code: public static <T> TreeSet<T> newSortedSet(@Nullable Comparator<? super T> comparator) {
  		return Sets.newTreeSet(comparator);
  	}
docs: 根据等号左边的类型，构造类型正确的TreeSet, 并设置comparator.
  
  @see com.google.common.collect.Sets#newTreeSet(Comparator)