Java-HashTable源码分析

Hashtable

initial capacity：初始容量

load factor：负载因子     the default load factor (.75) offers a good tradeoff between time and space costs.

 * Java Collections Framework</a>.  Unlike the new collection

 * implementations, {@code Hashtable} is synchronized.  If a

 * thread-safe implementation is not needed, it is recommended to use

 * {@link HashMap} in place of {@code Hashtable}.  If a thread-safe

 * highly-concurrent implementation is desired, then it is recommended

 * to use {@link java.util.concurrent.ConcurrentHashMap} in place of

 * {@code Hashtable}.

如果不考虑线程安全，推荐使用HashMap，如果考虑线程安全推荐使用ConcurrentHashMap

ublic class Hashtable<K,V>

    extends Dictionary<K,V>

    implements Map<K,V>, Cloneable, java.io.Serializable 

与HashTable的继承类不同，实现接口相同

public Hashtable() {
this(11, 0.75f);
}由构造函数可知，默认容量为11，负载因子为0.75

返回大小，方法是同步的

public synchronized int size() {
return count;
}判断某个值是否在HashTable中
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;
}判断某个key是否在HashTable中，与HashMap中对比可知其取table下标的方法不同
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;
}获取某个key对应的value值
@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;
}

HashTable扩容，有源码可知，其容量为原来的2倍+1，
@SuppressWarnings("unchecked")
protected void rehash() {
int oldCapacity = table.length;
Entry<?,?>[] oldMap = table;

// overflow-conscious code
int newCapacity = (oldCapacity << 1) + 1;
if (newCapacity - MAX_ARRAY_SIZE > 0) {
if (oldCapacity == MAX_ARRAY_SIZE)
// Keep running with MAX_ARRAY_SIZE buckets
return;
newCapacity = MAX_ARRAY_SIZE;
}
Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];

modCount++;
threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
table = newMap;

for (int i = oldCapacity ; i-- > 0 ;) {
for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) {
Entry<K,V> e = old;
old = old.next;

int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = (Entry<K,V>)newMap[index];
newMap[index] = e;
}
}
}

Neither the key nor thevalue can be <code>null</code>.   HashTable的key和value均不能为null值

put方法

public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
}

// Makes sure the key is not already in the hashtable.
Entry<?,?> tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry<K,V> entry = (Entry<K,V>)tab[index];
for(; entry != null ; entry = entry.next) {
if ((entry.hash == hash) && entry.key.equals(key)) {
V old = entry.value;
entry.value = value;
return old;
}
}

addEntry(hash, key, value, index);
return null;
}

remove方法
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;
}

putAll方法
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());
}

clear方法
public synchronized void clear() {
Entry<?,?> tab[] = table;
modCount++;
for (int index = tab.length; --index >= 0; )
tab[index] = null;
count = 0;
}

HashTable中的clone（）方法为浅复制，只是复制了结构，key和value值并没有复制

需要重写Object中hashcode(),equals()等方法。

由于HashTable的每个方法前面都有synchronized 关键字，所以其是线程安全的。