/*

 * Copyright (c) 1994, 2011, 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;

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;

    /**

     * 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)(initialCapacity * loadFactor);

    }

    /**

     * 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<K,V> 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 = key.hashCode();

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

        for (Entry<K,V> 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)

     */

    public synchronized V get(Object key) {

        Entry tab[] = table;

        int hash = key.hashCode();

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

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

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

                return 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.

     */

    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)(newCapacity * loadFactor);

        table = newMap;

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

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

                Entry<K,V> e = old;

                old = old.next;

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

                e.next = 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 = key.hashCode();

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

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

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

                V old = e.value;

                e.value = value;

                return old;

            }

        }

        modCount++;

        if (count >= threshold) {

            // Rehash the table if the threshold is exceeded

            rehash();

            tab = table;

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

        }

        // Creates the new entry.

        Entry<K,V> e = 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

     * @return  the value to which the key had been mapped in this hashtable,

     *          or <code>null</code> if the key did not have a mapping

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

     */

    public synchronized V remove(Object key) {

        Entry tab[] = table;

        int hash = key.hashCode();

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

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

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

                modCount++;

                if (prev != null) {

                    prev.next = e.next;

                } else {

                    tab[index] = e.next;

                }

                count--;

                V oldValue = e.value;

                e.value = null;

                return oldValue;

            }

        }

        return null;

    }

    /**

     * Copies all of the mappings from the specified map to this hashtable.

     * These mappings will replace any mappings that this hashtable had for any

     * of the keys currently in the specified map.

     *

     * @param t mappings to be stored in this map

     * @throws NullPointerException if the specified map is null

     * @since 1.2

     */

    public synchronized void putAll(Map<? extends K, ? extends V> t) {

        for (Map.Entry<? extends K, ? extends V> e : t.entrySet())

            put(e.getKey(), e.getValue());

    }

    /**

     * Clears this hashtable so that it contains no keys.

     */

    public synchronized void clear() {

        Entry tab[] = table;

        modCount++;

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

            tab[index] = null;

        count = 0;

    }

    /**

     * Creates a shallow copy of this hashtable. All the structure of the

     * hashtable itself is copied, but the keys and values are not cloned.

     * This is a relatively expensive operation.

     *

     * @return  a clone of the hashtable

     */

    public synchronized Object clone() {

        try {

            Hashtable<K,V> t = (Hashtable<K,V>) super.clone();

            t.table = new Entry[table.length];

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

                t.table[i] = (table[i] != null)

                    ? (Entry<K,V>) table[i].clone() : null;

            }

            t.keySet = null;

            t.entrySet = null;

            t.values = null;

            t.modCount = 0;

            return t;

        } catch (CloneNotSupportedException e) {

            // this shouldn't happen, since we are Cloneable

            throw new InternalError();

        }

    }

    /**

     * Returns a string representation of this <tt>Hashtable</tt> object

     * in the form of a set of entries, enclosed in braces and separated

     * by the ASCII characters "<tt>,&nbsp;</tt>" (comma and space). Each

     * entry is rendered as the key, an equals sign <tt>=</tt>, and the

     * associated element, where the <tt>toString</tt> method is used to

     * convert the key and element to strings.

     *

     * @return  a string representation of this hashtable

     */

    public synchronized String toString() {

        int max = size() - 1;

        if (max == -1)

            return "{}";

        StringBuilder sb = new StringBuilder();

        Iterator<Map.Entry<K,V>> it = entrySet().iterator();

        sb.append('{');

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

            Map.Entry<K,V> e = it.next();

            K key = e.getKey();

            V value = e.getValue();

            sb.append(key   == this ? "(this Map)" : key.toString());

            sb.append('=');

            sb.append(value == this ? "(this Map)" : value.toString());

            if (i == max)

                return sb.append('}').toString();

            sb.append(", ");

        }

    }

    private <T> Enumeration<T> getEnumeration(int type) {

        if (count == 0) {

            return Collections.emptyEnumeration();

        } else {

            return new Enumerator<>(type, false);

        }

    }

    private <T> Iterator<T> getIterator(int type) {

        if (count == 0) {

            return Collections.emptyIterator();

        } else {

            return new Enumerator<>(type, true);

        }

    }

    // Views

    /**

     * Each of these fields are initialized to contain an instance of the

     * appropriate view the first time this view is requested.  The views are

     * stateless, so there's no reason to create more than one of each.

     */

    private transient volatile Set<K> keySet = null;

    private transient volatile Set<Map.Entry<K,V>> entrySet = null;

    private transient volatile Collection<V> values = null;

    /**

     * Returns a {@link Set} view of the keys contained in this map.

     * The set is backed by the map, so changes to the map are

     * reflected in the set, and vice-versa.  If the map is modified

     * while an iteration over the set is in progress (except through

     * the iterator's own <tt>remove</tt> operation), the results of

     * the iteration are undefined.  The set supports element removal,

     * which removes the corresponding mapping from the map, via the

     * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,

     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>

     * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>

     * operations.

