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SortedSet<E> is the super interface of NavigableSet<E> and is a sub interface of Set<E>
or in other words, Iterable<E> -> Collection<E> -> Set <E> -> SortedSet<E> -> NavigableSet<E> -> TreeSet<E> is the inheritance tree.
TreeSet<E> implements NavigableSet<E>

SortedSet<E> ss = TreeSet<E> ts. headSet(E element) // This returns a SortedSet
where as
NavigableSet<E> ns = TreeSet<E> ts.headSet(E element, boolean b) // this returns a Navigable Set

this is same with tailSet() and subSet() which are in SCJP6 objectives!