java集合框架中Hashtable源码（基于JDK1.6）

此类实现一个哈希表，该哈希表将键映射到相应的值。任何非

null

对象都可以用作键或值。

为了成功地在哈希表中存储和获取对象，用作键的对象必须实现

hashCode

方法和

equals

方法。

Hashtable

的实例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。注意，哈希表的状态为open：在发生“哈希冲突”的情况下，单个桶会存储多个条目，这些条目必须按顺序搜索。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。初始容量和加载因子这两个参数只是对该实现的提示。关于何时以及是否调用
rehash 方法的具体细节则依赖于该实现。

通常，默认加载因子(.75)在时间和空间成本上寻求一种折衷。加载因子过高虽然减少了空间开销，但同时也增加了查找某个条目的时间（在大多数Hashtable 操作中，包括get 和
put 操作，都反映了这一点）。

初始容量主要控制空间消耗与执行

rehash

操作所需要的时间损耗之间的平衡。如果初始容量大于Hashtable 所包含的最大条目数除以加载因子，则永远 不会发生

rehash

操作。但是，将初始容量设置太高可能会浪费空间。

如果很多条目要存储在一个

Hashtable

中，那么与根据需要执行自动 rehashing 操作来增大表的容量的做法相比，使用足够大的初始容量创建哈希表或许可以更有效地插入条目。

下面这个示例创建了一个数字的哈希表。它将数字的名称用作键：

[code]Hashtable<String, Integer> numbers
= new Hashtable<String, Integer>();
numbers.put("one", 1);
numbers.put("two", 2);
numbers.put("three", 3);

[/code]
要获取一个数字，可以使用以下代码：

[code]Integer n = numbers.get("two");
if (n != null) {
System.out.println("two = " + n);
}

}[/code]
由所有类的“collection 视图方法”返回的 collection 的 iterator 方法返回的迭代器都是快速失败 的：在创建 Iterator 之后，如果从结构上对 Hashtable 进行修改，除非通过 Iterator 自身的remove 方法，否则在任何时间以任何方式对其进行修改，Iterator 都将抛出

ConcurrentModificationException

。因此，面对并发的修改，Iterator
很快就会完全失败，而不冒在将来某个不确定的时间发生任意不确定行为的风险。由 Hashtable 的键和元素方法返回的 Enumeration不 是快速失败的。

注意，迭代器的快速失败行为无法得到保证，因为一般来说，不可能对是否出现不同步并发修改做出任何硬性保证。快速失败迭代器会尽最大努力抛出ConcurrentModificationException。因此，为提高这类迭代器的正确性而编写一个依赖于此异常的程序是错误做法：迭代器的快速失败行为应该仅用于检测程序错误。

从Java 2 平台 v1.2起，此类就被改进以实现

Map

接口，使它成为 Java Collections Framework 中的一个成员。不像新的 collection 实现，

Hashtable

是同步的 。

以下为Hashtable的源码：

package java.util;
import java.util.*; // for javadoc (till 6280605 is fixed)
import java.io.*;

public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable {

/**
* The hash table data.
*/
private transient Entry[] table;

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

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

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

/**
* The number of times this Hashtable has been structurally modified
* Structural modifications are those that change the number of entries in
* the Hashtable or otherwise modify its internal structure (e.g.,
* rehash).  This field is used to make iterators on Collection-views of
* the Hashtable fail-fast.  (See ConcurrentModificationException).
*/
private transient int modCount = 0;

/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = 1421746759512286392L;

/**
* Constructs a new, empty hashtable with the specified initial
* capacity and the specified load factor.
*
* @param      initialCapacity   the initial capacity of the hashtable.
* @param      loadFactor        the load factor of the hashtable.
* @exception  IllegalArgumentException  if the initial capacity is less
*             than zero, or if the load factor is nonpositive.
*/
public Hashtable(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal Load: "+loadFactor);

if (initialCapacity==0)
initialCapacity = 1;
this.loadFactor = loadFactor;
table = new Entry[initialCapacity];
threshold = (int)(initialCapacity * loadFactor);
}

