package java.util;
import java.io.*;
import java.util.concurrent.ThreadLocalRandom;
import java.util.function.BiConsumer;
import java.util.function.Function;
import java.util.function.BiFunction;
import sun.misc.SharedSecrets;
/**
* 注意Hashtable t是小写，文件名小写会Eclipse构建会报错,未知原因
* Hashtable存储的内容是键值对(key-value)映射，其底层实现是一个Entry数组+链表；
* Hashtable和HashMap一样也是散列表，存储元素也是键值对；
* HashMap允许key和value都为null，而Hashtable都不能为null，Hashtable中的映射不是有序的；
* Hashtable和HashMap扩容的方法不一样，Hashtable中数组默认大小11，扩容方式是 old*2+1。
* HashMap中数组的默认大小是16，而且一定是2的指数，增加为原来的2倍。
* Hashtable继承于Dictionary类（Dictionary类声明了操作键值对的接口方法），实现Map接口（定义键值对接口）；
* Hashtable大部分类用synchronized修饰，证明Hashtable是线程安全的。
*/
public class Hashtable<K, V>
extends Dictionary<K, V>
implements Map<K, V>, Cloneable, java.io.Serializable {
/**
* 键值对/Entry数组，每个Entry本质上是一个单向链表的表头
*/
private transient Entry<?, ?>[] table;
/**
* 当前表中的Entry数量，如果超过了阈值，就会扩容，即调用rehash方法
*/
private transient int count;
/**
* rehash阈值
*
* @serial
*/
private int threshold;
/**
* 负载因子
*
* @serial
*/
private float loadFactor;
/**
* 用来实现"fail-fast"机制的（也就是快速失败）。所谓快速失败就是在并发集合中，其进行
* 迭代操作时，若有其他线程对其进行结构性的修改，这时迭代器会立马感知到，并且立即抛出
* ConcurrentModificationException异常，而不是等到迭代完成之后才告诉你（你已经出错了）。
*
* 注意，这个异常不仅可能出现在多线程中，在单线程可能因为对非同步集合同时进行插入删除而抛出该异常,而迭代器只能保证删除正常.
*/
private transient int modCount = 0;
/**
* 版本序列号
*/
private static final long serialVersionUID = 1421746759512286392L;
/**
* 指定容量大小和加载因子的构造函数
*
* @param initialCapacity
* 容量大小
* @param loadFactor
* 负载因子
* @throws IllegalArgumentException
* if the initial capacity is less than zero, or if the load
* factor is nonpositive.
*/
public Hashtable(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: " +
initialCapacity);
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal Load: " + loadFactor);
if (initialCapacity == 0)
initialCapacity = 1;
this.loadFactor = loadFactor;
table = new Entry<?, ?>[initialCapacity];
threshold = (int) Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
}
/**
* 指定容量大小的构造函数
*
* @param initialCapacity
* 容量大小
* @throws IllegalArgumentException
* if the initial capacity is less than zero.
*/
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);
}
/**
* 默认构造函数
*/
public Hashtable() {
// 默认构造函数，指定的容量大小是11；加载因子是0.75
this(11, 0.75f);
}
/**
* 包含子Map的构造函数
*
* @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);
}
/**
* 返回容量大小
*
* @return the number of keys in this hashtable.
*/
public synchronized int size() {
return count;
}
/**
* 判空
*
* @return <code>true</code> if this hashtable maps no keys to values;
* <code>false</code> otherwise.
*/
public synchronized boolean isEmpty() {
return count == 0;
}
/**
* 返回所有key的枚举对象
*
* @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);
}
/**
* 返回所有value的枚举对象
*
* @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);
}
/**
* 判断是否含有该value的键值对，在Hashtable中hashCode相同的Entry用链表组织，hashCode不同的存储在Entry数组table中；
*
* @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.
