Java集合之Hashtable

和HashMap一样，Hashtable也是一个散列表，存储的内容也是键值对key-value映射。它继承了Dictionary，并实现了Map、Cloneable、io、Serializable接口。Hashtable是线程安全的，并且key和value不能为空，并且不是有序的。

Hashtable有两个参数影响其性能：初始容量和加载因子。容量是哈希表中桶的数量，初始容量就是哈希表创建时的容量，加载因子是对哈希表在其容量自动增加之前可以达到多满的一个尺度。默认的加载因子是0.75。

Hashtable结构图：

从图中可以看出：

（1）Hashtable继承于Dictionary类，实现了Map接口。

（2）Hashtable通过 拉链法（对冲突的采用链表的方式）实现的哈希表。包括几个重要的成员变量：

        table是一个Entry[]数组类型，Entry是一个单向链表，哈希表的key-value都存储在Entry数组中的。

        count是Hashtable的大小，是Hashtable保存键值对的数量。

        threshold是Hashtable的阈值，用于判断是否需要调整Hashtable的容量，threshold的值=容量乘以加载因子。

        loadFactor是加载因子。

        modCount是用来实现fail-fast机制的。

Hashtable遍历方式：

（1）遍历Hashtable的键值对：首先获取Hashtable的键值对set集合，之后通过迭代器对集合进行迭代遍历。

Integer integ = null;
Iterator iter = table.entrySet().iterator();
while(iter.hasNext())
{
    Map.Entry entry = (Map.Entry)iter.next();
    // 获取key
    key = (String)entry.getKey();
   // 获取value
   integ = (Integer)entry.getValue();
}

（2）遍历Hashtable的键：通过keySet()获得键集合，通过Iterator迭代器遍历获得值。

String key = null;
Integer integ = null;
Iterator iter = table.keySet().iterator();
while (iter.hasNext()) {
    // 获取key
    key = (String)iter.next();
    // 根据key，获取value
    integ = (Integer)table.get(key);
}

（3）遍历Hashtable的值：通过values()获得Hashtable的值集合，通过Iterator迭代器遍历获得值

Integer value = null;
Collection c = table.values();
Iterator iter= c.iterator();
while (iter.hasNext()) 
{
    value = (Integer)iter.next();
}

（4）通过Enumeration遍历Hashtable的键或者值：首先获得键或者值的集合，通过Enumeration遍历得到值。

Enumeration enu = table.keys();
while(enu.hasMoreElements()) 
{
    System.out.println(enu.nextElement());
}
Enumeration enu = table.elements();
while(enu.hasMoreElements()) 
{
    System.out.println(enu.nextElement());
}

Hashtable示例代码：

public class Hello {

    public static void main(String[] args) {
        testHashtableAPIs();
    }

    private static void testHashtableAPIs() {
        // 初始化随机种子
        Random r = new Random();
        // 新建Hashtable
        Hashtable table = new Hashtable();
        // 添加操作
        table.put("one", r.nextInt(10));
        table.put("two", r.nextInt(10));
        table.put("three", r.nextInt(10));

        // 打印出table
        System.out.println("table:"+table );

        // 通过Iterator遍历key-value
        Iterator iter = table.entrySet().iterator();
        while(iter.hasNext()) {
            Map.Entry entry = (Map.Entry)iter.next();
            System.out.println("next : "+ entry.getKey() +" - "+entry.getValue());
        }

        // Hashtable的键值对个数        
        System.out.println("size:"+table.size());

        // containsKey(Object key) :是否包含键key
        System.out.println("contains key two : "+table.containsKey("two"));
        System.out.println("contains key five : "+table.containsKey("five"));

        // containsValue(Object value) :是否包含值value
        System.out.println("contains value 0 : "+table.containsValue(new Integer(0)));

        // remove(Object key) ： 删除键key对应的键值对
        table.remove("three");

        System.out.println("table:"+table );

        // clear() ： 清空Hashtable
        table.clear();

     // isEmpty() : Hashtable是否为空
        System.out.println((table.isEmpty()?"table is empty":"table is not empty") );
    }

