/*

 * Copyright (c) 1994, 2019, 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.*;

import java.util.function.BiConsumer;

import java.util.function.Function;

import java.util.function.BiFunction;

import jdk.internal.access.SharedSecrets;

/**

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

 * non-{@code null} 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}

 * method and the {@code equals} method. <p>

 *

 * An instance of {@code Hashtable} 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

 * {@code Hashtable} operations, including {@code get} and {@code put}).<p>

 *

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

 * need for {@code rehash} operations, which are time-consuming.

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

 * capacity is greater than the maximum number of entries the

 * {@code Hashtable} 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},

 * 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 {@code iterator} 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

 * {@code remove} 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 {@link #keys keys} and

 * {@link #elements elements} methods are <em>not</em> fail-fast; if the

 * Hashtable is structurally modified at any time after the enumeration is

 * created then the results of enumerating are undefined.

 *

 * <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 {@code ConcurrentModificationException} 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}/java.base/java/util/package-summary.html#CollectionsFramework">

 *

 * 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}.

 *

 * @param <K> the type of keys maintained by this map

 * @param <V> the type of mapped values

 *

 * @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 1.0

 */

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 */

    @java.io.Serial

    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.

     * @throws     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.

     * @throws    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);

    }

    /**

     * A constructor chained from {@link Properties} keeps Hashtable fields

     * uninitialized since they are not used.

     *

     * @param dummy a dummy parameter

     */

    Hashtable(Void dummy) {}

    /**

     * 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} if this hashtable maps no keys to values;

     *          {@code false} otherwise.

     */

    public synchronized boolean isEmpty() {

        return count == 0;

    }

    /**

     * Returns an enumeration of the keys in this hashtable.

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

     * sequentially. If the hashtable is structurally modified while enumerating

     * over the keys then the results of enumerating are undefined.

     *

     * @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. If the hashtable is structurally modified while enumerating

     * over the values then the results of enumerating are undefined.

     *

     * @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} if and only if some key maps to the

     *             {@code value} argument in this hashtable as

     *             determined by the {@code equals} method;

     *             {@code false} otherwise.

     * @throws     NullPointerException  if the value is {@code null}

     */

    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 {@code true} if this map maps one or more keys to the

     *         specified value

     * @throws NullPointerException  if the value is {@code null}

     * @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} if and only if the specified object

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

     *          {@code equals} method; {@code false} otherwise.

     * @throws  NullPointerException  if the key is {@code null}

     * @see     #contains(Object)

     */

    public synchronized boolean containsKey(Object key) {

        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.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 = key.hashCode();

        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;

            }

        }

    }

    private void addEntry(int hash, K key, V value, int index) {

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

        if (count >= threshold) {

            // Rehash the table if the threshold is exceeded

            rehash();

            tab = table;

            hash = key.hashCode();

            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++;

        modCount++;

    }

    /**

     * Maps the specified {@code key} to the specified

     * {@code value} in this hashtable. Neither the key nor the

     * value can be {@code null}. <p>

     *

     * The value can be retrieved by calling the {@code get} 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} if it did not have one

     * @throws     NullPointerException  if the key or value is

     *               {@code null}

     * @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;

        @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;

            }

        }

        addEntry(hash, key, value, index);

        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} if the key did not have a mapping

     * @throws  NullPointerException  if the key is {@code null}

     */

    public synchronized V remove(Object key) {

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

        int hash = key.hashCode();

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

        @SuppressWarnings("unchecked")

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

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

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

                if (prev != null) {

                    prev.next = e.next;

                } else {

                    tab[index] = e.next;

                }

                modCount++;

                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;

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

            tab[index] = null;

        modCount++;

        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() {

        Hashtable<?,?> t = cloneHashtable();

        t.table = new Entry<?,?>[table.length];

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

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

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

        }

        t.keySet = null;

        t.entrySet = null;

        t.values = null;

        t.modCount = 0;

        return t;

    }

    /** Calls super.clone() */

    final Hashtable<?,?> cloneHashtable() {

        try {

            return (Hashtable<?,?>)super.clone();

        } catch (CloneNotSupportedException e) {

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

            throw new InternalError(e);

        }

    }

    /**

     * Returns a string representation of this {@code Hashtable} object

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

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

     * entry is rendered as the key, an equals sign {@code =}, and the

     * associated element, where the {@code toString} 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;

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

    private transient volatile Collection<V> values;

    /**

     * 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 {@code remove} operation), the results of

