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public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable {
private transient Entry<?,?>[] table;
private transient int count;
private int threshold;
private float loadFactor;
private transient int modCount = 0;
private static final long serialVersionUID = 1421746759512286392L;
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);
}
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);
}
public Hashtable() {
this(11, 0.75f);
}
public Hashtable(Map<? extends K, ? extends V> t) {
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}
public synchronized int size() {
return count;
}
public synchronized boolean isEmpty() {
return count == 0;
}
public synchronized Enumeration<K> keys() {
return this.<K>getEnumeration(KEYS);
}
public synchronized Enumeration<V> elements() {
return this.<V>getEnumeration(VALUES);
}
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;
}
public boolean containsValue(Object value) {
return contains(value);
}
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;
}
@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;
}
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
@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++;
}
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;
}
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;
}
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());
}
public synchronized void clear() {
Entry<?,?> tab[] = table;
modCount++;
for (int index = tab.length; --index >= 0; )
tab[index] = null;
count = 0;
}
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);
}
}
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
public Set<K> keySet() {
if (keySet == null)
keySet = Collections.synchronizedSet(new KeySet(), this);
return keySet;
}
public Set<Map.Entry<K,V>> entrySet() {
if (entrySet==null)
entrySet = Collections.synchronizedSet(new EntrySet(), this);
return entrySet;
}
public Collection<V> values() {
if (values==null)
values = Collections.synchronizedCollection(new ValueCollection(),
this);
return values;
}
// Comparison and hashing
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 {
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;
}
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)) {
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, 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;
}
}
V newValue = mappingFunction.apply(key);
if (newValue != null) {
addEntry(hash, key, newValue, index);
}
return newValue;
}
@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)) {
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, 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)) {
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, 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)) {
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 <i>capacity</i> of the Hashtable (the length of the
* bucket array) is emitted (int), followed by the
* <i>size</i> of the Hashtable (the number of key-value
* mappings), followed by the key (Object) and value (Object)
* for each key-value mapping represented by the Hashtable
* The key-value mappings are emitted in no particular order.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws IOException {
Entry<Object, Object> entryStack = null;
synchronized (this) {
// Write out the 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 (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;
}
}
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException
{
// Read in the threshold and loadFactor
s.defaultReadObject();
// Validate loadFactor (ignore threshold - it will be re-computed)
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new StreamCorruptedException("Illegal Load: " + loadFactor);
// Read the original length of the array and number of elements
int origlength = s.readInt();
int elements = s.readInt();
// Validate # of elements
if (elements < 0)
throw new StreamCorruptedException("Illegal # of Elements: " + elements);
// Clamp original length to be more than elements / loadFactor
// (this is the invariant enforced with auto-growth)
origlength = Math.max(origlength, (int)(elements / loadFactor) + 1);
// Compute new length with a bit of room 5% + 3 to grow but
// no larger than the clamped original length. 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 + elements / 20) / loadFactor) + 3;
if (length > elements && (length & 1) == 0)
length--;
length = Math.min(length, origlength);
if (length < 0) { // overflow
length = origlength;
}
// Check Map.Entry[].class since it's the nearest public type to
// what we're actually creating.
SharedSecrets.getJavaOISAccess().checkArray(s, Map.Entry[].class, length);
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();
// sync is eliminated for performance
reconstitutionPut(table, key, value);
}
}
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++;
}
// Types of Enumerations/Iterations
private static final int KEYS = 0;
private static final int VALUES = 1;
private static final int ENTRIES = 2;
}