/**
* Constructs a new, empty hashtable with the specified initial capacity
* and default load factor (0.75).
*
* @param     initialCapacity   the initial capacity of the hashtable.
* @exception IllegalArgumentException if the initial capacity is less
*              than zero.
*/
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);
}

/**
* Constructs a new, empty hashtable with a default initial capacity (11)
* and load factor (0.75).
*/
public Hashtable() {
this(11, 0.75f);
}

/**
* Constructs a new hashtable with the same mappings as the given
* Map.  The hashtable is created with an initial capacity sufficient to
* hold the mappings in the given Map and a default load factor (0.75).
*
* @param t the map whose mappings are to be placed in this map.
* @throws NullPointerException if the specified map is null.
* @since   1.2
*/
public Hashtable(Map<? extends K, ? extends V> t) {
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}

/**
* Returns the number of keys in this hashtable.
*
* @return  the number of keys in this hashtable.
*/
public synchronized int size() {
return count;
}

/**
* Tests if this hashtable maps no keys to values.
*
* @return  <code>true</code> if this hashtable maps no keys to values;
*          <code>false</code> otherwise.
*/
public synchronized boolean isEmpty() {
return count == 0;
}

/**
* Returns an enumeration of the keys in this hashtable.
*
* @return  an enumeration of the keys in this hashtable.
* @see     Enumeration
* @see     #elements()
* @see	#keySet()
* @see	Map
*/
public synchronized Enumeration<K> keys() {
return this.<K>getEnumeration(KEYS);
}

/**
* Returns an enumeration of the values in this hashtable.
* Use the Enumeration methods on the returned object to fetch the elements
* sequentially.
*
* @return  an enumeration of the values in this hashtable.
* @see     java.util.Enumeration
* @see     #keys()
* @see	#values()
* @see	Map
*/
public synchronized Enumeration<V> elements() {
return this.<V>getEnumeration(VALUES);
}

/**
* Tests if some key maps into the specified value in this hashtable.
* This operation is more expensive than the {@link #containsKey
* containsKey} method.
*
* <p>Note that this method is identical in functionality to
* {@link #containsValue containsValue}, (which is part of the
* {@link Map} interface in the collections framework).
*
* @param      value   a value to search for
* @return     <code>true</code> if and only if some key maps to the
*             <code>value</code> argument in this hashtable as
*             determined by the <tt>equals</tt> method;
*             <code>false</code> otherwise.
* @exception  NullPointerException  if the value is <code>null</code>
*/
public synchronized boolean contains(Object value) {
if (value == null) {
throw new NullPointerException();
}

Entry tab[] = table;
for (int i = tab.length ; i-- > 0 ;) {
for (Entry<K,V> e = tab[i] ; e != null ; e = e.next) {
if (e.value.equals(value)) {
return true;
}
}
}
return false;
}

/**
* Returns true if this hashtable maps one or more keys to this value.
*
* <p>Note that this method is identical in functionality to {@link
* #contains contains} (which predates the {@link Map} interface).
*
* @param value value whose presence in this hashtable is to be tested
* @return <tt>true</tt> if this map maps one or more keys to the
*         specified value
* @throws NullPointerException  if the value is <code>null</code>
* @since 1.2
*/
public boolean containsValue(Object value) {
return contains(value);
}

/**
* Tests if the specified object is a key in this hashtable.
*
* @param   key   possible key
* @return  <code>true</code> if and only if the specified object
*          is a key in this hashtable, as determined by the
*          <tt>equals</tt> method; <code>false</code> otherwise.
* @throws  NullPointerException  if the key is <code>null</code>
* @see     #contains(Object)
*/
public synchronized boolean containsKey(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return true;
}
}
return false;
}

/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key.equals(k))},
* then this method returns {@code v}; otherwise it returns
* {@code null}.  (There can be at most one such mapping.)
*
* @param key the key whose associated value is to be returned
* @return the value to which the specified key is mapped, or
*         {@code null} if this map contains no mapping for the key
* @throws NullPointerException if the specified key is null
* @see     #put(Object, Object)
*/
public synchronized V get(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return e.value;
}
}
return null;
}

