Java集合：Hashtable使用详解及源码分析

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Hashtablesynchronized Object              clone() //复制Hashtableboolean             contains(Object value) //判断是否包含valuesynchronized boolean             containsKey(Object key) //是否包含keysynchronized boolean             containsValue(Object value) //是否包含valuesynchronized 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entrysynchronized int                 hashCode() //获取hashCodesynchronized 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/onehashtable的大小为: 4hashtable是否包含key为4的元素: truehashtable是否包含value为two的元素: true删除key为2的元素: twohashtable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/oneclonetable的元素为: {6=six, 5=five, 4=four, 3=three, 1=one}hashtable的key有: 6 5 4 3 1hashtable是否为空: true

2 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