### Question:
SimpleEventCalendar implements EventCalendar { @Override public boolean isEmpty() { return minIndex == NULL; } SimpleEventCalendar(ClockReader clockReader, Comparator<SimulationEvent> eventComparator); @Override boolean isEmpty(); @Override SimulationEvent peekFirstEvent(); @Override SimulationEvent removeFirstEvent(); @Override List<SimulationEvent> getEvents(); @Override void addEvent(SimulationEvent event); @Override void clear(); void addEvents(Collection<SimulationEvent> simulationEvents);  }### Answer:
@Test public void testIsEmpty() throws Exception { EventCalendar calendar = new SimpleEventCalendar(clock, comparator); assertTrue(calendar.isEmpty()); SimulationEvent event = calendar.removeFirstEvent(); assertNull(event); }
@Test public void testIsEmpty() throws Exception { EventCalendar calendar = new SimpleEventCalendar(clock, comparator); assertThat(calendar.isEmpty()).isTrue(); SimulationEvent event = calendar.removeFirstEvent(); assertThat(event).isNull(); }