/**
* Increases the capacity of and internally reorganizes this
* hashtable, in order to accommodate and access its entries more
* efficiently.  This method is called automatically when the
* number of keys in the hashtable exceeds this hashtable's capacity
* and load factor.
*/
protected void rehash() {
int oldCapacity = table.length;
Entry[] oldMap = table;

int newCapacity = oldCapacity * 2 + 1;
Entry[] newMap = new Entry[newCapacity];

modCount++;
threshold = (int)(newCapacity * loadFactor);
table = newMap;

for (int i = oldCapacity ; i-- > 0 ;) {
for (Entry<K,V> old = oldMap[i] ; old != null ; ) {
Entry<K,V> e = old;
old = old.next;

int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = newMap[index];
newMap[index] = e;
}
}
}

/**
* Maps the specified <code>key</code> to the specified
* <code>value</code> in this hashtable. Neither the key nor the
* value can be <code>null</code>. <p>
*
* The value can be retrieved by calling the <code>get</code> method
* with a key that is equal to the original key.
*
* @param      key     the hashtable key
* @param      value   the value
* @return     the previous value of the specified key in this hashtable,
*             or <code>null</code> if it did not have one
* @exception  NullPointerException  if the key or value is
*               <code>null</code>
* @see     Object#equals(Object)
* @see     #get(Object)
*/
public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
}

// Makes sure the key is not already in the hashtable.
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
V old = e.value;
e.value = value;
return old;
}
}

modCount++;
if (count >= threshold) {
// Rehash the table if the threshold is exceeded
rehash();

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

// Creates the new entry.
Entry<K,V> e = tab[index];
tab[index] = new Entry<K,V>(hash, key, value, e);
count++;
return null;
}

/**
* Removes the key (and its corresponding value) from this
* hashtable. This method does nothing if the key is not in the hashtable.
*
* @param   key   the key that needs to be removed
* @return  the value to which the key had been mapped in this hashtable,
*          or <code>null</code> if the key did not have a mapping
* @throws  NullPointerException  if the key is <code>null</code>
*/
public synchronized V remove(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index], prev = null ; e != null ; prev = e, e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
modCount++;
if (prev != null) {
prev.next = e.next;
} else {
tab[index] = e.next;
}
count--;
V oldValue = e.value;
e.value = null;
return oldValue;
}
}
return null;
}

/**
* Copies all of the mappings from the specified map to this hashtable.
* These mappings will replace any mappings that this hashtable had for any
* of the keys currently in the specified map.
*
* @param t mappings to be stored in this map
* @throws NullPointerException if the specified map is null
* @since 1.2
*/
public synchronized void putAll(Map<? extends K, ? extends V> t) {
for (Map.Entry<? extends K, ? extends V> e : t.entrySet())
put(e.getKey(), e.getValue());
}

/**
* Clears this hashtable so that it contains no keys.
*/
public synchronized void clear() {
Entry tab[] = table;
modCount++;
for (int index = tab.length; --index >= 0; )
tab[index] = null;
count = 0;
}

/**
* Creates a shallow copy of this hashtable. All the structure of the
* hashtable itself is copied, but the keys and values are not cloned.
* This is a relatively expensive operation.
*
* @return  a clone of the hashtable
*/
public synchronized Object clone() {
try {
Hashtable<K,V> t = (Hashtable<K,V>) super.clone();
t.table = new Entry[table.length];
for (int i = table.length ; i-- > 0 ; ) {
t.table[i] = (table[i] != null)
? (Entry<K,V>) table[i].clone() : null;
}
t.keySet = null;
t.entrySet = null;
t.values = null;
t.modCount = 0;
return t;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError();
}
}

/**
* Returns a string representation of this <tt>Hashtable</tt> object
* in the form of a set of entries, enclosed in braces and separated
* by the ASCII characters "<tt>, </tt>" (comma and space). Each
* entry is rendered as the key, an equals sign <tt>=</tt>, and the
* associated element, where the <tt>toString</tt> method is used to
* convert the key and element to strings.
*
* @return  a string representation of this hashtable
*/
public synchronized String toString() {
int max = size() - 1;
if (max == -1)
return "{}";