     *

     * @since 1.2

     */

    public Set<K> keySet() {

        if (keySet == null)

            keySet = Collections.synchronizedSet(new KeySet(), this);

        return keySet;

    }

    private class KeySet extends AbstractSet<K> {

        public Iterator<K> iterator() {

            return getIterator(KEYS);

        }

        public int size() {

            return count;

        }

        public boolean contains(Object o) {

            return containsKey(o);

        }

        public boolean remove(Object o) {

            return Hashtable.this.remove(o) != null;

        }

        public void clear() {

            Hashtable.this.clear();

        }

    }

    /**

     * Returns a {@link Set} view of the mappings contained in this map.

     * The set is backed by the map, so changes to the map are

     * reflected in the set, and vice-versa.  If the map is modified

     * while an iteration over the set is in progress (except through

     * the iterator's own <tt>remove</tt> operation, or through the

     * <tt>setValue</tt> operation on a map entry returned by the

     * iterator) the results of the iteration are undefined.  The set

     * supports element removal, which removes the corresponding

     * mapping from the map, via the <tt>Iterator.remove</tt>,

     * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and

     * <tt>clear</tt> operations.  It does not support the

     * <tt>add</tt> or <tt>addAll</tt> operations.

     *

     * @since 1.2

     */

    public Set<Map.Entry<K,V>> entrySet() {

        if (entrySet==null)

            entrySet = Collections.synchronizedSet(new EntrySet(), this);

        return entrySet;

    }

    private class EntrySet extends AbstractSet<Map.Entry<K,V>> {

        public Iterator<Map.Entry<K,V>> iterator() {

            return getIterator(ENTRIES);

        }

        public boolean add(Map.Entry<K,V> o) {

            return super.add(o);

        }

        public boolean contains(Object o) {

            if (!(o instanceof Map.Entry))

                return false;

            Map.Entry entry = (Map.Entry)o;

            Object key = entry.getKey();

            Entry[] tab = table;

            int hash = key.hashCode();

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

            for (Entry e = tab[index]; e != null; e = e.next)

                if (e.hash==hash && e.equals(entry))

                    return true;

            return false;

        }

        public boolean remove(Object o) {

            if (!(o instanceof Map.Entry))

                return false;

            Map.Entry<K,V> entry = (Map.Entry<K,V>) o;

            K key = entry.getKey();

            Entry[] tab = table;

            int hash = key.hashCode();

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

            for (Entry<K,V> e = tab[index], prev = null; e != null;

                 prev = e, e = e.next) {

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

                    modCount++;

                    if (prev != null)

                        prev.next = e.next;

                    else

                        tab[index] = e.next;

                    count--;

                    e.value = null;

                    return true;

                }

            }

            return false;

        }

        public int size() {

            return count;

        }

        public void clear() {

            Hashtable.this.clear();

        }

    }

    /**

     * Returns a {@link Collection} view of the values contained in this map.

     * The collection is backed by the map, so changes to the map are

     * reflected in the collection, and vice-versa.  If the map is

     * modified while an iteration over the collection is in progress

     * (except through the iterator's own <tt>remove</tt> operation),

     * the results of the iteration are undefined.  The collection

     * supports element removal, which removes the corresponding

     * mapping from the map, via the <tt>Iterator.remove</tt>,

     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,

     * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not

     * support the <tt>add</tt> or <tt>addAll</tt> operations.

     *

     * @since 1.2

     */

    public Collection<V> values() {

        if (values==null)

            values = Collections.synchronizedCollection(new ValueCollection(),

                                                        this);

        return values;

    }

    private class ValueCollection extends AbstractCollection<V> {

        public Iterator<V> iterator() {

            return getIterator(VALUES);

        }

        public int size() {

            return count;

        }

        public boolean contains(Object o) {

            return containsValue(o);

        }

        public void clear() {

            Hashtable.this.clear();

        }

    }

    // Comparison and hashing

    /**

     * Compares the specified Object with this Map for equality,

     * as per the definition in the Map interface.

     *

     * @param  o object to be compared for equality with this hashtable

     * @return true if the specified Object is equal to this Map

     * @see Map#equals(Object)

     * @since 1.2

     */

    public synchronized boolean equals(Object o) {

        if (o == this)

            return true;

        if (!(o instanceof Map))

            return false;

        Map<K,V> t = (Map<K,V>) o;

        if (t.size() != size())

            return false;

        try {

            Iterator<Map.Entry<K,V>> i = entrySet().iterator();

            while (i.hasNext()) {

                Map.Entry<K,V> e = i.next();

                K key = e.getKey();

                V value = e.getValue();

                if (value == null) {

                    if (!(t.get(key)==null && t.containsKey(key)))

                        return false;

                } else {

                    if (!value.equals(t.get(key)))

                        return false;

                }

            }

        } catch (ClassCastException unused)   {

            return false;

        } catch (NullPointerException unused) {

            return false;

        }

        return true;

    }

    /**

     * Returns the hash code value for this Map as per the definition in the

     * Map interface.

