1 使用方法
Hashtable是和HashMap類似的散列表，存儲的內容爲key-value鍵值對，key的值是唯一的，和HashMap不同的是key和value都不能爲null。Hashtable和HashMap的關係可以參考Vector和ArrayList的關係，操作和功能大部分相同，Hashtable是線程安全的但是速度較慢，建議使用HashMap，如果遇到多線程情況則使用concurrentHashMap或者Collections提供靜態函數SynchronizedMap等來保證線程安全。
public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable {}HashMap繼承了Dictionary並實現了Map、Cloneable以及Serializable接口，所以HashMap支持clone和序列化。
1.1 方法介紹
Hashtable提供的API主要有如下幾種：
synchronized void clear() //清空Hashtable
synchronized Object clone() //複製Hashtable
boolean contains(Object value) //判斷是否包含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() //獲取value組成的枚舉
synchronized Set<Entry<K, V>> entrySet() //獲取entry組成的Set集合
synchronized boolean equals(Object object) //判斷相等
synchronized V get(Object key) //獲取鍵值爲key的entry
synchronized int hashCode() //獲取hashCode
synchronized boolean isEmpty() //判空
synchronized Set<K> keySet() //獲取key組成的Set集合
synchronized Enumeration<K> keys() //獲取key組成的枚舉
synchronized V put(K key, V value) //添加元素
synchronized void putAll(Map<? extends K, ? extends V> map) //添加一組元素
synchronized V remove(Object key) //刪除鍵爲key的元素
synchronized int size() //獲取大小
synchronized String toString() //返回Hashtable鍵值組成的字符串
synchronized Collection<V> values() //獲取值組成的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);
//獲取大小
System.out.println("hashtable的大小爲: " + hashtable.size());
//是否包含key爲4的元素
System.out.println("hashtable是否包含key爲4的元素: " + hashtable.containsKey(4));
//是否包含值爲5的元素
System.out.println("hashtable是否包含value爲two的元素: " + hashtable.containsValue("two"));
hashtable.put(5, "five");
hashtable.put(6, "six");
//刪除元素
System.out.println("刪除key爲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的元素爲: " + cloneTable);
//打印克隆table的keys
this.printHashtableKeysByEnum(cloneTable);
//清空hashtable
hashtable.clear();
//判空
System.out.println("hashtable是否爲空: " + hashtable.isEmpty());
}
/**
* 根據entrySet()獲取Entry集合,然後遍歷Set集合獲取鍵值對
* @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獲取key的Set集合,利用key獲取值
* @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();
}
/**
* 使用枚舉獲取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();
}
}運行結果如下：
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的大小爲: 4
hashtable是否包含key爲4的元素: true
hashtable是否包含value爲two的元素: true
刪除key爲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的元素爲: {6=six, 5=five, 4=four, 3=three, 1=one}
hashtable的key有: 6 5 4 3 1
hashtable是否爲空: true2 源碼分析
2.1構造函數
Hashtable有四個構造函數，每個構造函數的不同之處和hashMap構造函數類似在於初始容量和加載因子不同。初始容量爲申請的Hashtable初始大小，當加入元素後的容量大於加載因子和當前容量的乘積是，Hashtable需要再hash增大容量。
/**
* 構造一個空的Hashtable,容量爲initialCapacity,加載因子爲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) //非法參數檢查
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal Load: "+loadFactor);
if (initialCapacity==0) //最少容量爲1
initialCapacity = 1;
this.loadFactor = loadFactor;
table = new Entry<?,?>[initialCapacity]; //元素數組
//再hash閾值,和HashMap不同.HashMap構造時閾值爲大於或者等於initialCapacity的最小的2的倍數
threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
}
/**
* 構造一個初始容量爲initialCapacity,加載因子爲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);
}
/**
* 構造並使用t初始化一個Hashtable,大小爲t大小兩倍和者11中的較大數,加載因子爲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; //獲取下標
for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) { //遍歷鏈表
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) { //不是鏈表的第一個元素, 跳過要刪除的節點
prev.next = e.next;
} else {
tab[index] = e.next; //第一個節點
}
count--;
V oldValue = e.value;
e.value = null; //刪除節點的value, help GC
return oldValue;
}
}
return null;
}參考：
[1] http://www.cnblogs.com/skywang12345/p/3310887.html
[2] http://blog.csdn.net/ns_code/article/details/36191279
[3] 《Java編程思想》第4版
