code: @Override
      public final HolidayCalendar<E> setHolidays(final Set<E> holidays) {
  
          if (holidays == null) {
              this.holidays = Collections.emptyMap();
              return this;
          }
  
          final Map<String, E> newSet = new TreeMap<>();
          for (final E e : holidays) {
              newSet.put(toString(e), e);
          }
          this.holidays = Collections.unmodifiableMap(newSet);
          return this;
      }
docs: /*
  (non-Javadoc)
  
  @see net.objectlab.kit.datecalc.common.HolidayCalendar#setHolidays(java.util.Set)