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

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

     * {@code Iterator.remove}, {@code Set.remove},

     * {@code removeAll}, {@code retainAll}, and {@code clear}

     * operations.  It does not support the {@code add} or {@code addAll}

     * 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 {@code remove} operation, or through the

     * {@code setValue} 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 {@code Iterator.remove},

     * {@code Set.remove}, {@code removeAll}, {@code retainAll} and

     * {@code clear} operations.  It does not support the

     * {@code add} or {@code addAll} 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<?, ?> entry))

                return false;

            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<?, ?> entry))

                return false;

            Object key = entry.getKey();

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

            int hash = key.hashCode();

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

            @SuppressWarnings("unchecked")

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

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

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

                    if (prev != null)

                        prev.next = e.next;

                    else

                        tab[index] = e.next;

                    e.value = null; // clear for gc.

                    modCount++;

                    count--;

                    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 {@code remove} operation),

     * the results of the iteration are undefined.  The collection

     * supports element removal, which removes the corresponding

     * mapping from the map, via the {@code Iterator.remove},

     * {@code Collection.remove}, {@code removeAll},

     * {@code retainAll} and {@code clear} operations.  It does not

     * support the {@code add} or {@code addAll} 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<?, ?> t))

            return false;

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

            return false;

        try {

            for (Map.Entry<K, V> e : entrySet()) {

                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 (Entry<?,?> entry : tab) {

            while (entry != null) {

                h += entry.hashCode();

                entry = entry.next;

            }

        }

        loadFactor = -loadFactor;  // Mark hashCode computation complete

        return h;

    }

    @Override

    public synchronized V getOrDefault(Object key, V defaultValue) {

        V result = get(key);

        return (null == result) ? defaultValue : result;

    }

    @SuppressWarnings("unchecked")

    @Override

    public synchronized void forEach(BiConsumer<? super K, ? super V> action) {

        Objects.requireNonNull(action);     // explicit check required in case

                                            // table is empty.

        final int expectedModCount = modCount;

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

        for (Entry<?, ?> entry : tab) {

            while (entry != null) {

                action.accept((K)entry.key, (V)entry.value);

                entry = entry.next;

                if (expectedModCount != modCount) {

                    throw new ConcurrentModificationException();

                }

            }

        }

    }

    @SuppressWarnings("unchecked")

    @Override

    public synchronized void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {

        Objects.requireNonNull(function);     // explicit check required in case

                                              // table is empty.

        final int expectedModCount = modCount;

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

        for (Entry<K, V> entry : tab) {

            while (entry != null) {

                entry.value = Objects.requireNonNull(

                    function.apply(entry.key, entry.value));

                entry = entry.next;

                if (expectedModCount != modCount) {

                    throw new ConcurrentModificationException();

                }

            }

        }

    }

    @Override

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

        Objects.requireNonNull(value);

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

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

        int hash = key.hashCode();

        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;

                if (old == null) {

                    entry.value = value;

                }

                return old;

            }

        }

        addEntry(hash, key, value, index);

        return null;

    }

    @Override

    public synchronized boolean remove(Object key, Object value) {

        Objects.requireNonNull(value);

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

        int hash = key.hashCode();

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

        @SuppressWarnings("unchecked")

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

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

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

                if (prev != null) {

                    prev.next = e.next;

                } else {

                    tab[index] = e.next;

                }

                e.value = null; // clear for gc

                modCount++;

                count--;

                return true;

            }

        }

        return false;

    }

    @Override

    public synchronized boolean replace(K key, V oldValue, V newValue) {

        Objects.requireNonNull(oldValue);

        Objects.requireNonNull(newValue);

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

        int hash = key.hashCode();

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

        @SuppressWarnings("unchecked")

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

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

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

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

                    e.value = newValue;

                    return true;

                } else {

                    return false;

                }

            }

        }

        return false;

    }

    @Override

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

        Objects.requireNonNull(value);

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

        int hash = key.hashCode();

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

        @SuppressWarnings("unchecked")

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

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

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

                V oldValue = e.value;

                e.value = value;

                return oldValue;

            }

        }

        return null;

    }

    /**

     * {@inheritDoc}

     *

     * <p>This method will, on a best-effort basis, throw a

     * {@link java.util.ConcurrentModificationException} if the mapping

     * function modified this map during computation.