private transient Entry<?,?>[] table：hash table 数据private transient int count：hash table 中 entry 总数private int threshold：当它的大小超过这个阈值，刷新表（这个字段的值是 (int)(capacity * loadFactor)）private float loadFactor：hashtable 的负载因子private transient int modCount = 0：这个 Hashtable 结构上修改的次数。结构修改是更改 Hashtable 的 entry 数量或其他修改它的内部结构（例如，rehash）这个字段被用于使 Hashtable 集合视图上的 iterators fail-fast（看 ConcurrentModificationException）private static final long serialVersionUID = 1421746759512286392L：从 JDK 1.0.2 使用 serialVersionUID 实现互操作性public Hashtable(int initialCapacity, float loadFactor)：用指定的 initial capacity 和指定的 load factor 构造一个新的空的 hashtablepublic Hashtable(int initialCapacity)：用指定的 initial capacity 和 load factor (0.75) 构造一个新的空的 hashtablepublic Hashtable()：使用默认的 initial capacity (11) 和默认的 load factor (0.75) 构造一个新的空的 hashtablepublic Hashtable(Map<? extends K, ? extends V> t)：使用和给定 Map 相同的映射构造一个新的 hashtable。通过足够保存给定 Map 的映射的 initial capacity 和默认 load factor (0.75) 创建 hashtablepublic synchronized int size()：返回这个 hashtable 中 key 的数量public synchronized boolean isEmpty()：测试这个 hashtable 是否没有 key 映射到 valuepublic synchronized Enumeration<K> keys()：返回这个 hashtable 枚举public synchronized Enumeration<V> elements()：返回这个 hashtable 中值的枚举public synchronized boolean contains(Object value)：测试是否某些 key 映射到这个 hashtable 指定 value。这个操作比 containsKey 方法更昂贵- 请注意，这个方法在功能上和 containsValue 相同（集合框架 Map 接口中的一部分）
public boolean containsValue(Object value)：返回 true，如果这个 hashtable 映射一个或多个 key 到这个 value- 请注意，这个方法在功能上和 contains （早于 Map 接口）
public synchronized boolean containsKey(Object key)：测试指定对象是否这个 hashtable 的 keypublic synchronized V get(Object key)：返回指定 key 映射的 value，或者 null，如果这个 map 没有包含 key 的映射- 更准确地，如果这个 map 包含从 key 到 value 的映射，使得 (key.equals(k))，然后这个方法返回 v，否则它返回 null（最多一个这样的映射）
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8：分配数组的最大大小。一些 VM 在数组保存一些头字。尝试分配更大的数组可能导致 OutOfMemoryError：请求数组大小超过 VM 限制protected void rehash()：增加这个 hashtable 容量和内部重组，以便更高效地容纳和访问它的 entry。当 hashtable 数量超过这个 hashtable 的容量和负载因子，这个方法自动调用private void addEntry(int hash, K key, V value, int index)public synchronized V put(K key, V value)：在这个 hashtable 映射指定 key 到指定 value。- 可以使用和原始 key 相同的 key 调用 get 方法检索 value
public synchronized V remove(Object key)：从这个 hashtable 移除 key（和它的相应 value）。如果 key 不在这个 hashtable，这个方法不做任何事public synchronized void putAll(Map<? extends K, ? extends V> t)：从指定 map 复制全部映射到这个 hashtable。 这些映射将代替这个 Hashtable 对当前指定 map 的任何 key 的任何映射public synchronized void clear()：清除这个 hashtable，以便它不包含 keypublic synchronized Object clone()：创建这个 hashtable 的浅拷贝。复制 hashtable 它自己的全部结构，但 key 和 value 不克隆。这是相对昂贵的操作public synchronized String toString()：返回 string 描述这个以一组 entry 形式的 Hashtable 对象，用大括号括起来和用 ASCII 字符“， ” 分开（逗号和空格）。每个 entry 表现为 key，等于号=，和关联元素，toString 方法被用于转换 key 和 element 为 stringprivate <T> Enumeration<T> getEnumeration(int type)private <T> Iterator<T> getIterator(int type)
看
这些字段的每一个在第一次请求这个视图时初始化包含相应的视图实例。视图是无状态，因此没有理由去创建多于个
private transient volatile Set<K> keySetprivate transient volatile Set<Map.Entry<K,V>> entrySetprivate transient volatile Collection<V> valuespublic Set<K> keySet()：返回这个 map 包含的 key 的 Set 视图。set 由 map 支持，所以改变 map 反映在 set，反之亦然。如果在遍历 set 的时候修改 map（除非通过迭代器自己的 remove 操作），迭代的结果是未定义的。set 支持元素移除，从 map 移除相应的映射，通过 Iterator.remove，Set.remove，removeAll，retainAll 和 clear 操作。它不支持 add 或 addAll 操作public Set<Map.Entry<K,V>> entrySet()：返回这个 map 包含的映射的 Set 视图。set 由 map 支持，所以改变 map 反映在 set，反之亦然。如果在迭代 set 的时候修改 map （除了通过迭代自己的 remove 操作，或者通过迭代器返回的 map entry 的 setValue 操作）迭代的结果是未定义的。set 支持元素移除，从 map 移除相应的映射，通过 Iterator.remove，Set.remove，removeAll，retainAll 和 clear 操作。它不支持 add 或 addAll 操作public Collection<V> values()：返回这个 map 包含的 value 的 Collection 视图。集合由 map 支持，所以改变 map 反映在 collection，反之亦然。如果在迭代 collection 的时修改 map（除了通过迭代器自己的 remove 操作），迭代的结果是未定义的。collection 支持元素移除，从 map 移除相应的映射，通过 Iterator.remove，Collection.remove，removeAll，retainAll 和 clear 操作。它不支持 add 或 addAll 操作
比较和哈希
public synchronized boolean equals(Object o)：根据 Map 接口的定义，将指定对象和这个 Map 进行相等性比较public synchronized int hashCode()：根据 Map 接口定义返回哈希编码值public synchronized V getOrDefault(Object key, V defaultValue)public synchronized void forEach(BiConsumer<? super K, ? super V> action)public synchronized void replaceAll(BiFunction<? super K, ? super V, ? extends V> function)public synchronized V putIfAbsent(K key, V value)public synchronized boolean remove(Object key, Object value)public synchronized boolean replace(K key, V oldValue, V newValue)public synchronized V replace(K key, V value)public synchronized V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)public synchronized V computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)public synchronized V compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)public synchronized V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)private void writeObject(java.io.ObjectOutputStream s)throws IOException：保存 Hashtable 的状态到流（即，序列化它）- 发出 Hashtable 的容量（bucket 数组的长度）（int），然后是 Hashtable 的大小（key-value 映射的数量），然后是 Hashtable 代表的每一个 key-value 映射的 key（对象） 和 value（对象），key-value 映射不按指定的顺序发出
private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException：从流重建这个 Hashtable（即，反序列化它）private void reconstitutionPut(Entry<?,?>[] tab, K key, V value) throws StreamCorruptedException：readObject 使用 put 方法。这是因为 put 重写，并且不应该在 readObject 中调用，因为子类还没有初始化- 这与通常的 put 方法有几个方面不同。不需要检查重新哈希，因为表中初始化 element 数量是已知的。。modCount 没有增加，并且没有同步，因为我们正在创建一个新实例。同样，不需要返回值
// 枚举/迭代的类型
private static final int KEYS = 0;private static final int VALUES = 1;private static final int ENTRIES = 2;
