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  *

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 package java.util;

 /**

  * A {@link NavigableSet} implementation based on a {@link TreeMap}.

  * The elements are ordered using their {@linkplain Comparable natural

  * ordering}, or by a {@link Comparator} provided at set creation

  * time, depending on which constructor is used.

  *

  * <p>This implementation provides guaranteed log(n) time cost for the basic

  * operations ({@code add}, {@code remove} and {@code contains}).

  *

  * <p>Note that the ordering maintained by a set (whether or not an explicit

  * comparator is provided) must be <i>consistent with equals</i> if it is to

  * correctly implement the {@code Set} interface.  (See {@code Comparable}

  * or {@code Comparator} for a precise definition of <i>consistent with

  * equals</i>.)  This is so because the {@code Set} interface is defined in

  * terms of the {@code equals} operation, but a {@code TreeSet} instance

  * performs all element comparisons using its {@code compareTo} (or

  * {@code compare}) method, so two elements that are deemed equal by this method

  * are, from the standpoint of the set, equal.  The behavior of a set

  * <i>is</i> well-defined even if its ordering is inconsistent with equals; it

  * just fails to obey the general contract of the {@code Set} interface.

  *

  * <p><strong>Note that this implementation is not synchronized.</strong>

  * If multiple threads access a tree set concurrently, and at least one

  * of the threads modifies the set, it <i>must</i> be synchronized

  * externally.  This is typically accomplished by synchronizing on some

  * object that naturally encapsulates the set.

  * If no such object exists, the set should be "wrapped" using the

  * {@link Collections#synchronizedSortedSet Collections.synchronizedSortedSet}

  * method.  This is best done at creation time, to prevent accidental

  * unsynchronized access to the set: <pre>

  *   SortedSet s = Collections.synchronizedSortedSet(new TreeSet(...));</pre>

  *

  * <p>The iterators returned by this class's {@code iterator} method are

  * <i>fail-fast</i>: if the set is modified at any time after the iterator is

  * created, in any way except through the iterator's own {@code remove}

  * method, the iterator will throw a {@link ConcurrentModificationException}.

  * Thus, in the face of concurrent modification, the iterator fails quickly

  * and cleanly, rather than risking arbitrary, non-deterministic behavior at

  * an undetermined time in the future.

  *

  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed

  * as it is, generally speaking, impossible to make any hard guarantees in the

  * presence of unsynchronized concurrent modification.  Fail-fast iterators

  * throw {@code ConcurrentModificationException} on a best-effort basis.

  * Therefore, it would be wrong to write a program that depended on this

  * exception for its correctness:   <i>the fail-fast behavior of iterators

  * should be used only to detect bugs.</i>

  *

  * <p>This class is a member of the

  * <a href="{@docRoot}/../technotes/guides/collections/index.html">

  * Java Collections Framework</a>.

  *

  * @param <E> the type of elements maintained by this set

  *

  * @author  Josh Bloch

  * @see     Collection

  * @see     Set

  * @see     HashSet

  * @see     Comparable

  * @see     Comparator

  * @see     TreeMap

  * @since   1.2

  */

 public class TreeSet<E> extends AbstractSet<E>

     implements NavigableSet<E>, Cloneable, java.io.Serializable

 {

     /**

      * The backing map.

      */

     private transient NavigableMap<E,Object> m;

     // Dummy value to associate with an Object in the backing Map

     private static final Object PRESENT = new Object();

     /**

      * Constructs a set backed by the specified navigable map.

      */

     TreeSet(NavigableMap<E,Object> m) {

         this.m = m;

     }

     /**

      * Constructs a new, empty tree set, sorted according to the

      * natural ordering of its elements.  All elements inserted into

      * the set must implement the {@link Comparable} interface.

