Java集合类之Hashtable

Hashtable低层的数据结构为数组和单链表，它使用除留余数法确定下标，使用拉链法解决哈希冲突，不允许任何记录的键或者值为null。

存储结构

• 数组

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

• 单链表

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;
    }
    ...
}

构造方法

Hashtable默认的初始容量为11，负载因子与HashMap相同，均为0.75。

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

插入操作

在Hashtable中，键和值都不能为null。否则，将抛出NullPointerException。

0x7FFFFFFF=2^31-1，即int类型的最大值（首位为符号位）。

当键的哈希码hash为正数时，hash & 0x7FFFFFFF = hash；

当键的哈希码hash为负数时，hash & 0x7FFFFFFF = -hash。

因此，hash & 0x7FFFFFFF的目的在于将hash转换为正数。

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();
    // 1.除留余数法——确定下标
    int index = (hash & 0x7FFFFFFF) % tab.length;
    // 2.拉链法——解决哈希冲突
    @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;
}

addEntry方法采用头插法，实现了Hashtable中的插入操作。

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++;
}

扩容操作

扩容后的大小为原来容量的2倍+1。

@SuppressWarnings("unchecked")
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;
        }
    }
}

按键取值

在按键取值操作中，若给定的key为null，将抛出NullPointerException。

@SuppressWarnings("unchecked")
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;
}

删除操作

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;
}