一 特性

• key和value都不能为null，这个是不同于hashMap的，后者允许有一个键值对为null
• HashTable容器使用synchronized来保证读写的线程安全

• 内部初始数组容量是11，每次扩容为原来容量*2+1，负载则是8

  二 属性

  ```java

  // Hashtable是采用拉链法实现的，每一个Entry本质上是一个单向链表 private transient Entry[] table;

  // Hashtable中元素的实际数量 private transient int count;

  // 阈值，用于判断是否需要调整Hashtable的容量（threshold = 容量*加载因子） private int threshold;

  // 加载因子 private float loadFactor;

  // Hashtable被改变的次数 private transient int modCount = 0;

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# 三 重要方法
- rehash
   - 扩容方法
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# 四 构造方法
```java
   //使用给定的容量初始化，这里
    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);
    }
    //初始容量是11
    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);
    }

五 添加元素

//使用synchronized关键字保证线程安全    
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];
    //发生hash冲突 链表解决，
        for(; entry != null ; entry = entry.next) {
            //重复插入返回之前的值
            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];
        //这里构造函数采用头插法插入链表
        tab[index] = new Entry<>(hash, key, value, e);
        count++;
    }

六 删除元素

    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];
        //找到对应的位置 并进行链表删除操作
        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;
    }

七 获取元素

//也是线程安全的方法
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;
    }

八 扩容方法

    @SuppressWarnings("unchecked")
    protected void rehash() {
        int oldCapacity = table.length;
        Entry<?,?>[] oldMap = table;
        // overflow-conscious code
        //这里扩容大小为原始容量的两倍+1
        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++;
        //重新赋值伐值
        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;
                //重新计算位置
                int index = (e.hash & 0x7FFFFFFF) % newCapacity;
                e.next = (Entry<K,V>)newMap[index];
                newMap[index] = e;
            }
        }
    }