java.util.Hashtable實現了一個哈希表，存儲key-value。任何非null對象都可以用作鍵或值。要成功地從哈希表存儲和檢索對象，用作鍵的對象必須實現hashCode方法和equals方法。
Hashtable的實例有兩個影響其性能的參數：初始容量和負載因子。容量是哈希表中buckets的數目，初始容量只是創建哈希表時的容量。注意，在“散列衝突”的情況下，一個bucket存儲多個條目，這些條目必須按順序搜索。加載因子是在哈希表的容量自動增加之前允許獲得的滿容量的度量。初始容量和負載因子參數只是用於實現。關於何時以及是否調用rehash方法的確切細節取決於實現。
類名
public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable
變量
// hash表
private transient Entry<?,?>[] table;
// hash表中的條目總數.
private transient int count;
/**
* 當表的大小超過此閾值時，將重新hash該表。
*（此字段的值爲（int）（容量*負載因子）)
*/
private int threshold;
// 負載因子
private float loadFactor;
//此字段用於使哈希表集合視圖上的迭代器fail-fast.
private transient int modCount = 0;
put(K key, V value)
public synchronized V put(K key, V value) {
// value不能爲null
if (value == null) {
throw new NullPointerException();
}
// 如果key存在，替換value值，並返回舊的value值
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;
}
}
// 添加key-value
addEntry(hash, key, value, index);
return null;
}
// 添加key-value
private void addEntry(int hash, K key, V value, int index) {
modCount++;
Entry<?,?> tab[] = table;
if (count >= threshold) {
// 重寫hash
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++;
}
從put方法我們可以看出Hashtable的key-value都不可以爲空。首先判斷value是不是爲null，然後循環table，當key已經存在的時候，修改value值。當key不存在的時候，添加一個Entry。當table的容量不足時，則調用rehash方法。
rehash()
protected void rehash() {
int oldCapacity = table.length;
Entry<?,?>[] oldMap = table;
// 容量 * 2 + 1
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;
}
//新建一個size = newCapacity 的HashTable
Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];
modCount++;
//重新計算閥值
threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
table = newMap;
//把oldTab中的節點reHash到newTab 中去
for (int i = oldCapacity ; i-- > 0 ;) {
for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) {
Entry<K,V> e = old;
//下一個Entry
old = old.next;
//新的index
int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = (Entry<K,V>)newMap[index];
newMap[index] = e;
}
}
}
get(Object key)
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) {
// 判斷hash和equals
if ((e.hash == hash) && e.key.equals(key)) {
return (V)e.value;
}
}
return null;
}
HashTable與HashMap的區別
-
HashTable基於Dictionary類，而HashMap是基於AbstractMap。Dictionary是任何可將鍵映射到相應值的類的抽象父類，而AbstractMap是基於Map接口的實現，它以最大限度地減少實現此接口所需的工作。
-
HashMap可以允許存在一個爲null的key和任意個爲null的value，但是HashTable中的key和value都不允許爲null。
-
Hashtable的方法是同步的，而HashMap的方法不是。
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;
public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable {
/**
* hash表數據.
*/
private transient Entry<?,?>[] table;
/**
* 哈希表中的條目總數.
*/
private transient int count;
/**
* 當表的大小超過此閾值時，將重新hash該表。
*（此字段的值爲（int）（容量*負載因子）)
*/
private int threshold;
/**
* 負載因子
*/
private float loadFactor;
/**
* 此字段用於使哈希表集合視圖上的迭代器fail-fast.
*/
private transient int modCount = 0;
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = 1421746759512286392L;
/**
* 指定初始容量和負載因子構造實例
*/
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);
}
/**
* 指定初始容量構造實例
*/
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);
}
/**
* 使用默認參數構造實例
*/
public Hashtable() {
this(11, 0.75f);
}
/**
* 使用與給定映射相同的映射構造實例
*/
public Hashtable(Map<? extends K, ? extends V> t) {
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}
/**
* 返回此哈希表中的鍵數.
*/
public synchronized int size() {
return count;
}
/**
* 測試此哈希表是否有數據。
*/
public synchronized boolean isEmpty() {
return count == 0;
}
/**
* 返回此哈希表中鍵的枚舉
*/
public synchronized Enumeration<K> keys() {
return this.<K>getEnumeration(KEYS);
}
/**
* 返回此哈希表中值的枚舉
*/
public synchronized Enumeration<V> elements() {
return this.<V>getEnumeration(VALUES);
}
/**
* 測試是否包含指定值。
* 此操作比{containsKey}方法更昂貴
*/
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);
}
/**
* 測試指定的對象是否是此哈希表中的鍵。
*/
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;
}
/**
* 返回指定鍵映射到的值，如果此映射不包含鍵的映射，則返回null.
