Java中的HashTable

HashTable是基于hash表实现的存储key-value数据的集合，它的所有方法都添加了同步锁，是线程安全的

特点

1. 元素是无序的：元素存放位置是根据key值和数组长度运算的来的，无法确定顺序
2. 元素的key不能重复，value可以重复
3. 元素的key和value都不能为空：否则会抛出异常
4. hash冲突：如果出现hash冲突，会在计算出的索引位形成链表存储
5. 数组动态扩容：数组默认初始化容量为11，扩容比例因子是0.75。当数组元素个数达到容量的75%时会扩容，扩容为原始数组的两倍+1
6. 链表不会转树：可能会出现链表过长的情况，如果数组没有达到扩容条件，链表的长度一直增加，会导致链表过长
7. 线程安全：每个方法都加了同步锁，支持多线程访问

方法

构造方法

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

    //存储元素的数组
    private transient Entry<?,?>[] table;

    //实际元素个数
    private transient int count;

    //扩容阈值(临界值)
    private int threshold;

    //比例因子   
    private float loadFactor;

    //修改次数(用于fail-fast判断)
    private transient int modCount = 0;

    
    private static final long serialVersionUID = 1421746759512286392L;

   //创建一个Hashtable，手动设置初始化容量和比例因子
    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);
    }

    //创建一个Hashtable，手动设置初始化容量
    public Hashtable(int initialCapacity) {
        this(initialCapacity, 0.75f);
    }

    //创建一个Hashtable，默认初始化容量和比例因子
    public Hashtable() {
        this(11, 0.75f);
    }

    //创建一个Hashtable，将传入集合元素复制到新数组
    public Hashtable(Map<? extends K, ? extends V> t) {
        this(Math.max(2*t.size(), 11), 0.75f);
        putAll(t);
    }
}

foreach

    public synchronized void forEach(BiConsumer<? super K, ? super V> action) {
        Objects.requireNonNull(action);     
                                            
        final int expectedModCount = modCount;

        Entry<?, ?>[] tab = table;
        //底层使用迭代器获取容器中的节点，调用传入函数操作
        for (Entry<?, ?> entry : tab) {
            while (entry != null) {
                action.accept((K)entry.key, (V)entry.value);
                entry = entry.next;
                //如果循环时容器内节点被修改，直接抛出并发异常
                if (expectedModCount != modCount) {
                    throw new ConcurrentModificationException();
                }
            }
        }
    }

获取元素

     //根据key获取value值  
     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;
    }

添加元素

    //添加元素，如果key已经存在，则直接更新value值
    //key不存在，则添加到hashtbale中
    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;
    }

    //添加元素，如果达到阈值，则扩容，扩容后重新计算节点的索引位
    private void addEntry(int hash, K key, V value, int index) {
        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++;
        modCount++;
    }

扩容

    //扩容：按照旧容量的2倍加1扩容成新数组的长度，重新计算临界值
    //将旧数组中的数据复制到新数组
    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;
            }
        }
    }

删除

    //遍历查找key，删除节点
    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)) {
                //不是在链表头部：将查找到的节点从链中解除绑定，将e的前一个节点指向e的后一个节点
                if (prev != null) {
                    prev.next = e.next;
                } 
                //如果是链表头部或只有索引位有值，直接将e.next放入索引位
                else {
                    tab[index] = e.next;
                }
                modCount++;
                count--;
                V oldValue = e.value;
                e.value = null;
                return oldValue;
            }
        }
        return null;
    }