Java集合--HashTable

简单看一下 HashTable 内部结构

public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, java.io.Serializable {

    private transient Entry<?,?>[] table;

    private transient int count;

    private int threshold;

    private float loadFactor;

    private transient int modCount = 0;

    public Hashtable(int initialCapacity, float loadFactor) {
        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;
        this.loadFactor = loadFactor;
        table = new Entry<?,?>[initialCapacity];
        threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
    }

    public Hashtable(int initialCapacity) {
        this(initialCapacity, 0.75f);
    }

    public Hashtable() {
        this(11, 0.75f);
    }

    public Hashtable(Map<? extends K, ? extends V> t) {
        this(Math.max(2*t.size(), 11), 0.75f);
        putAll(t);
    }

    public synchronized int size() {...}

    public synchronized boolean isEmpty() {...}

    public synchronized Enumeration<K> keys() {...}

    public synchronized Enumeration<V> elements() {...}

    public synchronized boolean contains(Object value) {...}

    public synchronized boolean containsKey(Object key) {...}

    private static class Entry<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        V value;
        Entry<K,V> next;

        protected Entry(int hash, K key, V value, Entry<K,V> next) {
            this.hash = hash;
            this.key =  key;
            this.value = value;
            this.next = next;
        }

        @SuppressWarnings("unchecked")
        protected Object clone() {
            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) {
            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==null ? e.getKey()==null : key.equals(e.getKey())) &&
               (value==null ? e.getValue()==null : value.equals(e.getValue()));
        }

        public int hashCode() {
            return hash ^ Objects.hashCode(value);
        }

        public String toString() {
            return key.toString()+"="+value.toString();
        }
    }

    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;
    }

    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;
    }

    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;
    }

    public synchronized void putAll(Map<? extends K, ? extends V> t) {...}

    public synchronized void clear() {...}

    public synchronized Object clone() {...}

    public synchronized String toString() {...}

    ....
}

看过 HashMap 内部结构之后再来看 HashTable，就会发现结构非常相似。

默认构造方法初始化容量为11，负载因子为0.75，几乎所有的方法都用了 synchronized 修饰了，所有的操作都需要把整个map锁起来，数据是非常安全的，在多线程情况下不会有什么问题，但是性能应该也不怎么样。

HashTable 确定一个数组的位置用的方法为

int index = (hash & 0x7FFFFFFF) % tab.length;

hash 值可能为负数，0x7FFFFFFF的意思是首位为0 剩下的都是1。二进制的第一位为符号为，hash & 0x7FFFFFFF % tab.length 的意思也就是把hash值转换为正数在和tab.length取模，得到数组的位置。

get/put/remove 都是先锁方法，然后通过这个方法获取位置，再对位置上的节点操作。

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}.

这是官网注解，如果不需要线程安全的话，用 HashMap。如果需要线程安全的话，用 ConcurrentHashMap。