### Question:
FixedMillisecond extends RegularTimePeriod implements Serializable { @Override public long getFirstMillisecond() { return this.time; } FixedMillisecond();  FixedMillisecond(long millisecond);  FixedMillisecond(Date time); Date getTime(); @Override void peg(Calendar calendar); @Override RegularTimePeriod previous(); @Override RegularTimePeriod next(); @Override boolean equals(Object object); @Override int hashCode(); @Override int compareTo(TimePeriod o1); @Override long getFirstMillisecond(); @Override long getFirstMillisecond(Calendar calendar); @Override long getLastMillisecond(); @Override long getLastMillisecond(Calendar calendar); @Override long getMiddleMillisecond(); @Override long getMiddleMillisecond(Calendar calendar); @Override long getSerialIndex();  }### Answer:
@Test public void testImmutability() { Date d = new Date(20L); FixedMillisecond fm = new FixedMillisecond(d); d.setTime(22L); assertEquals(20L, fm.getFirstMillisecond()); }