### Question:
Day implements Comparable { public static Set<String> getDayString(Set<Day> days) { return getDayStringTreated(days, ""); } Day(String dayStr);  Day(String dayStr, List<Integer> hours); String getDayStr(); List<Integer> getHours(); static List<Day> buildSortedDays(List<Range> ranges); static Set<String> getDayString(Set<Day> days); static Set<String> getDayStringTreated(Set<Day> days, String toBeRemoved); @Override int hashCode(); @Override boolean equals(Object dayObject); static String treatDayStr(String dayStr); @Override int compareTo(Object o);  }### Answer:
@Test public void getDayString() { String dayStr = "20180105", dayStr2 = "20180106", dayStr3 = "20180107"; Day day = new Day(dayStr), day2 = new Day(dayStr2), day3 = new Day(dayStr3); Set<Day> days = new HashSet(); days.add(day); days.add(day2); days.add(day3); Set<String> exp = new HashSet(); exp.add("2018-01-05"); exp.add("2018-01-06"); exp.add("2018-01-07"); assertEquals(exp, Day.getDayString(days)); }