1 使用方法
Hashtable是和HashMap类似的散列表，存储的内容为key-value键值对，key的值是唯一的，和HashMap不同的是key和value都不能为null。Hashtable和HashMap的关系可以参考Vector和ArrayList的关系，操作和功能大部分相同，Hashtable是线程安全的但是速度较慢，建议使用HashMap，如果遇到多线程情况则使用concurrentHashMap或者Collections提供静态函数SynchronizedMap等来保证线程安全。
public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, java.io.Serializable {}HashMap继承了Dictionary并实现了Map、Cloneable以及Serializable接口，所以HashMap支持clone和序列化。
1.1 方法介绍
Hashtable提供的API主要有如下几种：
synchronized void clear() //清空Hashtable
synchronized Object clone() //复制Hashtable
boolean contains(Object value) //判断是否包含value
synchronized boolean containsKey(Object key) //是否包含key
synchronized boolean containsValue(Object value) //是否包含value
synchronized Enumeration<V> elements() //获取value组成的枚举
synchronized Set<Entry<K, V>> entrySet() //获取entry组成的Set集合
synchronized boolean equals(Object object) //判断相等
synchronized V get(Object key) //获取键值为key的entry
synchronized int hashCode() //获取hashCode
synchronized boolean isEmpty() //判空
synchronized Set<K> keySet() //获取key组成的Set集合
synchronized Enumeration<K> keys() //获取key组成的枚举
synchronized V put(K key, V value) //添加元素
synchronized void putAll(Map<? extends K, ? extends V> map) //添加一组元素
synchronized V remove(Object key) //删除键为key的元素
synchronized int size() //获取大小
synchronized String toString() //返回Hashtable键值组成的字符串
synchronized Collection<V> values() //获取值组成的Collection集合1.2 使用示例
public void testHashtable() {
//新建hashtable
Hashtable hashtable = new Hashtable();
//添加元素
hashtable.put(1, "one");
hashtable.put(2, "two");
hashtable.put(3, "three");
hashtable.put(4, "four");
//打印元素
this.printMapByEntrySet(hashtable);
//获取大小
System.out.println("hashtable的大小为: " + hashtable.size());
//是否包含key为4的元素
System.out.println("hashtable是否包含key为4的元素: " + hashtable.containsKey(4));
//是否包含值为5的元素
System.out.println("hashtable是否包含value为two的元素: " + hashtable.containsValue("two"));
hashtable.put(5, "five");
hashtable.put(6, "six");
//删除元素
System.out.println("删除key为2的元素: " + hashtable.remove(2));
//打印元素
this.printMapByKeySet(hashtable);
//clone
Hashtable cloneTable = (Hashtable) hashtable.clone();
//打印克隆table
System.out.println("clonetable的元素为: " + cloneTable);
//打印克隆table的keys
this.printHashtableKeysByEnum(cloneTable);
//清空hashtable
hashtable.clear();
//判空
System.out.println("hashtable是否为空: " + hashtable.isEmpty());
}
/** * 根据entrySet()获取Entry集合,然后遍历Set集合获取键值对 * @param hashtable */
private void printMapByEntrySet(Hashtable hashtable) {
Integer key = null;
String value = null;
Iterator iterator = hashtable.entrySet().iterator(); //
System.out.print("hashtable中含有的元素有: ");
while (iterator.hasNext()) {
Map.Entry entry = (Map.Entry) iterator.next();
key = (Integer) entry.getKey();
value = (String) entry.getValue();
System.out.print("key/value : " + key + "/" + value + " ");
}
System.out.println();
}
/** * 使用keySet获取key的Set集合,利用key获取值 * @param hashtable */
private void printMapByKeySet(Hashtable hashtable) {
Integer key = null;
String value = null;
Iterator iterator = hashtable.keySet().iterator();
System.out.print("hashtable中含有的元素有: ");
while (iterator.hasNext()) {
key = (Integer) iterator.next();
value = (String) hashtable.get(key);
System.out.print("key/value : " + key + "/" + value + " ");
}
System.out.println();
}
/** * 使用枚举获取hashtable的keys * @param hashtable */
private void printHashtableKeysByEnum(Hashtable hashtable) {
Enumeration enumeration = hashtable.keys();
System.out.print("hashtable的key有: ");
while (enumeration.hasMoreElements()) {
System.out.print(enumeration.nextElement() + " ");
}
System.out.println();
}
}运行结果如下：
hashtable中含有的元素有: key/value : 4/four key/value : 3/three key/value : 2/two key/value : 1/one
hashtable的大小为: 4
hashtable是否包含key为4的元素: true
hashtable是否包含value为two的元素: true
删除key为2的元素: two
hashtable中含有的元素有: key/value : 6/six key/value : 5/five key/value : 4/four key/value : 3/three key/value : 1/one
clonetable的元素为: {6=six, 5=five, 4=four, 3=three, 1=one}
hashtable的key有: 6 5 4 3 1
hashtable是否为空: true2 源码分析
2.1构造函数
Hashtable有四个构造函数，每个构造函数的不同之处和hashMap构造函数类似在于初始容量和加载因子不同。初始容量为申请的Hashtable初始大小，当加入元素后的容量大于加载因子和当前容量的乘积是，Hashtable需要再hash增大容量。
/** * 构造一个空的Hashtable,容量为initialCapacity,加载因子为loadFactor * * @param initialCapacity the initial capacity of the hashtable. * @param loadFactor the load factor of the hashtable. * @exception 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) //非法参数检查
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal Load: "+loadFactor);
if (initialCapacity==0) //最少容量为1
initialCapacity = 1;
this.loadFactor = loadFactor;
table = new Entry<?,?>[initialCapacity]; //元素数组
//再hash阈值,和HashMap不同.HashMap构造时阈值为大于或者等于initialCapacity的最小的2的倍数
threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
}
/** * 构造一个初始容量为initialCapacity,加载因子为0.75的Hashtable. * * @param initialCapacity the initial capacity of the hashtable. * @exception IllegalArgumentException if the initial capacity is less * than zero. */
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);
}
/** * Constructs a new, empty hashtable with a default initial capacity (11) * and load factor (0.75). */
public Hashtable() {
this(11, 0.75f);
}
/** * 构造并使用t初始化一个Hashtable,大小为t大小两倍和者11中的较大数,加载因子为0.75. * * @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);
}2.2 put方法
/** * 向Hashtable中添加元素 * * @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 * @exception 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) {
// 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;
}
private void addEntry(int hash, K key, V value, int index) {
modCount++;
Entry<?,?> tab[] = table;
if (count >= threshold) {
// Rehash the table if the threshold is exceeded
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++;
}2.3 get方法
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key.equals(k))},
* then this method returns {@code v}; otherwise it returns
* {@code null}. (There can be at most one such mapping.)
*
* @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; //获取下标
for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) { //遍历链表
if ((e.hash == hash) && e.key.equals(key)) {
return (V)e.value;
}
}
return null;
}2.4 remove方法
/** * Removes the key (and its corresponding value) from this * hashtable. This method does nothing if the key is not in the hashtable. * * @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) { //不是链表的第一个元素, 跳过要删除的节点
prev.next = e.next;
} else {
tab[index] = e.next; //第一个节点
}
count--;
V oldValue = e.value;
e.value = null; //删除节点的value, help GC
return oldValue;
}
}
return null;
}参考：
[1] http://www.cnblogs.com/skywang12345/p/3310887.html
[2] http://blog.csdn.net/ns_code/article/details/36191279
[3] 《Java编程思想》第4版
