public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}
//计算hash
static final int hash(Object key) {
    int h;
    //用key的hashCode值右移16位，与hashCode异或，原值不大于2的16次方的话，相当于按位取反
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    if ((tab = table) == null || (n = tab.length) == 0)
        // table是map的内置数组，若为空就要初始化
        n = (tab = resize()).length;
    if ((p = tab[i = (n - 1) & hash]) == null)
        //通过n-1&hash计算该hash值在tab中的下标hash%(n-1)，若该位置数组元素为空，直接放入
        tab[i] = newNode(hash, key, value, null);
    else {
        //若原位置不为空，则分3种情况
        Node<K,V> e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            //p.key与key相同，则把p的引用赋给e
            e = p;
        else if (p instanceof TreeNode)
            //如果p是个树节点，遍历树，如果存在键为key的Node就返回，不存在就挂树上
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            //把p视为链表，遍历之，大致同上
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        //链表长度>=8(可以改jvm参数)时，把链表重构成红黑树
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                //如果原值为空或 onlyIfAbsent=false 则替换成新值
                e.value = value;
             //后处理，LinkedHashMap用来把node移至末尾
            afterNodeAccess(e);
            //返回原值
            return oldValue;
        }
    }
    //如果是新插入节点
    ++modCount;
    //size超过阈值
    if (++size > threshold)
        resize();
    //节点插入后处理
    afterNodeInsertion(evict);
    return null;
}

//返回node数组
final Node<K,V>[] resize() {
    获取原数组大小
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    //原阈值
    int oldThr = threshold;
    int newCap, newThr = 0;
    if (oldCap > 0) {
        if (oldCap >= MAXIMUM_CAPACITY) {//已经是最大值，只能扩容阈值
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        else if (//翻倍(newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
                 //初始容量->最大容量/2之间，将容量&阈值翻倍
            newThr = oldThr << 1; // double threshold
    }
    else if (oldThr > 0) // 原容量<=0,阈值>0时，将阈值作为新容量 initial capacity was placed in threshold
        newCap = oldThr;
    else {               // 初始化 zero initial threshold signifies using defaults
        newCap = DEFAULT_INITIAL_CAPACITY;
        //默认初始化，阈值=0.75*容量
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    //根据新容量计算新阈值
    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"})
    //创建新的Node数组
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    if (oldTab != null) {
        //分三种情况，将旧数组中的数据按hash装到新数组中
        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)
                    //将原树上的节点按(hash&oldCap)==0划分到两棵新树上，若新树size<8,转成链表
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                else { // preserve order
                    //将原链表拆成2个新链表
                    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;
}