public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.SerializableNOTE: This class is obsolete. New implementations should * implement the Map interface, rather than extending this class.
public abstract
class Dictionary<K,V> {
public Dictionary() {
}
abstract public int size();
abstract public boolean isEmpty();
abstract public Enumeration<K> keys();
abstract public Enumeration<V> elements();
abstract public V get(Object key);
abstract public V put(K key, V value);
abstract public V remove(Object key);
}没啥好说的，下面直接看Hashtable源码，首先依然是成员变量：
private transient Entry<K,V>[] table;//存储键值对对象的桶数组
/**
* The total number of entries in the hash table.
*键值对总数
*/
private transient int count;
/**
* The table is rehashed when its size exceeds this threshold. (The
* value of this field is (int)(capacity * loadFactor).)
*容量的阈值，超过此容量将会导致扩容。值为容量*负载因子
*/
private int threshold;
/**
* The load factor for the hashtable.
*负载因子
*/
private float loadFactor;
/**
* hashtable被改变的次数，用于快速失败机制
*/
private transient int modCount = 0;
public Hashtable(int initialCapacity, float loadFactor) {//可指定初始容量和加载因子
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;//初始容量最小值为1
this.loadFactor = loadFactor;
table = new Entry[initialCapacity];//创建桶数组
threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);//初始化容量阈值
useAltHashing = sun.misc.VM.isBooted() &&
(initialCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
}
/**
* Constructs a new, empty hashtable with the specified initial capacity
* and default load factor (0.75).
*/
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);//默认负载因子为0.75
}
public Hashtable() {
this(11, 0.75f);//默认容量为11，负载因子为0.75
}
/**
* Constructs a new hashtable with the same mappings as the given
* Map. The hashtable is created with an initial capacity sufficient to
* hold the mappings in the given Map and a default load factor (0.75).
*/
public Hashtable(Map<? extends K, ? extends V> t) {
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}
private static class Entry<K,V> implements Map.Entry<K,V> {//键值对对象
int hash;//哈希值
final K key;//键
V value;//值
Entry<K,V> next;//指向下一个
protected Entry(int hash, K key, V value, Entry<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
protected Object clone() {//直接通过new的方式克隆
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) {//可设置值
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.equals(e.getKey()) && value.equals(e.getValue());
}
public int hashCode() {
return hash ^ value.hashCode();
}
public String toString() {
return key.toString()+"="+value.toString();
}
}
public synchronized V put(K key, V value) {//向哈希表中添加键值对
// Make sure the value is not null
if (value == null) {//确保值不能为空
throw new NullPointerException();
}
// Makes sure the key is not already in the hashtable.
Entry tab[] = table;
int hash = hash(key);//根据键生成hash值---->若key为null，此方法会抛异常
int index = (hash & 0x7FFFFFFF) % tab.length;//通过hash值找到其存储位置
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {/遍历链表
if ((e.hash == hash) && e.key.equals(key)) {//若键相同，则新值覆盖旧值
V old = e.value;
e.value = value;
return old;
}
}
modCount++;
if (count >= threshold) {//当前容量超过阈值。需要扩容
// Rehash the table if the threshold is exceeded
rehash();//重新构建桶数组，并对数组中所有键值对重哈希，耗时！
tab = table;
hash = hash(key);
index = (hash & 0x7FFFFFFF) % tab.length;//这里是取摸运算
}
// Creates the new entry.
Entry<K,V> e = tab[index];
//将新结点插到链表首部
tab[index] = new Entry<>(hash, key, value, e);//生成一个新结点
count++;
return null;
}
private int hash(Object k) {
if (useAltHashing) {
if (k.getClass() == String.class) {
return sun.misc.Hashing.stringHash32((String) k);
} else {
int h = hashSeed ^ k.hashCode();
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}
} else {
return k.hashCode();//此处可能抛空指针异常
}
}
protected void rehash() {
int oldCapacity = table.length;//记录旧容量
Entry<K,V>[] oldMap = table;//记录旧的桶数组
// overflow-conscious code
int newCapacity = (oldCapacity << 1) + 1;//新容量为老容量的2倍加1
if (newCapacity - MAX_ARRAY_SIZE > 0) {
if (oldCapacity == MAX_ARRAY_SIZE)//容量不得超过约定的最大值
// Keep running with MAX_ARRAY_SIZE buckets
return;
newCapacity = MAX_ARRAY_SIZE;
}
Entry<K,V>[] newMap = new Entry[newCapacity];//创建新的数组
modCount++;
threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
boolean currentAltHashing = useAltHashing;
useAltHashing = sun.misc.VM.isBooted() &&
(newCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
boolean rehash = currentAltHashing ^ useAltHashing;
table = newMap;
for (int i = oldCapacity ; i-- > 0 ;) {//转移键值对到新数组
for (Entry<K,V> old = oldMap[i] ; old != null ; ) {
Entry<K,V> e = old;
old = old.next;
if (rehash) {
e.hash = hash(e.key);
}
int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = newMap[index];
newMap[index] = e;
}
}
}
public synchronized V get(Object key) {//根据键取出对应索引
Entry tab[] = table;
int hash = hash(key);//先根据key计算hash值
int index = (hash & 0x7FFFFFFF) % tab.length;//再根据hash值找到索引
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {//遍历entry链
if ((e.hash == hash) && e.key.equals(key)) {//若找到该键
return e.value;//返回对应的值
}
}
return null;//否则返回null
}
public synchronized V remove(Object key) {//删除指定键值对
Entry tab[] = table;
int hash = hash(key);//计算hash值
int index = (hash & 0x7FFFFFFF) % tab.length;//计算索引
for (Entry<K,V> e = tab[index], prev = null ; e != null ; prev = e, e = e.next) {//遍历entry链
if ((e.hash == hash) && e.key.equals(key)) {//找到指定键
modCount++;
if (prev != null) {//修改相关指针
prev.next = e.next;
} else {
tab[index] = e.next;
}
count--;
V oldValue = e.value;
e.value = null;
return oldValue;
}
}
return null;
} public synchronized void clear() {//清空桶数组
Entry tab[] = table;
modCount++;
for (int index = tab.length; --index >= 0; )
tab[index] = null;//直接置空
count = 0;
}public Set<K> keySet() {
if (keySet == null)//通过Collections的包装，返回的是线程安全的键集
keySet = Collections.synchronizedSet(new KeySet(), this);
return keySet;
}
public Set<Map.Entry<K,V>> entrySet() {
if (entrySet==null)//通过Collections的包装，返回的是线程安全的键值集
entrySet = Collections.synchronizedSet(new EntrySet(), this);
return entrySet;
} 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();
}
}
【源码】Hashtable源码剖析,布布扣,bubuko.com
原文地址：http://blog.csdn.net/chdjj/article/details/38581035
