//根据hash值及key值查找元素
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) {
            //第一个元素key值相同，直接返回第一个元素
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            //桶中有多个元素
            if ((e = first.next) != null) {
                //如果是TreeNode类型，在红黑树中查找
                if (first instanceof TreeNode)
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                do {
                    //链表中查找元素
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
}

//HashMap的扩容，每次都按2倍扩容
final Node<K,V>[] resize() {
        //table是一个hash桶数组，oldTab指向该桶
        Node<K,V>[] oldTab = table;
        //原始数组容量
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        //原始阈值
        int oldThr = threshold;
        int newCap, newThr = 0;
        //如果桶数组不为空
        if (oldCap > 0) {
            //如果超过最大容量2的30次方，已经无法扩容
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            //按2的幂次方扩容，新的数组容量和阈值都增大一倍
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold
        }
       //初始容量设置为阈值
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
           //如果桶数组为空，阈值也为0,初始化容量和阈值，一般为首次初始化并且没有指定初始容量的情况下，初始化容量为16
            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"})
            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        //以下进行真正的扩容操作
        if (oldTab != null) {
            //遍历原始数组
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                //如果桶数组对象不为空，先缓存到e，原始对象置为空，方便垃圾回收
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    //桶中只有一个元素，则计算扩容后索引位置，并放置到新数组中
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    //如果对象是TreeNode，加入到红黑树
                    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;
                        //遍历链表，放入到新桶数组中
                        do {
                            next = e.next;
                            //索引不变的情况
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            //索引变为'原索引+oldCap'情况
                            else {
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        //索引不变的情况
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        //索引变为'原索引+oldCap'情况
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }