private void grow(int minCapacity)
            {
            // oldCapacity为旧容量，newCapacity为新容量
            int oldCapacity = elementData.length;
            int newCapacity = oldCapacity + (oldCapacity >> 1);
            // 将oldCapacity 右移一位，其效果相当于oldCapacity 除2
            // 位运算快于整除运算
            if (newCapacity - minCapacity < 0)
                newCapacity = minCapacity;
            // 检查扩容后容量是否满足最小需要容量
            if (newCapacity - MAX_ARRAY_SIZE > 0)
                newCapacity = hugeCapacity(minCapacity);
            // 检查扩容后容量是否大于规定的数组最大容量
            // 如果超过了使用hugeCapacity()方法比较minCapacity和 MAX_ARRAY_SIZE
            // 如果minCapacity大于MAX_ARRAY_SIZE，则新容量则为Integer.MAX_VALUE
            // 否则，新容量大小则为 MAX_ARRAY_SIZE。
            elementData = Arrays.copyOf(elementData, newCapacity);
        }

  

    /**
    *   复制数组
    * @param src 源数组
    * @param srcPos 源数组中的起始位置
    * @param dest 目标数组
    * @param destPos 目标数组中的起始位置
    * @param length 要复制的数组元素的数量
    */
    public static native void arraycopy(Object src,  int  srcPos,
                                        Object dest, int destPos,
                                        int length);

  

    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

    /**
     * Implements Map.put and related methods.
     *
     * @param hash key的hash值
     * @param key the key
     * @param value the value to put
     * @param onlyIfAbsent 如果为真，只有在不存在key时才会put
     * @param evict if false, the table is in creation mode.
     * @return previous value, or null if none
     */
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        //第一次put会使用resize方法检查是否需要扩容
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        //给节点赋值
        //如果没有节点，就创建一个新的节点
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        //eles如果这个节点已经有数据了
        else {
            Node<K,V> e; K k;
            // 首先，判断该位置的第一个数据和我们要插入的数据，key 是不是"相等"
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                //如果是，取出这个节点
                e = p;
            // 如果该节点是代表红黑树的节点，调用红黑树的插值方法
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            //else数组该位置不是红黑树节点，而是链表
            else {
                for (int binCount = 0; ; ++binCount) {
                    //插入到链表后面
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        //TREEIFY_THRESHOLD = 8
                        //如果新插入的值时链表中的第8个
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            //treeifyBin()方法会将链表转化为红黑树
                            treeifyBin(tab, hash);
                        break;
                    }
                    //如果在链表中找到了相等的key
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        //break 此时e为链表中与要插入新值的key相等的node
                        break;
                    p = e;
                }
            }
            //e != null 说明旧值的key与插入的key相等
            //值覆盖，返回旧值
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        //如果插入后size超过阈值，就对其扩容
        //从整个方法来看，先插入再考虑扩容
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

  

    /**
     * Initializes or doubles table size.  If null, allocates in
     * accord with initial capacity target held in field threshold.
     * Otherwise, because we are using power-of-two expansion, the
     * elements from each bin must either stay at same index, or move
     * with a power of two offset in the new table.
     *
     * @return the table
     */
    final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        //扩容
        if (oldCap > 0) {
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            //数组扩大一倍
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                //阈值扩大一倍
                newThr = oldThr << 1; // double threshold
        }
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        if (newThr == 0) {
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
        //用新的数组大小初始化新的数组
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        //初始化数组结束
        table = newTab;
        
        //数据迁移
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        do {
                            next = e.next;
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            else {
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

  

    /**
     * Returns a power of two size for the given target capacity.
     */
    static final int tableSizeFor(int cap) {
        int n = cap - 1;
        n |= n >>> 1;
        n |= n >>> 2;
        n |= n >>> 4;
        n |= n >>> 8;
        n |= n >>> 16;
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }

  

    public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

    /**
     * Implements Map.get and related methods.
     *
     * @param hash hash for key
     * @param key the key
     * @return the node, or null if none
     */
    final Node<K,V> getNode(int hash, Object key) {
        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            //判断第一个节点是不是要找的
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
                //判断是否为红黑树
                if (first instanceof TreeNode)
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                do {
                    //遍历链表
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }