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

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 * 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.

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 * 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

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 * 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,

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 *

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

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 */

package java.util;

import java.io.*;

/**

 * This class implements a hash table, which maps keys to values. Any

 * non-<code>null</code> object can be used as a key or as a value. <p>

 *

 * To successfully store and retrieve objects from a hashtable, the

 * objects used as keys must implement the <code>hashCode</code>

 * method and the <code>equals</code> method. <p>

 *

 * An instance of <code>Hashtable</code> has two parameters that affect its

 * performance: <i>initial capacity</i> and <i>load factor</i>.  The

 * <i>capacity</i> is the number of <i>buckets</i> in the hash table, and the

 * <i>initial capacity</i> is simply the capacity at the time the hash table

 * is created.  Note that the hash table is <i>open</i>: in the case of a "hash

 * collision", a single bucket stores multiple entries, which must be searched

 * sequentially.  The <i>load factor</i> is a measure of how full the hash

 * table is allowed to get before its capacity is automatically increased.

 * The initial capacity and load factor parameters are merely hints to

 * the implementation.  The exact details as to when and whether the rehash

 * method is invoked are implementation-dependent.<p>

 *

 * Generally, the default load factor (.75) offers a good tradeoff between

 * time and space costs.  Higher values decrease the space overhead but

 * increase the time cost to look up an entry (which is reflected in most

 * <tt>Hashtable</tt> operations, including <tt>get</tt> and <tt>put</tt>).<p>

 *

 * The initial capacity controls a tradeoff between wasted space and the

 * need for <code>rehash</code> operations, which are time-consuming.

 * No <code>rehash</code> operations will <i>ever</i> occur if the initial

 * capacity is greater than the maximum number of entries the

 * <tt>Hashtable</tt> will contain divided by its load factor.  However,

 * setting the initial capacity too high can waste space.<p>

 *

 * If many entries are to be made into a <code>Hashtable</code>,

 * creating it with a sufficiently large capacity may allow the

 * entries to be inserted more efficiently than letting it perform

 * automatic rehashing as needed to grow the table. <p>

 *

 * This example creates a hashtable of numbers. It uses the names of

 * the numbers as keys:

 * <pre>   {@code

 *   Hashtable<String, Integer> numbers

 *     = new Hashtable<String, Integer>();

 *   numbers.put("one", 1);

 *   numbers.put("two", 2);

 *   numbers.put("three", 3);}</pre>

 *

 * <p>To retrieve a number, use the following code:

 * <pre>   {@code

 *   Integer n = numbers.get("two");

 *   if (n != null) {

 *     System.out.println("two = " + n);

 *   }}</pre>

 *

 * <p>The iterators returned by the <tt>iterator</tt> method of the collections

 * returned by all of this class's "collection view methods" are

 * <em>fail-fast</em>: if the Hashtable is structurally modified at any time

 * after the iterator is created, in any way except through the iterator's own

 * <tt>remove</tt> 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.

 * The Enumerations returned by Hashtable's keys and elements methods are

 * <em>not</em> fail-fast.

 *

 * <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>As of the Java 2 platform v1.2, this class was retrofitted to

 * implement the {@link Map} interface, making it a member of the

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

 *

 * Java Collections Framework</a>.  Unlike the new collection

 * implementations, {@code Hashtable} is synchronized.  If a

 * thread-safe implementation is not needed, it is recommended to use

 * {@link HashMap} in place of {@code Hashtable}.  If a thread-safe

 * highly-concurrent implementation is desired, then it is recommended

 * to use {@link java.util.concurrent.ConcurrentHashMap} in place of

 * {@code Hashtable}.

 *

 * @author  Arthur van Hoff

 * @author  Josh Bloch

 * @author  Neal Gafter

 * @see     Object#equals(java.lang.Object)

 * @see     Object#hashCode()

 * @see     Hashtable#rehash()

 * @see     Collection

 * @see     Map

 * @see     HashMap

 * @see     TreeMap

 * @since JDK1.0

 */

public class Hashtable<K,V>

    extends Dictionary<K,V>

    implements Map<K,V>, Cloneable, java.io.Serializable {

    /**

     * The hash table data.

     */

    private transient Entry<?,?>[] table;

    /**

     * The total number of entries in the hash table.

     */

    private transient int count;

    /**

     * The table is rehashed when its size exceeds this threshold.  (The

     * value of this field is (int)(capacity * loadFactor).)

     *

     * @serial

     */

    private int threshold;

    /**

     * The load factor for the hashtable.

     *

     * @serial

     */

    private float loadFactor;

    /**

     * The number of times this Hashtable has been structurally modified

     * Structural modifications are those that change the number of entries in

     * the Hashtable or otherwise modify its internal structure (e.g.,

     * rehash).  This field is used to make iterators on Collection-views of

     * the Hashtable fail-fast.  (See ConcurrentModificationException).

     */

    private transient int modCount = 0;

    /** use serialVersionUID from JDK 1.0.2 for interoperability */

    private static final long serialVersionUID = 1421746759512286392L;

    private static class Holder {

            // Unsafe mechanics

        /**

         *

         */

        static final sun.misc.Unsafe UNSAFE;

        /**

         * Offset of "final" hashSeed field we must set in

         * readObject() method.

         */

        static final long HASHSEED_OFFSET;

        static {

            try {

                UNSAFE = sun.misc.Unsafe.getUnsafe();

                HASHSEED_OFFSET = UNSAFE.objectFieldOffset(

                    Hashtable.class.getDeclaredField("hashSeed"));

            } catch (NoSuchFieldException | SecurityException e) {

                throw new InternalError("Failed to record hashSeed offset", e);

            }

        }

    }

    /**

     * A randomizing value associated with this instance that is applied to

     * hash code of keys to make hash collisions harder to find.

     */

    transient final int hashSeed = sun.misc.Hashing.randomHashSeed(this);

    private int hash(Object k) {

        if (k instanceof String) {

            return ((String)k).hash32();

    }

    /**

     * Constructs a new, empty hashtable with the specified initial

     * capacity and the specified load factor.

     *

     * @param      initialCapacity   the initial capacity of the hashtable.

     * @param      loadFactor        the load factor of the hashtable.

     * @exception  IllegalArgumentException  if the initial capacity is less

     *             than zero, or if the load factor is nonpositive.

     */

    public Hashtable(int initialCapacity, float loadFactor) {

        if (initialCapacity < 0)

            throw new IllegalArgumentException("Illegal Capacity: "+

                                               initialCapacity);

        if (loadFactor <= 0 || Float.isNaN(loadFactor))

            throw new IllegalArgumentException("Illegal Load: "+loadFactor);

        if (initialCapacity==0)

            initialCapacity = 1;

        this.loadFactor = loadFactor;

        table = new Entry<?,?>[initialCapacity];

        threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);

    }

    /**

     * Constructs a new, empty hashtable with the specified initial capacity

     * and default load factor (0.75).

     *

     * @param     initialCapacity   the initial capacity of the hashtable.

     * @exception IllegalArgumentException if the initial capacity is less

     *              than zero.

     */

    public Hashtable(int initialCapacity) {

        this(initialCapacity, 0.75f);

    }

    /**

     * Constructs a new, empty hashtable with a default initial capacity (11)

     * and load factor (0.75).

     */

    public Hashtable() {

        this(11, 0.75f);

    }

    /**

     * Constructs a new hashtable with the same mappings as the given

     * Map.  The hashtable is created with an initial capacity sufficient to

     * hold the mappings in the given Map and a default load factor (0.75).

     *

     * @param t the map whose mappings are to be placed in this map.

     * @throws NullPointerException if the specified map is null.

     * @since   1.2

     */

    public Hashtable(Map<? extends K, ? extends V> t) {

        this(Math.max(2*t.size(), 11), 0.75f);

        putAll(t);

    }

    /**

     * Returns the number of keys in this hashtable.

     *

     * @return  the number of keys in this hashtable.

     */

    public synchronized int size() {

        return count;

    }

    /**

     * Tests if this hashtable maps no keys to values.

     *

     * @return  <code>true</code> if this hashtable maps no keys to values;

     *          <code>false</code> otherwise.

     */

    public synchronized boolean isEmpty() {

        return count == 0;

    }

    /**

     * Returns an enumeration of the keys in this hashtable.

     *

     * @return  an enumeration of the keys in this hashtable.

     * @see     Enumeration

     * @see     #elements()

     * @see     #keySet()

     * @see     Map

     */

    public synchronized Enumeration<K> keys() {

        return this.<K>getEnumeration(KEYS);

    }

    /**

     * Returns an enumeration of the values in this hashtable.

     * Use the Enumeration methods on the returned object to fetch the elements

     * sequentially.

     *

     * @return  an enumeration of the values in this hashtable.

     * @see     java.util.Enumeration

     * @see     #keys()

     * @see     #values()

     * @see     Map

     */

    public synchronized Enumeration<V> elements() {

        return this.<V>getEnumeration(VALUES);

    }

    /**

     * Tests if some key maps into the specified value in this hashtable.

     * This operation is more expensive than the {@link #containsKey

     * containsKey} method.

     *

     * <p>Note that this method is identical in functionality to

     * {@link #containsValue containsValue}, (which is part of the

     * {@link Map} interface in the collections framework).

     *

     * @param      value   a value to search for

     * @return     <code>true</code> if and only if some key maps to the

     *             <code>value</code> argument in this hashtable as

     *             determined by the <tt>equals</tt> method;

     *             <code>false</code> otherwise.

     * @exception  NullPointerException  if the value is <code>null</code>

     */

    public synchronized boolean contains(Object value) {

        if (value == null) {

            throw new NullPointerException();

        }

        Entry<?,?> tab[] = table;

        for (int i = tab.length ; i-- > 0 ;) {

            for (Entry<?,?> e = tab[i] ; e != null ; e = e.next) {

                if (e.value.equals(value)) {

                    return true;

                }

            }

        }

        return false;

    }

    /**

     * Returns true if this hashtable maps one or more keys to this value.

     *

     * <p>Note that this method is identical in functionality to {@link

     * #contains contains} (which predates the {@link Map} interface).