}

运行结果：

table:{two=5, one=4, three=2}

next : two - 5

next : one - 4

next : three - 2

size:3

contains key two : true

contains key five : false

contains value 0 : false

table:{two=5, one=4}

table is empty

基于Java8的Dictionary源代码：

public abstract
class Dictionary<K,V> {
public Dictionary() {
    }
abstract public int size();
    abstract public boolean isEmpty();
    abstract public Enumeration<K> keys();
    abstract public Enumeration<V> elements();
    abstract public V get(Object key);
    abstract public V put(K key, V value);
    abstract public V remove(Object key);
}

基于Java8的Hashtable源码：

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;//entry表

    /**
     * The total number of entries in the hash table.
     */
    private transient int count;//entry数据

    /**
     * The table is rehashed when its size exceeds this threshold.  (The
     * value of this field is (int)(capacity * loadFactor).)
     *
     * @serial
     */
    private int threshold;//阈值

    /**
     * The load factor for the hashtable.
     *
     * @serial
     */
    private float loadFactor;//加载因子

    /**
     * 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;//fail-fast机制，记录改变的数目

    /** 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.
     */
    //还有初始大小和加载因子的构造函数
    public Hashtable(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                    initialCapacity);
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal Load: "+loadFactor);
        if (initialCapacity==0)
            initialCapacity = 1;
        this.loadFactor = loadFactor;
        table = new Entry,?>[initialCapacity];
        threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
    }

    /**
     * Constructs a new, empty hashtable with the specified initial capacity
     * and default load factor (0.75).
     *
     * @param     initialCapacity   the initial capacity of the hashtable.
     * @exception IllegalArgumentException if the initial capacity is less
     *              than zero.
     */
    //初始大小和默认的0.75加载因子的构造函数
    public Hashtable(int initialCapacity) {
        this(initialCapacity, 0.75f);
    }

    /**
     * Constructs a new, empty hashtable with a default initial capacity (11)
     * and load factor (0.75).
     */
    //使用默认的构造函数
    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(Mapextends 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.
     */
    //Hashtable中entry大小
    public synchronized int size() {
        return count;
    }

    /**
     * Tests if this hashtable maps no keys to values.
     *
     * @return  true if this hashtable maps no keys to values;
     *          false otherwise.
     */
    //判断是否为空
    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
     */
    //key值的枚举
    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
     */
    //value的枚举
    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.
     *
     * 
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     true if and only if some key maps to the
     *             value argument in this hashtable as
     *             determined by the equals method;
     *             false otherwise.
     * @exception  NullPointerException  if the value is null
     */
    //判断是否包含某个值
    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.
     *
     * 
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 true if this map maps one or more keys to the
     *         specified value
     * @throws NullPointerException  if the value is 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  true if and only if the specified object
     *          is a key in this hashtable, as determined by the
     *          equals method; false otherwise.
     * @throws  NullPointerException  if the key is null
     * @see     #contains(Object)
     */
    //判断是否包含某个key
    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.
     *
     * 
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)
     */
    //获得某个key对应的value
    @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) {
        modCount++;

        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++;
    }

    /**
     * Maps the specified key to the specified
     * value in this hashtable. Neither the key nor the
     * value can be null. 

     *
     * The value can be retrieved by calling the 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 null if it did not have one
     * @exception  NullPointerException  if the key or value is
     *               null
     * @see     Object#equals(Object)
     * @see     #get(Object)
     */
    //key和value都不为空
    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 null if the key did not have a mapping
     * @throws  NullPointerException  if the key is null
     */
    //删除某个key对应的value
    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)) {
                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(Mapextends K, ? extends V> t) {
        for (Map.Entryextends K, ? extends V> e : t.entrySet())
            put(e.getKey(), e.getValue());
    }

    /**
     * Clears this hashtable so that it contains no keys.
     */
    //清空
    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
     */
    //浅拷贝
    public synchronized Object clone() {
        try {
            Hashtable,?> t = (Hashtable,?>)super.clone();
            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;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }

    /**
     * Returns a string representation of this Hashtable object
     * in the form of a set of entries, enclosed in braces and separated
     * by the ASCII characters ", " (comma and space). Each
     * entry is rendered as the key, an equals sign =, and the
     * associated element, where the 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();
        IteratorK,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 SetK,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 remove operation), the results of
     * the iteration are undefined.  The set supports element removal,
     * which removes the corresponding mapping from the map, via the
     * Iterator.remove, Set.remove,
     * removeAll, retainAll, and clear
     * operations.  It does not support the add or 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 remove operation, or through the
     * 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 Iterator.remove,
     * Set.remove, removeAll, retainAll and
     * clear operations.  It does not support the
     * add or addAll operations.
     *
     * @since 1.2
     */
    public SetK,V>> entrySet() {
        if (entrySet==null)
            entrySet = Collections.synchronizedSet(new EntrySet(), this);
        return entrySet;
    }

    private class EntrySet extends AbstractSetK,V>> {
        public IteratorK,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,?> entry = (Map.Entry,?>) o;
            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)) {
                    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 remove operation),
     * the results of the iteration are undefined.  The collection
     * supports element removal, which removes the corresponding
     * mapping from the map, via the Iterator.remove,
     * Collection.remove, removeAll,
     * retainAll and clear operations.  It does not
     * support the add or 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))
            return false;
        Map,?> t = (Map,?>) o;
        if (t.size() != size())
            return false;

        try {
            IteratorK,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 (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(BiConsumersuper 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(BiFunctionsuper 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)) {
                modCount++;
                if (prev != null) {
                    prev.next = e.next;
                } else {
                    tab[index] = e.next;
                }
                count--;
                e.value = null;
                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;
    }

    @Override
    public synchronized V computeIfAbsent(K key, Functionsuper 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;
            }
        }

        V newValue = mappingFunction.apply(key);
        if (newValue != null) {
            addEntry(hash, key, newValue, index);
        }

        return newValue;
    }

    @Override
    public synchronized V computeIfPresent(K key, BiFunctionsuper 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)) {
                V newValue = remappingFunction.apply(key, e.value);
                if (newValue == null) {
                    modCount++;
                    if (prev != null) {
                        prev.next = e.next;
                    } else {
                        tab[index] = e.next;
                    }
                    count--;
                } else {
                    e.value = newValue;
                }
                return newValue;
            }
        }
        return null;
    }

    @Override
    public synchronized V compute(K key, BiFunctionsuper 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)) {
                V newValue = remappingFunction.apply(key, e.value);
                if (newValue == null) {
                    modCount++;
                    if (prev != null) {
                        prev.next = e.next;
                    } else {
                        tab[index] = e.next;
                    }
                    count--;
                } else {
                    e.value = newValue;
                }
                return newValue;
            }
        }

        V newValue = remappingFunction.apply(key, null);
        if (newValue != null) {
            addEntry(hash, key, newValue, index);
        }

        return newValue;
    }

    @Override
    public synchronized V merge(K key, V value, BiFunctionsuper 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)) {
                V newValue = remappingFunction.apply(e.value, value);
                if (newValue == null) {
                    modCount++;
                    if (prev != null) {
                        prev.next = e.next;
                    } else {
                        tab[index] = e.next;
                    }
                    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 capacity of the Hashtable (the length of the
     *             bucket array) is emitted (int), followed by the
     *             size 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> 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;
        table = new Entry,?>[length];
        threshold = (int)Math.min(length * loadFactor, MAX_ARRAY_SIZE + 1);
        count = 0;

        // Read the number of elements and then all the key/value objects
        for (; elements > 0; elements--) {
            @SuppressWarnings("unchecked")
            K key = (K)s.readObject();
            @SuppressWarnings("unchecked")
            V value = (V)s.readObject();
            // synch could be eliminated for performance
            reconstitutionPut(table, key, value);
        }
    }

    /**
     * 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.
     *
     * 
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,?> 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.
        @SuppressWarnings("unchecked")
        Entry<K,V> e = (Entry<K,V>)tab[index];
        tab[index] = new Entry<>(hash, key, value, e);
        count++;
    }

    /**
     * Hashtable bucket collision list entry
     */
    private static class Entry<K,V> implements Map.Entry<K,V> {
        final int hash;
        final 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;
        }

        @SuppressWarnings("unchecked")
        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 ^ Objects.hashCode(value);
        }

        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,?> entry;
        Entry,?> lastReturned;
        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,?> 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;
        }

        @SuppressWarnings("unchecked")
        public T nextElement() {
            Entry,?> 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,?> 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;

                @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 == lastReturned) {
                        modCount++;
                        expectedModCount++;
                        if (prev == null)
                            tab[index] = e.next;
                        else
                            prev.next = e.next;
                        count--;
                        lastReturned = null;
                        return;
                    }
                }
                throw new ConcurrentModificationException();
            }
        }
    }
}