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

sb.append('{');
for (int i = 0; ; i++) {
Map.Entry<K,V> e = it.next();
K key = e.getKey();
V value = e.getValue();
sb.append(key   == this ? "(this Map)" : key.toString());
sb.append('=');
sb.append(value == this ? "(this Map)" : value.toString());

if (i == max)
return sb.append('}').toString();
sb.append(", ");
}
}

private <T> Enumeration<T> getEnumeration(int type) {
if (count == 0) {
return (Enumeration<T>)emptyEnumerator;
} else {
return new Enumerator<T>(type, false);
}
}

private <T> Iterator<T> getIterator(int type) {
if (count == 0) {
return (Iterator<T>) emptyIterator;
} else {
return new Enumerator<T>(type, true);
}
}

// Views

/**
* Each of these fields are initialized to contain an instance of the
* appropriate view the first time this view is requested.  The views are
* stateless, so there's no reason to create more than one of each.
*/
private transient volatile Set<K> keySet = null;
private transient volatile Set<Map.Entry<K,V>> entrySet = null;
private transient volatile Collection<V> values = null;

/**
* Returns a {@link Set} view of the keys contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa.  If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own <tt>remove</tt> operation), the results of
* the iteration are undefined.  The set supports element removal,
* which removes the corresponding mapping from the map, via the
* <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
* operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>
* operations.
*
* @since 1.2
*/
public Set<K> keySet() {
if (keySet == null)
keySet = Collections.synchronizedSet(new KeySet(), this);
return keySet;
}

private class KeySet extends AbstractSet<K> {
public Iterator<K> iterator() {
return getIterator(KEYS);
}
public int size() {
return count;
}
public boolean contains(Object o) {
return containsKey(o);
}
public boolean remove(Object o) {
return Hashtable.this.remove(o) != null;
}
public void clear() {
Hashtable.this.clear();
}
}

/**
* Returns a {@link Set} view of the mappings contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa.  If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own <tt>remove</tt> operation, or through the
* <tt>setValue</tt> operation on a map entry returned by the
* iterator) the results of the iteration are undefined.  The set
* supports element removal, which removes the corresponding
* mapping from the map, via the <tt>Iterator.remove</tt>,
* <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
* <tt>clear</tt> operations.  It does not support the
* <tt>add</tt> or <tt>addAll</tt> operations.
*
* @since 1.2
*/
public Set<Map.Entry<K,V>> entrySet() {
if (entrySet==null)
entrySet = Collections.synchronizedSet(new EntrySet(), this);
return entrySet;
}

private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
public Iterator<Map.Entry<K,V>> iterator() {
return getIterator(ENTRIES);
}

public boolean add(Map.Entry<K,V> o) {
return super.add(o);
}

public boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry entry = (Map.Entry)o;
Object key = entry.getKey();
Entry[] tab = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;

for (Entry e = tab[index]; e != null; e = e.next)
if (e.hash==hash && e.equals(entry))
return true;
return false;
}

public boolean remove(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry<K,V> entry = (Map.Entry<K,V>) o;
K key = entry.getKey();
Entry[] tab = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;

for (Entry<K,V> e = tab[index], prev = null; e != null;
prev = e, e = e.next) {
if (e.hash==hash && e.equals(entry)) {
modCount++;
if (prev != null)
prev.next = e.next;
else
tab[index] = e.next;

count--;
e.value = null;
return true;
}
}
return false;
}

public int size() {
return count;
}

public void clear() {
Hashtable.this.clear();
}
}

/**
* Returns a {@link Collection} view of the values contained in this map.
* The collection is backed by the map, so changes to the map are
* reflected in the collection, and vice-versa.  If the map is
* modified while an iteration over the collection is in progress
* (except through the iterator's own <tt>remove</tt> operation),
* the results of the iteration are undefined.  The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the <tt>Iterator.remove</tt>,
* <tt>Collection.remove</tt>, <tt>removeAll</tt>,
* <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
* support the <tt>add</tt> or <tt>addAll</tt> operations.
*
* @since 1.2
*/
public Collection<V> values() {
if (values==null)
values = Collections.synchronizedCollection(new ValueCollection(),
this);
return values;
}

private class ValueCollection extends AbstractCollection<V> {
public Iterator<V> iterator() {
return getIterator(VALUES);
}
public int size() {
return count;
}
public boolean contains(Object o) {
return containsValue(o);
}
public void clear() {
Hashtable.this.clear();
}
}

