Create TreeSet objects
TreeSet()- Creates a new, empty tree set, sorted according to the natural ordering of its elements.
TreeSet(Collection<? extends E> c)- Creates a new tree set containing the elements in the specified collection, sorted according to the natural ordering of its elements.
TreeSet(Comparator<? super E> comparator)- Creates a new, empty tree set, sorted according to the specified comparator.
TreeSet(SortedSet<E> s)- Creates a new tree set containing the same elements and using the same ordering as the specified sorted set.