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 * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.

 * 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) {

        }

    }

    /**

     * Save the state of this <tt>HashSet</tt> instance to a stream (that is,

     * serialize it).

     *

     * @serialData The capacity of the backing <tt>HashMap</tt> instance

     *             (int), and its load factor (float) are emitted, followed by

     *             the size of the set (the number of elements it contains)

     *             (int), followed by all of its elements (each an Object) in

     *             no particular order.

     */

    private void writeObject(java.io.ObjectOutputStream s)

        throws java.io.IOException {

        // Write out any hidden serialization magic

        s.defaultWriteObject();

        // Write out HashMap capacity and load factor

        s.writeInt(map.capacity());

        s.writeFloat(map.loadFactor());

        // Write out size

        s.writeInt(map.size());

        // Write out all elements in the proper order.

        for (E e : map.keySet())

            s.writeObject(e);

    }

    /**

     * Reconstitute the <tt>HashSet</tt> instance from a stream (that is,

     * deserialize it).

     */

    private void readObject(java.io.ObjectInputStream s)

        throws java.io.IOException, ClassNotFoundException {

        // Read in any hidden serialization magic

        s.defaultReadObject();

        // Read in HashMap capacity and load factor and create backing HashMap

        int capacity = s.readInt();

        float loadFactor = s.readFloat();

        map = (((HashSet)this) instanceof LinkedHashSet ?

               new LinkedHashMap<E,Object>(capacity, loadFactor) :

               new HashMap<E,Object>(capacity, loadFactor));

        // Read in size

        int size = s.readInt();

        // Read in all elements in the proper order.

        for (int i=0; i<size; i++) {

            E e = (E) s.readObject();

            map.put(e, PRESENT);

        }

    }

}