### Question:
Day extends RegularTimePeriod implements Serializable { @Override public RegularTimePeriod previous() { int serial = this.serialDate.toSerial(); if (serial > SerialDate.SERIAL_LOWER_BOUND) { SerialDate yesterday = SerialDate.createInstance(serial - 1); return new Day(yesterday); } return null; } Day();  Day(int day, int month, int year);  Day(SerialDate serialDate);  Day(Date time);  Day(Date time, TimeZone zone, Locale locale); SerialDate getSerialDate(); 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 int hashCode(); @Override int compareTo(TimePeriod o1); @Override String toString(); static Day parseDay(String s);  }### Answer:
@Test public void test1Jan1900Previous() { Day jan1st1900 = new Day(1, MonthConstants.JANUARY, 1900); Day previous = (Day) jan1st1900.previous(); assertNull(previous); }