HashTable源代码剖析
2024-10-20 11:52:17
<span style="font-size:14px;font-weight: normal;">public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable {
//transient不能被序列化 数据部分
private transient Entry[] table;
// 元素个数
private transient int count;
//当HashTable的大小超过这个阈值时重Hash
private int threshold;
//装载因子 过大会导致冲突机会变大 过小会导致空间浪费
private float loadFactor;
//fail-fast机制 保证迭代时，其它线程不干扰
private transient int modCount = 0;
//构造函数 初始容量仅仅要大于0即可，不同于HashMap（系统优化为2的幂次）
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);
}
//默认0.75的装载因子
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);
}
//默认的构造函数
public Hashtable() {
this(11, 0.75f);
}
//map的初始容量必须大于等于11
public Hashtable(Map<? extends K, ? extends V> t) {
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}
//synchronized线程安全的原因
public synchronized int size() {
return count;
}
//是否为空
public synchronized boolean isEmpty() {
return count == 0;
}
//返回枚举迭代器 keys
public synchronized Enumeration<K> keys() {
return this.<K>getEnumeration(KEYS);
}
//返回枚举迭代器 values
public synchronized Enumeration<V> elements() {
return this.<V>getEnumeration(VALUES);
}
//是否包括此元素 从最后一列循环
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;
}
public boolean containsValue(Object value) {
return contains(value);
}
//与查看是否包括value的方法不同 直接依据hash值找到对应的一列
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;
}
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;
}
//重哈希新的大小 oldCapacity * 2 + 1
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;
}
}
}
//加入元素 hashTable不能加入空元素 与HashMap不同
public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
}
// 看该KEY是否已经存在
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++;
//大于阈值重哈希
if (count >= threshold) {
rehash();
tab = table;
index = (hash & 0x7FFFFFFF) % tab.length;
}
// 新加入的元素放在链表的第一个位置
Entry<K,V> e = tab[index];
tab[index] = new Entry<K,V>(hash, key, value, e);
count++;
return null;
}
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) {
// 这里就是重要的 比較像等时要同一时候比較equals和哈希值 覆写当中一个还有一个也要复写
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;
}
//批量加入
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());
}
//清空
public synchronized void clear() {
Entry tab[] = table;
modCount++;
for (int index = tab.length; --index >= 0; )
tab[index] = null;
count = 0;
}
//复制
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();
}
}
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);
}
}
private transient volatile Set<K> keySet = null;
private transient volatile Set<Map.Entry<K,V>> entrySet = null;
private transient volatile Collection<V> values = null;
//得到set集合
public Set<K> keySet() {
if (keySet == null)
keySet = Collections.synchronizedSet(new KeySet(), this);
return keySet;
}
//KeySet类 key的集合
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();
}
}
public Set<Map.Entry<K,V>> entrySet() {
if (entrySet==null)
entrySet = Collections.synchronizedSet(new EntrySet(), this);
return entrySet;
}
//EntrySet类
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();
}
}
public Collection<V> values() {
if (values==null)
values = Collections.synchronizedCollection(new ValueCollection(),
this);
return values;
}
//values类，含有values的迭代器
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();
}
}
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 {
//迭代一方，再还有一方中进行查找
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;
}
// h += e.key.hashCode() ^ e.value.hashCode()
public synchronized int hashCode() {
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;
}
//序列化 table不能序列化，之序列化里面的key value
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;
}
}
}
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;
Entry[] 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(table, key, value);
}
this.table = table;
}
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<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;
}
//java的clone是浅复制
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()));
}
//哈希码 通过 ^
public int hashCode() {
return hash ^ (value==null ? 0 : value.hashCode());
}
public String toString() {
return key.toString()+"="+value.toString();
}
}
//枚举迭代器
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;
boolean iterator;
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 */
//循环直到一个没有空的列
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 class EmptyEnumerator implements Enumeration<Object> {
EmptyEnumerator() {
}
public boolean hasMoreElements() {
return false;
}
public Object nextElement() {
throw new NoSuchElementException("Hashtable Enumerator");
}
}
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");
}
}
}</span>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable { //transient不能被序列化 数据部分
private transient Entry[] table; // 元素个数
private transient int count; //当HashTable的大小超过这个阈值时重Hash
private int threshold; //装载因子 过大会导致冲突机会变大 过小会导致空间浪费
private float loadFactor; //fail-fast机制 保证迭代时，其它线程不干扰
private transient int modCount = 0; //构造函数 初始容量仅仅要大于0即可，不同于HashMap（系统优化为2的幂次）
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);
} //默认0.75的装载因子
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);
} //默认的构造函数
public Hashtable() {
this(11, 0.75f);
} //map的初始容量必须大于等于11
public Hashtable(Map<? extends K, ? extends V> t) {
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}
//synchronized线程安全的原因
public synchronized int size() {
return count;
} //是否为空
public synchronized boolean isEmpty() {
return count == 0;
} //返回枚举迭代器 keys