     *

     * @param value value whose presence in this hashtable is to be tested

     * @return <tt>true</tt> if this map maps one or more keys to the

     *         specified value

     * @throws NullPointerException  if the value is <code>null</code>

     * @since 1.2

     */

    public boolean containsValue(Object value) {

        return contains(value);

    }

    /**

     * Tests if the specified object is a key in this hashtable.

     *

     * @param   key   possible key

     * @return  <code>true</code> if and only if the specified object

     *          is a key in this hashtable, as determined by the

     *          <tt>equals</tt> method; <code>false</code> otherwise.

     * @throws  NullPointerException  if the key is <code>null</code>

     * @see     #contains(Object)

     */

    public synchronized boolean containsKey(Object key) {

        Entry<?,?> tab[] = table;

        int hash = hash(key);

        int index = (hash & 0x7FFFFFFF) % tab.length;

        for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {

            if ((e.hash == hash) && e.key.equals(key)) {

                return true;

            }

        }

        return false;

    }

    /**

     * Returns the value to which the specified key is mapped,

     * or {@code null} if this map contains no mapping for the key.

     *

     * <p>More formally, if this map contains a mapping from a key

     * {@code k} to a value {@code v} such that {@code (key.equals(k))},

     * then this method returns {@code v}; otherwise it returns

     * {@code null}.  (There can be at most one such mapping.)

     *

     * @param key the key whose associated value is to be returned

     * @return the value to which the specified key is mapped, or

     *         {@code null} if this map contains no mapping for the key

     * @throws NullPointerException if the specified key is null

     * @see     #put(Object, Object)

     */

    @SuppressWarnings("unchecked")

    public synchronized V get(Object key) {

        Entry<?,?> tab[] = table;

        int hash = hash(key);

        int index = (hash & 0x7FFFFFFF) % tab.length;

        for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {

            if ((e.hash == hash) && e.key.equals(key)) {

                return (V)e.value;

            }

        }

        return null;

    }

    /**

     * The maximum size of array to allocate.

     * Some VMs reserve some header words in an array.

     * Attempts to allocate larger arrays may result in

     * OutOfMemoryError: Requested array size exceeds VM limit

     */

    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**

     * Increases the capacity of and internally reorganizes this

     * hashtable, in order to accommodate and access its entries more

     * efficiently.  This method is called automatically when the

     * number of keys in the hashtable exceeds this hashtable's capacity

     * and load factor.

     */

    @SuppressWarnings("unchecked")

    protected void rehash() {

        int oldCapacity = table.length;

        Entry<?,?>[] oldMap = table;

        // overflow-conscious code

        int newCapacity = (oldCapacity << 1) + 1;

        if (newCapacity - MAX_ARRAY_SIZE > 0) {

            if (oldCapacity == MAX_ARRAY_SIZE)

                // Keep running with MAX_ARRAY_SIZE buckets

                return;

            newCapacity = MAX_ARRAY_SIZE;

        }

        Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];

        modCount++;

        threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);

        table = newMap;

        for (int i = oldCapacity ; i-- > 0 ;) {

            for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) {

                Entry<K,V> e = old;

                old = old.next;

                int index = (e.hash & 0x7FFFFFFF) % newCapacity;

                e.next = (Entry<K,V>)newMap[index];

                newMap[index] = e;

            }

        }

    }

    /**

     * Maps the specified <code>key</code> to the specified

     * <code>value</code> in this hashtable. Neither the key nor the

     * value can be <code>null</code>. <p>

     *

     * The value can be retrieved by calling the <code>get</code> method

     * with a key that is equal to the original key.

     *

     * @param      key     the hashtable key

     * @param      value   the value

     * @return     the previous value of the specified key in this hashtable,

     *             or <code>null</code> if it did not have one

     * @exception  NullPointerException  if the key or value is

     *               <code>null</code>

     * @see     Object#equals(Object)

     * @see     #get(Object)

     */

    public synchronized V put(K key, V value) {

        // Make sure the value is not null

        if (value == null) {

            throw new NullPointerException();

        }

        // Makes sure the key is not already in the hashtable.

        Entry<?,?> tab[] = table;

        int hash = hash(key);

        int index = (hash & 0x7FFFFFFF) % tab.length;

        @SuppressWarnings("unchecked")

        Entry<K,V> entry = (Entry<K,V>)tab[index];

        for(; entry != null ; entry = entry.next) {

            if ((entry.hash == hash) && entry.key.equals(key)) {

                V old = entry.value;

                entry.value = value;

                return old;

            }

        }

        modCount++;

        if (count >= threshold) {

            // Rehash the table if the threshold is exceeded

            rehash();

            tab = table;

            hash = hash(key);

            index = (hash & 0x7FFFFFFF) % tab.length;

        }

        // Creates the new entry.

        @SuppressWarnings("unchecked")

        Entry<K,V> e = (Entry<K,V>)tab[index];

        tab[index] = new Entry<>(hash, key, value, e);

        count++;

        return null;

    }

    /**

     * Removes the key (and its corresponding value) from this

     * hashtable. This method does nothing if the key is not in the hashtable.

     *

     * @param   key   the key that needs to be removed