//如何做到的呢？通過右移和或運算，最終n = xxx11111。n+1 = xx100000，2的n次方，即為數(shù)組大小
    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 + 1;
    }

	//1. int h = key.hashCode 得到key的hashCode.
    //2. int j = h>>>16 右移16位
    //3. int hash = h^j 異或，將hashCode變?yōu)閔ash值。
    //通過hash算法將hashCode轉換為hash值，注意hash值和hashCode不是一回事。
    //4.int index = (n - 1) & hash，n是數(shù)組的長度，計算得到的index即為數(shù)組的角標。
	//有興趣的朋友，可以寫幾行代碼進行驗證。
    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }
	static final int index(int hash,int n){
		return (n - 1) & hash;
	}

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        //如果數(shù)組是null 或者長度為0，則創(chuàng)建數(shù)組 resize()
        if ((tab = table) == null || (n = tab.length) == 0)
            //resize既是創(chuàng)建，也是擴容
            n = (tab = resize()).length;
        //取出索引為i的Node賦值給p，如果為null，說明這個位置沒有節(jié)點
        if ((p = tab[i = (n - 1) & hash]) == null)
            //創(chuàng)建Node，并放在角標為i的位置。這個node是一個單鏈表結構
            tab[i] = newNode(hash, key, value, null);
        else {//如果i的位置有節(jié)點，則添加到鏈表中
            Node<K,V> e; K k;
            //先判斷hash是否一致，再判斷key，如果一樣，則說明是同一個Key，
            //直接將p賦值給e，這也就是hashMap和HashSet的key不能重復的原因
            if (p.hash == hash &&
                    ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)//紅黑樹
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                //創(chuàng)建新的node添加的鏈表的末尾
                for (int binCount = 0; ; ++binCount) {
                    //將下一個節(jié)點賦值給e。如果e==null，說明遍歷到最后一個節(jié)點，
                    if ((e = p.next) == null) {
                        //創(chuàng)建新的節(jié)點，添加到鏈表末尾
                        p.next = newNode(hash, key, value, null);
                        //static final int TREEIFY_THRESHOLD = 8;
                        //當鏈表長度大于等于8時，
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);//轉換成紅黑樹
                        break;
                    }
                    //去重
                    if (e.hash == hash &&
                            ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    //將當前的e賦值給p
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold) //添加元素個數(shù)大于數(shù)組長度時，進行擴容
            resize();
        afterNodeInsertion(evict);
        return null;
    }

public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }
    final Node<K,V> getNode(int hash, Object key) {
        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
        // 數(shù)組不為空且長度大于0，對應角標的第一個node，first
        if ((tab = table) != null && (n = tab.length) > 0 &&
                (first = tab[(n - 1) & hash]) != null) {
            if (first.hash == hash && //如果和第一個是同一個直接返回
                    ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {//和鏈表第一個節(jié)點不一致，則進行遍歷
                if (first instanceof TreeNode)//紅黑樹
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                do {
                    if (e.hash == hash &&
                            ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);//遍歷鏈表取出和key一致的node
            }
        }
        return null;
    }

 final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {//數(shù)組長度大于0
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                    oldCap >= DEFAULT_INITIAL_CAPACITY){
                newThr = oldThr << 1; // 數(shù)組大于默認大小時， 擴容閾值是原來的2倍
            }
        } else if (oldThr > 0) //初始化時，threshold已被設置（調用有參構造函數(shù)時）
            newCap = oldThr; //將數(shù)組長度設置為threshold值。
        else { //如果數(shù)組和閾值都為0 (調用無參構造函數(shù))
            newCap = DEFAULT_INITIAL_CAPACITY; //默認數(shù)組大小，
            //擴容閾值為默認數(shù)組大小的0.75倍
            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;
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {//遍歷數(shù)組
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {//取出數(shù)組元素，也就是鏈表的第一個節(jié)點
                    oldTab[j] = null;
                    if (e.next == null)//鏈表只有首個節(jié)點
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)//紅黑樹
                        ((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;
    }
}