public synchronized Enumeration<K> keys() {
return this.<K>getEnumeration(KEYS);
} //返回枚举迭代器 values
public synchronized Enumeration<V> elements() {
return this.<V>getEnumeration(VALUES);
} //是否包括此元素 从最后一列循环
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;
} public boolean containsValue(Object value) {
return contains(value);
} //与查看是否包括value的方法不同 直接依据hash值找到对应的一列
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;
} 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;
} //重哈希新的大小 oldCapacity * 2 + 1
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;
}
}
}
//加入元素 hashTable不能加入空元素 与HashMap不同
public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
} // 看该KEY是否已经存在
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++;
//大于阈值重哈希
if (count >= threshold) {
rehash(); tab = table;
index = (hash & 0x7FFFFFFF) % tab.length;
} // 新加入的元素放在链表的第一个位置
Entry<K,V> e = tab[index];
tab[index] = new Entry<K,V>(hash, key, value, e);
count++;
return null;
} 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) {
// 这里就是重要的 比較像等时要同一时候比較equals和哈希值 覆写当中一个还有一个也要复写
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;
} //批量加入
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());
} //清空
public synchronized void clear() {
Entry tab[] = table;
modCount++;
for (int index = tab.length; --index >= 0; )
tab[index] = null;
count = 0;
} //复制
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();
}
} 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);
}
} private transient volatile Set<K> keySet = null;
private transient volatile Set<Map.Entry<K,V>> entrySet = null;
private transient volatile Collection<V> values = null; //得到set集合
public Set<K> keySet() {
if (keySet == null)
keySet = Collections.synchronizedSet(new KeySet(), this);
return keySet;
}
//KeySet类 key的集合
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();
}
} public Set<Map.Entry<K,V>> entrySet() {
if (entrySet==null)
entrySet = Collections.synchronizedSet(new EntrySet(), this);
return entrySet;
}
//EntrySet类
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();
}
} public Collection<V> values() {
if (values==null)
values = Collections.synchronizedCollection(new ValueCollection(),
this);
return values;
}
//values类，含有values的迭代器
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();
}
} 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 {
//迭代一方，再还有一方中进行查找
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;
}
// h += e.key.hashCode() ^ e.value.hashCode()
public synchronized int hashCode() { 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;
}
//序列化 table不能序列化，之序列化里面的key value
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;
}
}
} 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; Entry[] 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(table, key, value);
}
this.table = table;
} 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<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;
}
//java的clone是浅复制
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()));
}
//哈希码 通过 ^
public int hashCode() {
return hash ^ (value==null ? 0 : value.hashCode());
} public String toString() {
return key.toString()+"="+value.toString();
}
} //枚举迭代器
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; boolean iterator; 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 */
//循环直到一个没有空的列
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 class EmptyEnumerator implements Enumeration<Object> { EmptyEnumerator() {
} public boolean hasMoreElements() {
return false;
} public Object nextElement() {
throw new NoSuchElementException("Hashtable Enumerator");
}
} 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");
} } }</span>
几点总结
针对Hashtable，我们相同给出几点比較重要的总结。但要结合与HashMap的比較来总结。
1、二者的存储结构和解决冲突的方法都是同样的。
2、HashTable在不指定容量的情况下的默认容量为11，而HashMap为16。Hashtable不要求底层数组的容量一定要为2的整数次幂，而HashMap则要求一定为2的整数次幂。
3、Hashtable中key和value都不同意为null，而HashMap中key和value都同意为null（key仅仅能有一个为null，而value则能够有多个为null）。可是假设在Hashtable中有类似put(null,null)的操作，编译相同能够通过，由于key和value都是Object类型。但执行时会抛出NullPointerException异常，这是JDK的规范规定的。
我们来看下ContainsKey方法和ContainsValue的源代码：
- // 推断Hashtable是否包括“值(value)”
- public synchronized boolean contains(Object value) {
- //注意。Hashtable中的value不能是null，
- // 若是null的话，抛出异常!
- if (value == null) {
- throw new NullPointerException();
- }
- // 从后向前遍历table数组中的元素(Entry)
- // 对于每一个Entry(单向链表)。逐个遍历。推断节点的值是否等于value
- 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;
- }
- public boolean containsValue(Object value) {
- return contains(value);
- }
- // 推断Hashtable是否包括key
- public synchronized boolean containsKey(Object key) {
- Entry tab[] = table;
- /计算hash值，直接用key的hashCode取代
- int hash = key.hashCode();
- // 计算在数组中的索引值
- int index = (hash & 0x7FFFFFFF) % tab.length;
- // 找到“key相应的Entry(链表)”，然后在链表中找出“哈希值”和“键值”与key都相等的元素
- for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
- if ((e.hash == hash) && e.key.equals(key)) {
- return true;
- }
- }
- return false;
- }
非常明显。假设value为null，会直接抛出NullPointerException异常。但源代码中并没有对key是否为null推断，有点小不解。只是NullPointerException属于RuntimeException异常。是能够由JVM自己主动抛出的，或许对key的值在JVM中有所限制吧。
4、Hashtable扩容时。将容量变为原来的2倍加1。而HashMap扩容时。将容量变为原来的2倍。
5、Hashtable计算hash值。直接用key的hashCode()，而HashMap又一次计算了key的hash值。Hashtable在求hash值相应的位置索引时，用取模运算。而HashMap在求位置索引时，则用与运算。且这里一般先用hash&0x7FFFFFFF后，再对length取模。&0x7FFFFFFF的目的是为了将负的hash值转化为正值，由于hash值有可能为负数。而&0x7FFFFFFF后。仅仅有符号外改变。而后面的位都不变。
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