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  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.  Oracle designates this

  * particular file as subject to the "Classpath" exception as provided

  * by Oracle in the LICENSE file that accompanied this code.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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  */

 package java.util;

 /**

  * This class implements the <tt>Set</tt> interface, backed by a hash table

  * (actually a <tt>HashMap</tt> instance).  It makes no guarantees as to the

  * iteration order of the set; in particular, it does not guarantee that the

  * order will remain constant over time.  This class permits the <tt>null</tt>

  * element.

  *

  * <p>This class offers constant time performance for the basic operations

  * (<tt>add</tt>, <tt>remove</tt>, <tt>contains</tt> and <tt>size</tt>),

  * assuming the hash function disperses the elements properly among the

  * buckets.  Iterating over this set requires time proportional to the sum of

  * the <tt>HashSet</tt> instance's size (the number of elements) plus the

  * "capacity" of the backing <tt>HashMap</tt> instance (the number of

  * buckets).  Thus, it's very important not to set the initial capacity too

  * high (or the load factor too low) if iteration performance is important.

  *

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

  * If multiple threads access a hash 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#synchronizedSet Collections.synchronizedSet}

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

  * unsynchronized access to the set:<pre>

  *   Set s = Collections.synchronizedSet(new HashSet(...));</pre>

  *

  * <p>The iterators returned by this class's <tt>iterator</tt> 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 <tt>remove</tt>

  * method, the Iterator throws 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 <tt>ConcurrentModificationException</tt> 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

  * @author  Neal Gafter

  * @see     Collection

  * @see     Set

  * @see     TreeSet

  * @see     HashMap

  * @since   1.2

  */

 public class HashSet<E>

     extends AbstractSet<E>

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

 {

     static final long serialVersionUID = -5024744406713321676L;

     private transient HashMap<E,Object> map;

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

     private static final Object PRESENT = new Object();

     /**

      * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has

      * default initial capacity (16) and load factor (0.75).

      */

     public HashSet() {

         map = new HashMap<>();

     }

     /**

      * Constructs a new set containing the elements in the specified

      * collection.  The <tt>HashMap</tt> is created with default load factor

      * (0.75) and an initial capacity sufficient to contain the elements in

      * the specified collection.

      *

      * @param c the collection whose elements are to be placed into this set

      * @throws NullPointerException if the specified collection is null

      */

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

         map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));

         addAll(c);

     }

     /**

      * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has

      * the specified initial capacity and the specified load factor.

      *

      * @param      initialCapacity   the initial capacity of the hash map

      * @param      loadFactor        the load factor of the hash map

      * @throws     IllegalArgumentException if the initial capacity is less

      *             than zero, or if the load factor is nonpositive

      */

     public HashSet(int initialCapacity, float loadFactor) {

         map = new HashMap<>(initialCapacity, loadFactor);

     }

     /**

      * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has

      * the specified initial capacity and default load factor (0.75).

      *

      * @param      initialCapacity   the initial capacity of the hash table

      * @throws     IllegalArgumentException if the initial capacity is less

      *             than zero

      */

     public HashSet(int initialCapacity) {

         map = new HashMap<>(initialCapacity);

     }

     /**

      * Constructs a new, empty linked hash set.  (This package private

      * constructor is only used by LinkedHashSet.) The backing

      * HashMap instance is a LinkedHashMap with the specified initial

      * capacity and the specified load factor.

      *

      * @param      initialCapacity   the initial capacity of the hash map

      * @param      loadFactor        the load factor of the hash map

      * @param      dummy             ignored (distinguishes this

      *             constructor from other int, float constructor.)

      * @throws     IllegalArgumentException if the initial capacity is less

      *             than zero, or if the load factor is nonpositive

      */

     HashSet(int initialCapacity, float loadFactor, boolean dummy) {

         map = new LinkedHashMap<>(initialCapacity, loadFactor);

     }

     /**

      * Returns an iterator over the elements in this set.  The elements

      * are returned in no particular order.

      *

      * @return an Iterator over the elements in this set

      * @see ConcurrentModificationException

      */

     public Iterator<E> iterator() {

         return map.keySet().iterator();

     }

     /**

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

      *

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

      */

     public int size() {

         return map.size();

     }

     /**

      * Returns <tt>true</tt> if this set contains no elements.

      *

      * @return <tt>true</tt> if this set contains no elements

      */

     public boolean isEmpty() {

         return map.isEmpty();

     }

     /**

      * Returns <tt>true</tt> if this set contains the specified element.

      * More formally, returns <tt>true</tt> if and only if this set

      * contains an element <tt>e</tt> such that

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

      *

      * @param o element whose presence in this set is to be tested

      * @return <tt>true</tt> if this set contains the specified element

      */

     public boolean contains(Object o) {

         return map.containsKey(o);

     }

     /**

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

      * More formally, adds the specified element <tt>e</tt> to this set if

      * this set contains no element <tt>e2</tt> 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 <tt>false</tt>.

      *

      * @param e element to be added to this set

      * @return <tt>true</tt> if this set did not already contain the specified

      * element

      */

     public boolean add(E e) {

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

     }

     /**

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

      * More formally, removes an element <tt>e</tt> such that

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

      * if this set contains such an element.  Returns <tt>true</tt> 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 <tt>true</tt> if the set contained the specified element

      */

     public boolean remove(Object o) {

         return map.remove(o)==PRESENT;

     }

     /**

      * Removes all of the elements from this set.

      * The set will be empty after this call returns.

      */

     public void clear() {

         map.clear();

     }

     /**

      * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements

      * themselves are not cloned.

      *

      * @return a shallow copy of this set

      */

     public Object clone() {

         try {

             HashSet<E> newSet = (HashSet<E>) super.clone();

             newSet.map = (HashMap<E, Object>) map.clone();

             return newSet;

         } catch (CloneNotSupportedException e) {

             throw new InternalError();

         }

     }

 }