      * Furthermore, all such elements must be <i>mutually

      * comparable</i>: {@code e1.compareTo(e2)} must not throw a

      * {@code ClassCastException} for any elements {@code e1} and

      * {@code e2} in the set.  If the user attempts to add an element

      * to the set that violates this constraint (for example, the user

      * attempts to add a string element to a set whose elements are

      * integers), the {@code add} call will throw a

      * {@code ClassCastException}.

      */

     public TreeSet() {

         this(new TreeMap<E,Object>());

     }

     /**

      * Constructs a new, empty tree set, sorted according to the specified

      * comparator.  All elements inserted into the set must be <i>mutually

      * comparable</i> by the specified comparator: {@code comparator.compare(e1,

      * e2)} must not throw a {@code ClassCastException} for any elements

      * {@code e1} and {@code e2} in the set.  If the user attempts to add

      * an element to the set that violates this constraint, the

      * {@code add} call will throw a {@code ClassCastException}.

      *

      * @param comparator the comparator that will be used to order this set.

      *        If {@code null}, the {@linkplain Comparable natural

      *        ordering} of the elements will be used.

      */

     public TreeSet(Comparator<? super E> comparator) {

         this(new TreeMap<>(comparator));

     }

     /**

      * Constructs a new tree set containing the elements in the specified

      * collection, sorted according to the <i>natural ordering</i> of its

      * elements.  All elements inserted into the set must implement the

      * {@link Comparable} interface.  Furthermore, all such elements must be

      * <i>mutually comparable</i>: {@code e1.compareTo(e2)} must not throw a

      * {@code ClassCastException} for any elements {@code e1} and

      * {@code e2} in the set.

      *

      * @param c collection whose elements will comprise the new set

      * @throws ClassCastException if the elements in {@code c} are

      *         not {@link Comparable}, or are not mutually comparable

      * @throws NullPointerException if the specified collection is null

      */

     public TreeSet(Collection<? extends E> c) {

         this();

         addAll(c);

     }

     /**

      * Constructs a new tree set containing the same elements and

      * using the same ordering as the specified sorted set.

      *

      * @param s sorted set whose elements will comprise the new set

      * @throws NullPointerException if the specified sorted set is null

      */

     public TreeSet(SortedSet<E> s) {

         this(s.comparator());

         addAll(s);

     }

     /**

      * Returns an iterator over the elements in this set in ascending order.

      *

      * @return an iterator over the elements in this set in ascending order

      */

     public Iterator<E> iterator() {

         return m.navigableKeySet().iterator();

     }

     /**

      * Returns an iterator over the elements in this set in descending order.

      *

      * @return an iterator over the elements in this set in descending order

      * @since 1.6

      */

     public Iterator<E> descendingIterator() {

         return m.descendingKeySet().iterator();

     }

     /**

      * @since 1.6

      */

     public NavigableSet<E> descendingSet() {

         return new TreeSet<>(m.descendingMap());

     }

     /**

      * Returns the number of elements in this set (its cardinality).

      *

      * @return the number of elements in this set (its cardinality)

      */

     public int size() {

         return m.size();

     }

     /**

      * Returns {@code true} if this set contains no elements.

      *

      * @return {@code true} if this set contains no elements

      */

     public boolean isEmpty() {

         return m.isEmpty();

     }

     /**

      * Returns {@code true} if this set contains the specified element.

      * More formally, returns {@code true} if and only if this set

      * contains an element {@code e} such that

      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.

      *

      * @param o object to be checked for containment in this set

      * @return {@code true} if this set contains the specified element

      * @throws ClassCastException if the specified object cannot be compared

      *         with the elements currently in the set

      * @throws NullPointerException if the specified element is null

      *         and this set uses natural ordering, or its comparator

      *         does not permit null elements

      */

     public boolean contains(Object o) {

         return m.containsKey(o);

     }

     /**

      * Adds the specified element to this set if it is not already present.

      * More formally, adds the specified element {@code e} to this set if

      * the set contains no element {@code e2} such that

      * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.

      * If this set already contains the element, the call leaves the set

      * unchanged and returns {@code false}.