     *

     * @see Map#hashCode()

     * @since 1.2

     */

    public synchronized int hashCode() {

        /*

         * This code detects the recursion caused by computing the hash code

         * of a self-referential hash table and prevents the stack overflow

         * that would otherwise result.  This allows certain 1.1-era

         * applets with self-referential hash tables to work.  This code

         * abuses the loadFactor field to do double-duty as a hashCode

         * in progress flag, so as not to worsen the space performance.

         * A negative load factor indicates that hash code computation is

         * in progress.

         */

        int h = 0;

        if (count == 0 || loadFactor < 0)

            return h;  // Returns zero

        loadFactor = -loadFactor;  // Mark hashCode computation in progress

        Entry[] tab = table;

        for (int i = 0; i < tab.length; i++)

            for (Entry e = tab[i]; e != null; e = e.next)

                h += e.key.hashCode() ^ e.value.hashCode();

        loadFactor = -loadFactor;  // Mark hashCode computation complete

        return h;

    }

    /**

     * Hashtable collision list.

     */

    private static class Entry<K,V> implements Map.Entry<K,V> {

        int hash;

        K key;

        V value;

        Entry<K,V> next;

        protected Entry(int hash, K key, V value, Entry<K,V> next) {

            this.hash = hash;

            this.key = key;

            this.value = value;

            this.next = next;

        }

        protected Object clone() {

            return new Entry<>(hash, key, value,

                                  (next==null ? null : (Entry<K,V>) next.clone()));

        }

        // Map.Entry Ops

        public K getKey() {

            return key;

        }

        public V getValue() {

            return value;

        }

        public V setValue(V value) {

            if (value == null)

                throw new NullPointerException();

            V oldValue = this.value;

            this.value = value;

            return oldValue;

        }

        public boolean equals(Object o) {

            if (!(o instanceof Map.Entry))

                return false;

            Map.Entry e = (Map.Entry)o;

            return (key==null ? e.getKey()==null : key.equals(e.getKey())) &&

               (value==null ? e.getValue()==null : value.equals(e.getValue()));

        }

        public int hashCode() {

            return hash ^ (value==null ? 0 : value.hashCode());

        }

        public String toString() {

            return key.toString()+"="+value.toString();

        }

    }

    // Types of Enumerations/Iterations

    private static final int KEYS = 0;

    private static final int VALUES = 1;

    private static final int ENTRIES = 2;

    /**

     * A hashtable enumerator class.  This class implements both the

     * Enumeration and Iterator interfaces, but individual instances

     * can be created with the Iterator methods disabled.  This is necessary

     * to avoid unintentionally increasing the capabilities granted a user

     * by passing an Enumeration.

     */

    private class Enumerator<T> implements Enumeration<T>, Iterator<T> {

        Entry[] table = Hashtable.this.table;

        int index = table.length;

        Entry<K,V> entry = null;

        Entry<K,V> lastReturned = null;

        int type;

        /**

         * Indicates whether this Enumerator is serving as an Iterator

         * or an Enumeration.  (true -> Iterator).

         */

        boolean iterator;

        /**

         * The modCount value that the iterator believes that the backing

         * Hashtable should have.  If this expectation is violated, the iterator

         * has detected concurrent modification.

         */

        protected int expectedModCount = modCount;

        Enumerator(int type, boolean iterator) {

            this.type = type;

            this.iterator = iterator;

        }

        public boolean hasMoreElements() {

            Entry<K,V> e = entry;

            int i = index;

            Entry[] t = table;

            /* Use locals for faster loop iteration */

            while (e == null && i > 0) {

                e = t[--i];

            }

            entry = e;

            index = i;

            return e != null;

        }

        public T nextElement() {

            Entry<K,V> et = entry;

            int i = index;

            Entry[] t = table;

            /* Use locals for faster loop iteration */

            while (et == null && i > 0) {

                et = t[--i];

            }

            entry = et;

            index = i;

            if (et != null) {

                Entry<K,V> e = lastReturned = entry;

                entry = e.next;

                return type == KEYS ? (T)e.key : (type == VALUES ? (T)e.value : (T)e);

            }

            throw new NoSuchElementException("Hashtable Enumerator");

        }

        // Iterator methods

        public boolean hasNext() {

            return hasMoreElements();

        }

        public T next() {

            if (modCount != expectedModCount)

                throw new ConcurrentModificationException();

            return nextElement();

        }

        public void remove() {

            if (!iterator)

                throw new UnsupportedOperationException();

            if (lastReturned == null)

                throw new IllegalStateException("Hashtable Enumerator");

            if (modCount != expectedModCount)

                throw new ConcurrentModificationException();

            synchronized(Hashtable.this) {

                Entry[] tab = Hashtable.this.table;

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

                for (Entry<K,V> e = tab[index], prev = null; e != null;

                     prev = e, e = e.next) {

                    if (e == lastReturned) {

                        modCount++;

                        expectedModCount++;

                        if (prev == null)

                            tab[index] = e.next;

                        else