// Comparison and hashing

/**
* Compares the specified Object with this Map for equality,
* as per the definition in the Map interface.
*
* @param  o object to be compared for equality with this hashtable
* @return true if the specified Object is equal to this Map
* @see Map#equals(Object)
* @since 1.2
*/
public synchronized boolean equals(Object o) {
if (o == this)
return true;

if (!(o instanceof Map))
return false;
Map<K,V> t = (Map<K,V>) o;
if (t.size() != size())
return false;

try {
Iterator<Map.Entry<K,V>> i = entrySet().iterator();
while (i.hasNext()) {
Map.Entry<K,V> e = i.next();
K key = e.getKey();
V value = e.getValue();
if (value == null) {
if (!(t.get(key)==null && t.containsKey(key)))
return false;
} else {
if (!value.equals(t.get(key)))
return false;
}
}
} catch (ClassCastException unused)   {
return false;
} catch (NullPointerException unused) {
return false;
}

return true;
}

/**
* Returns the hash code value for this Map as per the definition in the
* Map interface.
*
* @see Map#hashCode()
* @since 1.2
*/
public synchronized int hashCode() {
/*
* This code detects the recursion caused by computing the hash code
* of a self-referential hash table and prevents the stack overflow
* that would otherwise result.  This allows certain 1.1-era
* applets with self-referential hash tables to work.  This code
* abuses the loadFactor field to do double-duty as a hashCode
* in progress flag, so as not to worsen the space performance.
* A negative load factor indicates that hash code computation is
* in progress.
*/
int h = 0;
if (count == 0 || loadFactor < 0)
return h;  // Returns zero

loadFactor = -loadFactor;  // Mark hashCode computation in progress
Entry[] tab = table;
for (int i = 0; i < tab.length; i++)
for (Entry e = tab[i]; e != null; e = e.next)
h += e.key.hashCode() ^ e.value.hashCode();
loadFactor = -loadFactor;  // Mark hashCode computation complete

return h;
}

/**
* Save the state of the Hashtable to a stream (i.e., serialize it).
*
* @serialData The <i>capacity</i> of the Hashtable (the length of the
*		   bucket array) is emitted (int), followed by the
*		   <i>size</i> of the Hashtable (the number of key-value
*		   mappings), followed by the key (Object) and value (Object)
*		   for each key-value mapping represented by the Hashtable
*		   The key-value mappings are emitted in no particular order.
*/
private synchronized void writeObject(java.io.ObjectOutputStream s)
throws IOException
{
// Write out the length, threshold, loadfactor
s.defaultWriteObject();

// Write out length, count of elements and then the key/value objects
s.writeInt(table.length);
s.writeInt(count);
for (int index = table.length-1; index >= 0; index--) {
Entry entry = table[index];

while (entry != null) {
s.writeObject(entry.key);
s.writeObject(entry.value);
entry = entry.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];
count = 0;

// Read the number of elements and then all the key/value objects
for (; elements > 0; elements--) {
K key = (K)s.readObject();
V value = (V)s.readObject();
// synch could be eliminated for performance
reconstitutionPut(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.
*
* <p>This differs from the regular put method in several ways. No
* checking for rehashing is necessary since the number of elements
* initially in the table is known. The modCount is not incremented
* because we are creating a new instance. Also, no return value
* is needed.
*/
private void reconstitutionPut(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.
Entry[] tab = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
throw new java.io.StreamCorruptedException();
}
}
// Creates the new entry.
Entry<K,V> e = tab[index];
tab[index] = new Entry<K,V>(hash, key, value, e);
count++;
}

/**
* Hashtable collision list.
*/
private static class Entry<K,V> implements Map.Entry<K,V> {
int hash;
K key;
V value;
Entry<K,V> next;

protected Entry(int hash, K key, V value, Entry<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}

protected Object clone() {
return new Entry<K,V>(hash, key, value,
(next==null ? null : (Entry<K,V>) next.clone()));
}

// Map.Entry Ops

public K getKey() {
return key;
}

public V getValue() {
return value;
}

public V setValue(V value) {
if (value == null)
throw new NullPointerException();