* @throws NullPointerException
* if the value is <code>null</code>
*/
public synchronized boolean contains(Object value) {
if (value == null) {
throw new NullPointerException();
}
Entry<?, ?> tab[] = table;
// 查找：遍历所有Entry链表
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;
}
/**
* 判断是否包含value值对象
*
* @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);
}
/**
* 判断是否包含key键值对象
*
* @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();
/**
* 计算index, % tab.length防止数组越界 index表示key对应entry所在链表表头
* 0x7FFFFFFF只有首位为0其余全1，是最大值，所以(hash & 0x7FFFFFFF)只与低位相关
* 类似HashMap中 (len-1) & hash求桶位置
*/
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;
}
/**
* 根据指定key查找对应value，查找原理与containsKey相同，查找成功返回value，否则返回null
*
* @param key
* the key whose associated value is to be returned
* @return the value to which the specified key is mapped, or {@code null}
* if this map contains no mapping for the key
* @throws NullPointerException
* if the specified key is null
* @see #put(Object, Object)
*/
@SuppressWarnings("unchecked")
public synchronized V get(Object key) {
Entry<?, ?> tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<?, ?> e = tab[index]; e != null; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return (V) e.value;
}
}
return null;
}
/**
* 规定的最大数组容量
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* 当Hashtable中键值对总数超过阈值（容量*装载因子）后，内部自动调用rehash()增加容量，重新计算每个键值对的hashCode int
* newCapacity = (oldCapacity << 1) + 1计算新容量 = 2 * 旧容量 + 1；并且根据新容量更新阈值
*/
@SuppressWarnings("unchecked")
protected void rehash() {
int oldCapacity = table.length;
Entry<?, ?>[] oldMap = table;
/**
* 新的大小为 原大小 * 2 + 1 虽然不保证capacity是一个质数，但至少保证它是一个奇数
*/
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;
// 拷贝每个Entry链表
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;
// 重新计算每个Entry链表的表头索引（rehash）
int index = (e.hash & 0x7FFFFFFF) % newCapacity;
// 开辟链表节点
e.next = (Entry<K, V>) newMap[index];
// 拷贝
newMap[index] = e;
}
}
}
/**
* 当键值对个数超过阈值，先进行rehash然后添加entry，否则直接添加entry
*/
private void addEntry(int hash, K key, V value, int index) {
modCount++;
Entry<?, ?> tab[] = table;
// 当前元素大于等于阈值，就扩容并且再计算hash值
if (count >= threshold) {
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];
// 和HashMap不同，Hashtable选择把新插入的元素放到链表最前边，而且没有使用红黑树
tab[index] = new Entry<>(hash, key, value, e);
count++;
}
/**
* 设置键值对，key和value都不可为null，设置顺序: 如果Hashtable含有key，设置(key, oldValue) -> (key,
* newValue)； 如果Hashtable不含有key, 调用addEntry(...)添加新的键值对；
*
* @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
* @throws 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) {
// value为空抛出空指针异常
if (value == null) {
throw new NullPointerException();
}
// Makes sure the key is not already in the hashtable.
Entry<?, ?> tab[] = table;
/**
* key的hashCode是调用Object的hashCode()方法， 是native的方法，如果为null，就会抛出空指针异常
*/
int hash = key.hashCode();
/**
* 因为hash可能为负数，所以就先和0x7FFFFFFF相与 在HashMap中，是用 (table.length - 1) & hash
* 计算要放置的位置
*/
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;
}
}
// 如果key对应的值不存在，就调用addEntry方法加入
addEntry(hash, key, value, index);
return null;
}
/**
* remove操作，计算key所在链表表头table[index]，然后进行单向链表的节点删除操作
*
* @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;
@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;
}
/**
* 把所有的 映射从指定的map复制到hashTable中
* 如果给定的map中的key值已经存在于hashTable中，则将会覆盖hashTable中key所对应的value（hashTable中key值不允许重复）
*
* @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) {
// foreach 循环map数据put到hashTable中
for (Map.Entry<? extends K, ? extends V> e : t.entrySet())
put(e.getKey(), e.getValue());
}
/**
* 清空Hashtable 将Hashtable的table数组的值全部设为null
*/
public synchronized void clear() {
Entry<?, ?> tab[] = table;
modCount++;
for (int index = tab.length; --index >= 0; )
tab[index] = null;
count = 0;
}
/**
* 对Hashtable的浅拷贝操作，浅拷贝所有bucket（单向链表组织形式）的表头
*
* @return a clone of the hashtable
*/
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);
}
}
/**
* 返回Hashtable对象的String表达方式，一系列以括号和逗号，空格分隔的Entry，如{key1=value1, key2=value2}
*
* @return a string representation of this hashtable
*/
public synchronized String toString() {
int max = size() - 1;
if (max == -1)
return "{}";
