### Question:
Timestamps { public static String format(long timestampMillis) { return FORMATTER.format(LocalTime.ofNanoOfDay(timestampMillis % (24 * 60 * 60 * 1000) * 1_000_000)); } private  Timestamps(); static String format(long timestampMillis);  }### Answer:
@Test public void testFirstMidnight() { long timestampMillis = 0L; assertEquals("00:00:00.000", Timestamps.format(timestampMillis)); }
@Test public void testFirstMidday() { long timestampMillis = 12L * 60 * 60 * 1000; assertEquals("12:00:00.000", Timestamps.format(timestampMillis)); }
@Test public void testSecondMidnight() { long timestampMillis = 24L * 60 * 60 * 1000; assertEquals("00:00:00.000", Timestamps.format(timestampMillis)); }