     *

     * @throws ConcurrentModificationException if it is detected that the

     * mapping function modified this map

     */

    @Override

    public synchronized V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {

        Objects.requireNonNull(mappingFunction);

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

        int hash = key.hashCode();

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

        @SuppressWarnings("unchecked")

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

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

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

                // Hashtable not accept null value

                return e.value;

            }

        }

        int mc = modCount;

        V newValue = mappingFunction.apply(key);

        if (mc != modCount) { throw new ConcurrentModificationException(); }

        if (newValue != null) {

            addEntry(hash, key, newValue, index);

        }

        return newValue;

    }

    /**

     * {@inheritDoc}

     *

     * <p>This method will, on a best-effort basis, throw a

     * {@link java.util.ConcurrentModificationException} if the remapping

     * function modified this map during computation.

     *

     * @throws ConcurrentModificationException if it is detected that the

     * remapping function modified this map

     */

    @Override

    public synchronized V computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) {

        Objects.requireNonNull(remappingFunction);

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

        int hash = key.hashCode();

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

        @SuppressWarnings("unchecked")

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

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

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

                int mc = modCount;

                V newValue = remappingFunction.apply(key, e.value);

                if (mc != modCount) {

                    throw new ConcurrentModificationException();

                }

                if (newValue == null) {

                    if (prev != null) {

                        prev.next = e.next;

                    } else {

                        tab[index] = e.next;

                    }

                    modCount = mc + 1;

                    count--;

                } else {

                    e.value = newValue;

                }

                return newValue;

            }

        }

        return null;

    }

    /**

     * {@inheritDoc}

     *

     * <p>This method will, on a best-effort basis, throw a

     * {@link java.util.ConcurrentModificationException} if the remapping

     * function modified this map during computation.

     *

     * @throws ConcurrentModificationException if it is detected that the

     * remapping function modified this map

     */

    @Override

    public synchronized V compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) {

        Objects.requireNonNull(remappingFunction);

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

        int hash = key.hashCode();

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

        @SuppressWarnings("unchecked")

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

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

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

                int mc = modCount;

                V newValue = remappingFunction.apply(key, e.value);

                if (mc != modCount) {

                    throw new ConcurrentModificationException();

                }

                if (newValue == null) {

                    if (prev != null) {

                        prev.next = e.next;

                    } else {

                        tab[index] = e.next;

                    }

                    modCount = mc + 1;

                    count--;

                } else {

                    e.value = newValue;

                }

                return newValue;

            }

        }

        int mc = modCount;

        V newValue = remappingFunction.apply(key, null);

        if (mc != modCount) { throw new ConcurrentModificationException(); }

        if (newValue != null) {

            addEntry(hash, key, newValue, index);

        }

        return newValue;

    }

    /**

     * {@inheritDoc}

     *

     * <p>This method will, on a best-effort basis, throw a

     * {@link java.util.ConcurrentModificationException} if the remapping

     * function modified this map during computation.

     *

     * @throws ConcurrentModificationException if it is detected that the

     * remapping function modified this map

     */

    @Override

    public synchronized V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) {

        Objects.requireNonNull(remappingFunction);

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

        int hash = key.hashCode();

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

        @SuppressWarnings("unchecked")

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

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

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

                int mc = modCount;

                V newValue = remappingFunction.apply(e.value, value);

                if (mc != modCount) {

                    throw new ConcurrentModificationException();

                }

                if (newValue == null) {

                    if (prev != null) {

                        prev.next = e.next;

                    } else {

                        tab[index] = e.next;

                    }

                    modCount = mc + 1;

                    count--;

                } else {

                    e.value = newValue;

                }

                return newValue;

            }

        }

        if (value != null) {

            addEntry(hash, key, value, index);

        }

        return value;

    }

    /**

     * 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.

     */

    @java.io.Serial

    private void writeObject(java.io.ObjectOutputStream s)

            throws IOException {

        writeHashtable(s);

    }

    /**

     * Perform serialization of the Hashtable to an ObjectOutputStream.

     * The Properties class overrides this method.

     */

    void writeHashtable(java.io.ObjectOutputStream s)

            throws IOException {

        Entry<Object, Object> entryStack = null;

        synchronized (this) {

            // Write out the threshold and loadFactor

            s.defaultWriteObject();

            // Write out the length and count of elements

            s.writeInt(table.length);

            s.writeInt(count);

            // Stack copies of the entries in the table

            for (Entry<?, ?> entry : table) {

                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;

        }

    }

    /**

     * Called by Properties to write out a simulated threshold and loadfactor.

     */

    final void defaultWriteHashtable(java.io.ObjectOutputStream s, int length,

            float loadFactor) throws IOException {

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

        this.loadFactor = loadFactor;

        s.defaultWriteObject();

    }

    /**

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

     */

    @java.io.Serial

    private void readObject(java.io.ObjectInputStream s)

            throws IOException, ClassNotFoundException {

        readHashtable(s);

    }

    /**