StringBuilder sb = new StringBuilder();
Iterator<Map.Entry<K, V>> it = entrySet().iterator();
sb.append('{');
for (int i = 0; ; i++) {
Map.Entry<K, V> e = it.next();
K key = e.getKey();
V value = e.getValue();
sb.append(key == this ? "(this Map)" : key.toString());
sb.append('=');
sb.append(value == this ? "(this Map)" : value.toString());
if (i == max)
return sb.append('}').toString();
sb.append(", ");
}
}
private <T> Enumeration<T> getEnumeration(int type) {
if (count == 0) {
return Collections.emptyEnumeration();
} else {
return new Enumerator<>(type, false);
}
}
/**
* 获得迭代器
*/
private <T> Iterator<T> getIterator(int type) {
if (count == 0) {
return Collections.emptyIterator();
} else {
return new Enumerator<>(type, true);
}
}
// 视图
/**
* 以下每个字段初始化后会包含一个首次请求后的指定视图，视图是无状态的，所以不必创建多个
*/
private transient volatile Set<K> keySet;
private transient volatile Set<Map.Entry<K, V>> entrySet;
private transient volatile Collection<V> values;
/**
* 返回一个被synchronizedSet封装后的KeySet对象
* synchronizedSet封装的目的是对KeySet的所有方法都添加synchronized，实现多线程同步
*/
public Set<K> keySet() {
if (keySet == null)
keySet = Collections.synchronizedSet(new KeySet(), this);
return keySet;
}
/**
* Hashtable的Key的Set集合 KeySet继承于AbstractSet，所以，KeySet中的元素没有重复的
*/
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();
}
}
/**
* 返回一个被synchronizedSet封装后的EntrySet对象
* synchronizedSet封装的目的是对EntrySet的所有方法都添加synchronized，实现多线程同步
*/
public Set<Map.Entry<K, V>> entrySet() {
if (entrySet == null)
entrySet = Collections.synchronizedSet(new EntrySet(), this);
return entrySet;
}
/**
* Hashtable的Entry的Set集合 EntrySet继承于AbstractSet，所以，EntrySet中的元素没有重复的
*/
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);
}
/**
* 查找EntrySet中是否包含Object(0) 首先，在table中找到o对应的Entry(Entry是一个单向链表)
* 然后，查找Entry链表中是否存在Object
*/
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;
}
/**
* 删除元素Object(0) 首先，在table中找到o对应的Entry(Entry是一个单向链表) 然后，删除链表中的元素Object
*/
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();
}
}
/**
* 返回一个被synchronizedCollection封装后的ValueCollection对象
* synchronizedCollection封装的目的是对ValueCollection的所有方法都添加synchronized，实现多线程同步
*/
public Collection<V> values() {
if (values == null)
values = Collections.synchronizedCollection(new ValueCollection(),
this);
return values;
}
/**
* Hashtable的value的Collection集合。
* ValueCollection继承于AbstractCollection，所以，ValueCollection中的元素可以重复的。
*/
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
/**
* 重新equals()函数 若两个Hashtable的所有key-value键值对都相等，则判断它们两个相等
*
* @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 {
/**
* 通过迭代器依次取出当前Hashtable的key-value键值对 并判断该键值对，存在于Hashtable(o)中。
* 若不存在，则立即返回false；否则，遍历完“当前Hashtable”并返回true。
*/
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;
}
/**
* 计算Hashtable的哈希值
*
* @see Map#hashCode()
* @since 1.2
*/
public synchronized int hashCode() {
int h = 0;
// 若 Hashtable的实际大小为0 或者 加载因子<0，则返回0
if (count == 0 || loadFactor < 0)
return h; // Returns zero
loadFactor = -loadFactor; // Mark hashCode computation in progress
Entry<?, ?>[] tab = table;
// 返回Hashtable中的每个Entry的key和value的异或值的总和
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;
}
/**
* 替换
*
* @param key
* @param value
* @return
*/
@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;
}
/**
* 将Hashtable的总的容量，实际容量，所有的Entry都写入到输出流中
*/
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;
}
}
/**
* 将Hashtable的总的容量，实际容量，所有的Entry依次读出
*/
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);
// 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);
}
}
/**
* readObject使用的put方法（重建put），因为put方法支持重写，并且子类尚未初始化的时候不能调用put方法，所以就提供了reconstitutionPut
* 它和常规put方法有几点不同，不检测rehash,因为初始元素数目已知。modCount不会自增，因为我们是在创建一个新的实例。
*/
private void reconstitutionPut(Entry<?, ?>[] tab, K key, V value)
throws StreamCorruptedException {
if (value == null) {
throw new java.io.StreamCorruptedException();
}
// 确保Key不在Hashtable中
// 反序列化过程中不应该 会发生的情况
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<?, ?> e = tab[index]; e != null; e = e.next) {
// 反序列化过程中如果出现Key值重复，抛出异常StreamCorruptedException
if ((e.hash == hash) && e.key.equals(key)) {
throw new java.io.StreamCorruptedException();
}
}
// 创建新的Entry.
