? startTime.hashCode() : 0);
    result = 31 * result + (endTime != null ? endTime.hashCode() : 0);
    return result;
  }
Return a cron expression from the schedule fields.
Example: "0 0 12 * * ?"
@return
ambari-server/src/main/java/org/apache/ambari/server/state/scheduler/Schedule.java