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

简介

HashTable和HashMap一样都是实现了哈希表，存储的内容是key-value映射。HashTable采用拉链法处理Hash冲突。

HashTable 是线程安全的。
key、value都不能为null。
不是有序的。

源码分析

属性

// 存放key-value的数组
   private transient Entry<?,?>[] table;
   // 表里保存的元素个数
   private transient int count;
   // 阈值，用于扩容rehashed判断
   private int threshold;
   private float loadFactor;
   private transient int modCount = 0;

构造方法

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);
}
/**
 * 默认初始表的初始容量为11，装填因子为0.75
 */
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);
}

Entry

HashTable中的每一个元素都是一个Entry，实际上table的每个桶里保存的都是一个单向链表，由Entry维护一个next引用。

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();
    }
}

方法

get

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;
}

put

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++;
}

rehash

jdk 1.8中HashMap对扩容做了优化，将原链表分为高低链表进行迁移，始终保证table的大小是偶数2的幂次方。
而在此HashTable中可以看出扩容后，仍然需要重新计算每个元素的hash值。rehash方法会把table大小翻倍并加1，始终保证table的大小是奇数。

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;
        }
    }
}

remove

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;
}

迭代和枚举

HashaTable的内部类Enumerator实现了迭代器Iterator和枚举Enumeration两个接口。

/**
 * 获取枚举对象
 */
private <T> Enumeration<T> getEnumeration(int type) {
    if (count == 0) {
        return Collections.emptyEnumeration();
    } else {
        return new Enumerator<>(type, false);
    }
}
/**
 * 获取迭代器
 */
private <T> Iterator<T> getIterator(int type) {
    if (count == 0) {
        return Collections.emptyIterator();
    } else {
        return new Enumerator<>(type, true);
    }
}