/*

  * 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

  * or visit www.oracle.com if you need additional information or have any

  * questions.

  */

 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;

     }

     /**

      * Save the state of the Hashtable to a stream (i.e., serialize it).

      *

      * @serialData The <i>capacity</i> of the Hashtable (the length of the

      *             bucket array) is emitted (int), followed by the

      *             <i>size</i> of the Hashtable (the number of key-value

      *             mappings), followed by the key (Object) and value (Object)

      *             for each key-value mapping represented by the Hashtable

      *             The key-value mappings are emitted in no particular order.

      */

     private void writeObject(java.io.ObjectOutputStream s)

             throws IOException {

         Entry<Object, Object> entryStack = null;

         synchronized (this) {

             // Write out the length, threshold, loadfactor

             s.defaultWriteObject();

             // Write out length, count of elements

             s.writeInt(table.length);

             s.writeInt(count);

             // Stack copies of the entries in the table

             for (int index = 0; index < table.length; index++) {

                 Entry entry = table[index];

                 while (entry != null) {

                     entryStack =

                         new Entry<>(0, entry.key, entry.value, entryStack);

                     entry = entry.next;

                 }

             }

         }

         // Write out the key/value objects from the stacked entries

         while (entryStack != null) {

             s.writeObject(entryStack.key);

             s.writeObject(entryStack.value);

             entryStack = entryStack.next;

         }

     }

     /**

      * Reconstitute the Hashtable from a stream (i.e., deserialize it).

      */

     private void readObject(java.io.ObjectInputStream s)

          throws IOException, ClassNotFoundException

     {

         // Read in the length, threshold, and loadfactor

         s.defaultReadObject();

         // Read the original length of the array and number of elements

         int origlength = s.readInt();

         int elements = s.readInt();

         // Compute new size with a bit of room 5% to grow but

         // no larger than the original size.  Make the length

         // odd if it's large enough, this helps distribute the entries.

         // Guard against the length ending up zero, that's not valid.

         int length = (int)(elements * loadFactor) + (elements / 20) + 3;

         if (length > elements && (length & 1) == 0)

             length--;

         if (origlength > 0 && length > origlength)

             length = origlength;

         Entry[] table = new Entry[length];

         count = 0;

         // Read the number of elements and then all the key/value objects

         for (; elements > 0; elements--) {

             K key = (K)s.readObject();

             V value = (V)s.readObject();

             // synch could be eliminated for performance

             reconstitutionPut(table, key, value);

         }

         this.table = table;

     }

     /**

      * The put method used by readObject. This is provided because put

      * is overridable and should not be called in readObject since the

      * subclass will not yet be initialized.

      *

      * <p>This differs from the regular put method in several ways. No

      * checking for rehashing is necessary since the number of elements

      * initially in the table is known. The modCount is not incremented

      * because we are creating a new instance. Also, no return value

      * is needed.

      */

     private void reconstitutionPut(Entry[] tab, K key, V value)

         throws StreamCorruptedException

     {

         if (value == null) {

             throw new java.io.StreamCorruptedException();

         }

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

         // This should not happen in deserialized version.

         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)) {

                 throw new java.io.StreamCorruptedException();

             }

         }

         // Creates the new entry.

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

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

         count++;

     }

     /**

      * 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

                             prev.next = e.next;

                         count--;

                         lastReturned = null;

                         return;

                     }

                 }

                 throw new ConcurrentModificationException();

             }

         }

     }

 }