V oldValue = this.value;
this.value = value;
return oldValue;
}

public boolean equals(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry)o;

return (key==null ? e.getKey()==null : key.equals(e.getKey())) &&
(value==null ? e.getValue()==null : value.equals(e.getValue()));
}

public int hashCode() {
return hash ^ (value==null ? 0 : value.hashCode());
}

public String toString() {
return key.toString()+"="+value.toString();
}
}

// Types of Enumerations/Iterations
private static final int KEYS = 0;
private static final int VALUES = 1;
private static final int ENTRIES = 2;

/**
* A hashtable enumerator class.  This class implements both the
* Enumeration and Iterator interfaces, but individual instances
* can be created with the Iterator methods disabled.  This is necessary
* to avoid unintentionally increasing the capabilities granted a user
* by passing an Enumeration.
*/
private class Enumerator<T> implements Enumeration<T>, Iterator<T> {
Entry[] table = Hashtable.this.table;
int index = table.length;
Entry<K,V> entry = null;
Entry<K,V> lastReturned = null;
int type;

/**
* Indicates whether this Enumerator is serving as an Iterator
* or an Enumeration.  (true -> Iterator).
*/
boolean iterator;

/**
* The modCount value that the iterator believes that the backing
* Hashtable should have.  If this expectation is violated, the iterator
* has detected concurrent modification.
*/
protected int expectedModCount = modCount;

Enumerator(int type, boolean iterator) {
this.type = type;
this.iterator = iterator;
}

public boolean hasMoreElements() {
Entry<K,V> e = entry;
int i = index;
Entry[] t = table;
/* Use locals for faster loop iteration */
while (e == null && i > 0) {
e = t[--i];
}
entry = e;
index = i;
return e != null;
}

public T nextElement() {
Entry<K,V> et = entry;
int i = index;
Entry[] t = table;
/* Use locals for faster loop iteration */
while (et == null && i > 0) {
et = t[--i];
}
entry = et;
index = i;
if (et != null) {
Entry<K,V> e = lastReturned = entry;
entry = e.next;
return type == KEYS ? (T)e.key : (type == VALUES ? (T)e.value : (T)e);
}
throw new NoSuchElementException("Hashtable Enumerator");
}

// Iterator methods
public boolean hasNext() {
return hasMoreElements();
}

public T next() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
return nextElement();
}

public void remove() {
if (!iterator)
throw new UnsupportedOperationException();
if (lastReturned == null)
throw new IllegalStateException("Hashtable Enumerator");
if (modCount != expectedModCount)
throw new ConcurrentModificationException();

synchronized(Hashtable.this) {
Entry[] tab = Hashtable.this.table;
int index = (lastReturned.hash & 0x7FFFFFFF) % tab.length;

for (Entry<K,V> e = tab[index], prev = null; e != null;
prev = e, e = e.next) {
if (e == lastReturned) {
modCount++;
expectedModCount++;
if (prev == null)
tab[index] = e.next;
else
prev.next = e.next;
count--;
lastReturned = null;
return;
}
}
throw new ConcurrentModificationException();
}
}
}

private static Enumeration emptyEnumerator = new EmptyEnumerator();
private static Iterator emptyIterator = new EmptyIterator();

/**
* A hashtable enumerator class for empty hash tables, specializes
* the general Enumerator
*/
private static class EmptyEnumerator implements Enumeration<Object> {

EmptyEnumerator() {
}

public boolean hasMoreElements() {
return false;
}

public Object nextElement() {
throw new NoSuchElementException("Hashtable Enumerator");
}
}

/**
* A hashtable iterator class for empty hash tables
*/
private static class EmptyIterator implements Iterator<Object> {

EmptyIterator() {
}

public boolean hasNext() {
return false;
}

public Object next() {
throw new NoSuchElementException("Hashtable Iterator");
}

public void remove() {
throw new IllegalStateException("Hashtable Iterator");
}

}

}

在熟悉运用的基础上，看看源码，分析各个方法“底层”运作流程，很有助于加深对方法的运用和理解，尤其是一些细节问题，小性能问题，从而有助于你更加合理的运用这些API！