*/
@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) {
// 判斷hash和equals
if ((e.hash == hash) && e.key.equals(key)) {
return (V)e.value;
}
}
return null;
}
/**
* 要分配的數組的最大大小.
* 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;
/**
* 增加哈希表的容量並在內部重新組織該哈希表，
* 以便更有效地容納和訪問其條目。
* 當哈希表中的鍵數超過此哈希表的容量和加載因子時將自動調用此方法。
*/
@SuppressWarnings("unchecked")
protected void rehash() {
int oldCapacity = table.length;
Entry<?,?>[] oldMap = table;
// 容量 * 2 + 1
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;
}
//新建一個size = newCapacity 的HashTable
Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];
modCount++;
//重新計算閥值
threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
table = newMap;
//把oldTab中的節點reHash到newTab 中去
for (int i = oldCapacity ; i-- > 0 ;) {
for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) {
Entry<K,V> e = old;
//下一個Entry
old = old.next;
//新的index
int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = (Entry<K,V>)newMap[index];
newMap[index] = e;
}
}
}
/**
* put key-value
*/
public synchronized V put(K key, V value) {
// value不能爲null
if (value == null) {
throw new NullPointerException();
}
// 如果key存在，替換value值，並返回舊的value值
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;
}
}
// 添加key-value
addEntry(hash, key, value, index);
return null;
}
// 添加key-value
private void addEntry(int hash, K key, V value, int index) {
modCount++;
Entry<?,?> tab[] = table;
if (count >= threshold) {
// 重寫hash
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++;
}
/**
* 從該哈希表中刪除鍵（及其相應的值）
*/
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 = e
//將e的下一個值賦值給e
prev.next = e.next;
} else { //第一次prev=null
//將e的下一個值賦值給當前索引位置
tab[index] = e.next;
}
count--;
V oldValue = e.value;
e.value = null;
return oldValue;
}
}
return null;
}
/**
* 將指定映射中的所有映射覆制到此哈希表.
*/
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());
}
/**
* 清除此哈希表
*/
public synchronized void clear() {
Entry<?,?> tab[] = table;
modCount++;
for (int index = tab.length; --index >= 0; )
tab[index] = null;
count = 0;
}
/**
* 創建此哈希表的淺層副本。
*/
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);
}
}
/**
* 重寫toString
*/
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
/**
* 這些字段中的每一個都初始化爲在第一次請求此視圖時包含相應視圖的實例。
* 視圖是無狀態的，因此沒有理由創建多個視圖。
*/
private transient volatile Set<K> keySet;
private transient volatile Set<Map.Entry<K,V>> entrySet;
private transient volatile Collection<V> values;
/**
* 返回此映射中包含的鍵的Set視圖
*/
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();
}
}
/**
* 返回此映射中包含的映射的Set視圖。
*/
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<?,?> 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();
}
}
/**
* 返回此映射中包含的值的Collection視圖。
*/
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
/**
* 根據映射接口中的定義，將指定對象與此映射進行相等性比較.
*/
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;
}
/**
* 根據映射接口中的定義返回此映射的hashcode.
*/
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;
}
/**
* 將哈希表的狀態保存到流中
*/
private void writeObject(java.io.ObjectOutputStream s)
throws IOException {
Entry<Object, Object> entryStack = null;
synchronized (this) {
// 寫出閾值和負載因子
s.defaultWriteObject();
// 寫出元素的長度和計數
s.writeInt(table.length);
s.writeInt(count);
// 表中條目的堆棧副本
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;
}
}
}
// 寫出鍵/值對象
while (entryStack != null) {
s.writeObject(entryStack.key);
s.writeObject(entryStack.value);
entryStack = entryStack.next;
}
}
/**
* 從流重建哈希表
*/
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException
{
//讀取閾值和負載因子
s.defaultReadObject();
// 驗證負載因子
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new StreamCorruptedException("Illegal Load: " + loadFactor);
// 讀取數組的原始長度和元素數
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);
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。這是因爲put是可重寫的，
* 不應在readObject中調用，因爲子類尚未初始化。
*/
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++;
}
/**
* 哈希表列表項
*/
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;
/**
* 哈希表枚舉器類。這個類實現了枚舉和迭代器接口，
* 但是可以在禁用迭代器方法的情況下創建單個實例。
* 這是必要的，以避免通過傳遞枚舉無意中增加授予用戶的功能。
*/
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();
}
}
}
}
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