### Question:
Hour extends RegularTimePeriod implements Serializable { @Override public RegularTimePeriod previous() { Hour result; if (this.hour != FIRST_HOUR_IN_DAY) { result = new Hour(this.hour - 1, this.day); } else { Day prevDay = (Day) this.day.previous(); if (prevDay != null) { result = new Hour(LAST_HOUR_IN_DAY, prevDay); } else { result = null; } } return result; } Hour();  Hour(int hour, Day day);  Hour(int hour, int day, int month, int year);  Hour(Date time);  Hour(Date time, TimeZone zone, Locale locale); int getHour(); Day getDay(); int getYear(); int getMonth(); int getDayOfMonth(); @Override long getFirstMillisecond(); @Override long getLastMillisecond(); @Override void peg(Calendar calendar); @Override RegularTimePeriod previous(); @Override RegularTimePeriod next(); @Override long getSerialIndex(); @Override long getFirstMillisecond(Calendar calendar); @Override long getLastMillisecond(Calendar calendar); @Override boolean equals(Object obj); @Override String toString(); @Override int hashCode(); @Override int compareTo(TimePeriod o1); static Hour parseHour(String s); static final int FIRST_HOUR_IN_DAY; static final int LAST_HOUR_IN_DAY; }### Answer:
@Test public void testFirstHourPrevious() { Hour first = new Hour(0, new Day(1, MonthConstants.JANUARY, 1900)); Hour previous = (Hour) first.previous(); assertNull(previous); }