      *

      * @param e element to be added to this set

      * @return {@code true} if this set did not already contain the specified

      *         element

      * @throws ClassCastException if the specified object cannot be compared

      *         with the elements currently in this set

      * @throws NullPointerException if the specified element is null

      *         and this set uses natural ordering, or its comparator

      *         does not permit null elements

      */

     public boolean add(E e) {

         return m.put(e, PRESENT)==null;

     }

     /**

      * Removes the specified element from this set if it is present.

      * More formally, removes an element {@code e} such that

      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>,

      * if this set contains such an element.  Returns {@code true} if

      * this set contained the element (or equivalently, if this set

      * changed as a result of the call).  (This set will not contain the

      * element once the call returns.)

      *

      * @param o object to be removed from this set, if present

      * @return {@code true} if this set contained the specified element

      * @throws ClassCastException if the specified object cannot be compared

      *         with the elements currently in this set

      * @throws NullPointerException if the specified element is null

      *         and this set uses natural ordering, or its comparator

      *         does not permit null elements

      */

     public boolean remove(Object o) {

         return m.remove(o)==PRESENT;

     }

     /**

      * Removes all of the elements from this set.

      * The set will be empty after this call returns.

      */

     public void clear() {

         m.clear();

     }

     /**

      * Adds all of the elements in the specified collection to this set.

      *

      * @param c collection containing elements to be added to this set

      * @return {@code true} if this set changed as a result of the call

      * @throws ClassCastException if the elements provided cannot be compared

      *         with the elements currently in the set

      * @throws NullPointerException if the specified collection is null or

      *         if any element is null and this set uses natural ordering, or

      *         its comparator does not permit null elements

      */

     public  boolean addAll(Collection<? extends E> c) {

         // Use linear-time version if applicable

         if (m.size()==0 && c.size() > 0 &&

             c instanceof SortedSet &&

             m instanceof TreeMap) {

             SortedSet<? extends E> set = (SortedSet<? extends E>) c;

             TreeMap<E,Object> map = (TreeMap<E, Object>) m;

             Comparator<? super E> cc = (Comparator<? super E>) set.comparator();

             Comparator<? super E> mc = map.comparator();

             if (cc==mc || (cc != null && cc.equals(mc))) {

                 map.addAllForTreeSet(set, PRESENT);

                 return true;

             }

         }

         return super.addAll(c);

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if {@code fromElement} or {@code toElement}

      *         is null and this set uses natural ordering, or its comparator

      *         does not permit null elements

      * @throws IllegalArgumentException {@inheritDoc}

      * @since 1.6

      */

     public NavigableSet<E> subSet(E fromElement, boolean fromInclusive,

                                   E toElement,   boolean toInclusive) {

         return new TreeSet<>(m.subMap(fromElement, fromInclusive,

                                        toElement,   toInclusive));

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if {@code toElement} is null and

      *         this set uses natural ordering, or its comparator does

      *         not permit null elements

      * @throws IllegalArgumentException {@inheritDoc}

      * @since 1.6

      */

     public NavigableSet<E> headSet(E toElement, boolean inclusive) {

         return new TreeSet<>(m.headMap(toElement, inclusive));

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if {@code fromElement} is null and

      *         this set uses natural ordering, or its comparator does

      *         not permit null elements

      * @throws IllegalArgumentException {@inheritDoc}

      * @since 1.6

      */

     public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {

         return new TreeSet<>(m.tailMap(fromElement, inclusive));

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if {@code fromElement} or

      *         {@code toElement} is null and this set uses natural ordering,

      *         or its comparator does not permit null elements

      * @throws IllegalArgumentException {@inheritDoc}

      */

     public SortedSet<E> subSet(E fromElement, E toElement) {

         return subSet(fromElement, true, toElement, false);

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if {@code toElement} is null

      *         and this set uses natural ordering, or its comparator does

      *         not permit null elements

      * @throws IllegalArgumentException {@inheritDoc}

      */

     public SortedSet<E> headSet(E toElement) {

         return headSet(toElement, false);