@SuppressWarnings("unchecked")
Entry<K, V> e = (Entry<K, V>) tab[index];
tab[index] = new Entry<>(hash, key, value, e);
count++;
}
/**
* Hashtable的Entry节点，它本质上是一个单向链表。 因此，我们能推断出Hashtable是由拉链法实现的散列表
*/
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;
}
// 进行判断value是否为空，即不允许value为空，其实key也不能为空
public V setValue(V value) {
if (value == null)
throw new NullPointerException();
V oldValue = this.value;
this.value = value;
return oldValue;
}
// 覆盖equals()方法，判断两个Entry是否相等。
// 若两个Entry的key和value都相等，则认为它们相等。
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() {
// 直接用hash进行异或，与HashMap不同
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;
/**
* Enumerator的作用是提供了通过elements()遍历Hashtable的接口和通过entrySet()遍历Hashtable的接口。
* 因为，它同时实现了 Enumerator接口和Iterator接口。
*/
private class Enumerator<T> implements Enumeration<T>, Iterator<T> {
// 指向Hashtable的table
Entry<?, ?>[] table = Hashtable.this.table;
// Hashtable的总的大小
int index = table.length;
Entry<?, ?> entry;
Entry<?, ?> lastReturned;
int type;
/**
* Enumerator是 迭代器(Iterator) 还是 枚举类(Enumeration)的标志
* iterator为true，表示它是迭代器；否则，是枚举类。
*/
boolean iterator;
/**
* 在将Enumerator当作迭代器使用时会用到，用来实现fail-fast机制。
*/
protected int expectedModCount = modCount;
Enumerator(int type, boolean iterator) {
this.type = type;
this.iterator = iterator;
}
/**
* 从遍历table的数组的末尾向前查找，直到找到不为null的Entry。
*/
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;
}
/**
* 获取下一个元素 注意：从hasMoreElements() 和nextElement()
* 可以看出Hashtable的elements()遍历方式
* 首先，从后向前的遍历table数组。table数组的每个节点都是一个单向链表(Entry)。 然后，依次向后遍历单向链表Entry。
*/
@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");
}
// 迭代器Iterator的判断是否存在下一个元素
// 实际上，它是调用的hasMoreElements()
public boolean hasNext() {
return hasMoreElements();
}
// 迭代器获取下一个元素
// 实际上，它是调用的nextElement()
public T next() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
return nextElement();
}
// 迭代器的remove()接口。
// 首先，它在table数组中找出要删除元素所在的Entry，
// 然后，删除单向链表Entry中的元素。
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;
// 获取该槽位第一个元素
@SuppressWarnings("unchecked")
Entry<K, V> e = (Entry<K, V>) tab[index];
// 从单链表的一端向后遍历
for (Entry<K, V> prev = null; e != null; prev = e, e = e.next) {
// 当前元素即为上一个返回元素
if (e == lastReturned) {
modCount++;
expectedModCount++;
// 删除上一个元素
if (prev == null)
tab[index] = e.next;
else
prev.next = e.next;
count--;
lastReturned = null;
return;
}
}
throw new ConcurrentModificationException();
}
}
}
}
