Collections - The Java Programming Language - page 853

21.5. Set and SortedSet
The Set<E> interface extends Collection<E> , providing a more specific con-
tract for its methods, but adds no new methods of its own. A collection
that is a Set contains no duplicate elements. If you add the same element
twice to a set (in other words, if you add two objects that are equal ), the
first invocation will return TRue , while the second will return false . If after
this, remove is similarly invoked twice, the first remove of the element will
return TRue since the set was changed by removing the element, while
the second will return false since the element was no longer present. A
set may also contain at most one null element.
The SortedSet<E> interface extends Set<E> to specify an additional contrac-
titerators on such a set will always return the elements in a specified or-
der. By default this will be the element's natural order. In the implement-
ations of SortedSet provided in java.util you can also specify a Comparator
object that will be used to order the elements instead of the natural or-
der.
SortedSet adds some methods that make sense in an ordered set:
public Comparator<? super E> comparator()
Returns the Comparator being used by this sorted set, or null if
the elements' natural order is being used. Note the use of the
lower bounded wildcard in the return type; any implementa-
tion of this interface should accept a comparator that is typed
the same way.
public E first()
Returns the first (lowest) object in this set.
public E last()
Returns the last (highest) object in this set.