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if {@code fromElement} is null

      *         and this set uses natural ordering, or its comparator does

      *         not permit null elements

      * @throws IllegalArgumentException {@inheritDoc}

      */

     public SortedSet<E> tailSet(E fromElement) {

         return tailSet(fromElement, true);

     }

     public Comparator<? super E> comparator() {

         return m.comparator();

     }

     /**

      * @throws NoSuchElementException {@inheritDoc}

      */

     public E first() {

         return m.firstKey();

     }

     /**

      * @throws NoSuchElementException {@inheritDoc}

      */

     public E last() {

         return m.lastKey();

     }

     // NavigableSet API methods

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if the specified element is null

      *         and this set uses natural ordering, or its comparator

      *         does not permit null elements

      * @since 1.6

      */

     public E lower(E e) {

         return m.lowerKey(e);

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if the specified element is null

      *         and this set uses natural ordering, or its comparator

      *         does not permit null elements

      * @since 1.6

      */

     public E floor(E e) {

         return m.floorKey(e);

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if the specified element is null

      *         and this set uses natural ordering, or its comparator

      *         does not permit null elements

      * @since 1.6

      */

     public E ceiling(E e) {

         return m.ceilingKey(e);

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if the specified element is null

      *         and this set uses natural ordering, or its comparator

      *         does not permit null elements

      * @since 1.6

      */

     public E higher(E e) {

         return m.higherKey(e);

     }

     /**

      * @since 1.6

      */

     public E pollFirst() {

         Map.Entry<E,?> e = m.pollFirstEntry();

         return (e == null) ? null : e.getKey();

     }

     /**

      * @since 1.6

      */

     public E pollLast() {

         Map.Entry<E,?> e = m.pollLastEntry();

         return (e == null) ? null : e.getKey();

     }

     /**

      * Returns a shallow copy of this {@code TreeSet} instance. (The elements

      * themselves are not cloned.)

      *

      * @return a shallow copy of this set

      */

     public Object clone() {

         TreeSet<E> clone = null;

         try {

             clone = (TreeSet<E>) super.clone();

         } catch (CloneNotSupportedException e) {

             throw new InternalError();

         }

         clone.m = new TreeMap<>(m);

         return clone;

     }

     /**

      * Save the state of the {@code TreeSet} instance to a stream (that is,

      * serialize it).

      *

      * @serialData Emits the comparator used to order this set, or

      *             {@code null} if it obeys its elements' natural ordering

      *             (Object), followed by the size of the set (the number of

      *             elements it contains) (int), followed by all of its

      *             elements (each an Object) in order (as determined by the

      *             set's Comparator, or by the elements' natural ordering if

      *             the set has no Comparator).

      */

     private void writeObject(java.io.ObjectOutputStream s)

         throws java.io.IOException {

         // Write out any hidden stuff

         s.defaultWriteObject();

         // Write out Comparator

         s.writeObject(m.comparator());

         // Write out size

         s.writeInt(m.size());

         // Write out all elements in the proper order.

         for (E e : m.keySet())

             s.writeObject(e);

     }

     /**

      * Reconstitute the {@code TreeSet} instance from a stream (that is,

      * deserialize it).

      */

     private void readObject(java.io.ObjectInputStream s)

         throws java.io.IOException, ClassNotFoundException {

         // Read in any hidden stuff

         s.defaultReadObject();

         // Read in Comparator

         Comparator<? super E> c = (Comparator<? super E>) s.readObject();

         // Create backing TreeMap

         TreeMap<E,Object> tm;

         if (c==null)

             tm = new TreeMap<>();

         else

             tm = new TreeMap<>(c);

         m = tm;

         // Read in size

         int size = s.readInt();

         tm.readTreeSet(size, s, PRESENT);

     }

     private static final long serialVersionUID = -2479143000061671589L;

 }