Map<String, String> map = new HashMap<>(3);
map.put("id", "1");
map.put("name", "riemann");
map.put("sex", "male");

public HashMap(int initialCapacity, float loadFactor) {
 // 初始化大小小于0，拋出異常
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal initial capacity: " +
                                           initialCapacity);
    // 初始大小最大為默認(rèn)最大值
    if (initialCapacity > MAXIMUM_CAPACITY)
        initialCapacity = MAXIMUM_CAPACITY;
    // 加載因子要在0到1之間
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal load factor: " +
                                           loadFactor);
    this.loadFactor = loadFactor;
    // threshold是根據(jù)當(dāng)前的初始化大小和加載因子算出來的邊界大小，
    // 當(dāng)桶中的鍵值對(duì)超過這個(gè)大小就進(jìn)行擴(kuò)容
    this.threshold = tableSizeFor(initialCapacity);
}

// 這個(gè)方法返回大于輸入?yún)?shù)且最接近的2的整數(shù)次冪的數(shù)
static final int tableSizeFor(int cap) {
    int n = cap - 1;
    // 無符號(hào)向右移動(dòng) 
    // 按位或
    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;
}

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    // 如果存儲(chǔ)元素的table為空，則進(jìn)行必要字段的初始化
    if ((tab = table) == null || (n = tab.length) == 0)
    	// 獲取長(zhǎng)度
        n = (tab = resize()).length;
    // 如果根據(jù)hash值獲取的結(jié)點(diǎn)為空，則新建一個(gè)結(jié)點(diǎn)
    // 此處 & 代替了 % （除法散列法進(jìn)行散列）
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    // 這里的p結(jié)點(diǎn)是根據(jù)hash值算出來對(duì)應(yīng)在數(shù)組中的元素
    else {
        Node<K,V> e; K k;
        // 如果新插入的結(jié)點(diǎn)和table中p結(jié)點(diǎn)的hash值，key值相同的話
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        // 如果是紅黑樹結(jié)點(diǎn)的話，進(jìn)行紅黑樹插入
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            for (int binCount = 0; ; ++binCount) {
            	// 代表這個(gè)單鏈表只有一個(gè)頭部結(jié)點(diǎn)，則直接新建一個(gè)結(jié)點(diǎn)即可
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    // 鏈表長(zhǎng)度大于8時(shí)，將鏈表轉(zhuǎn)紅黑樹
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                // 及時(shí)更新p
                p = e;
            }
        }
        // 如果存在這個(gè)映射就覆蓋
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            // 判斷是否允許覆蓋，并且value是否為空
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            // 回調(diào)以允許LinkedHashMap后置操作
            afterNodeAccess(e);
            return oldValue;
        }
    }
    // 更改操作次數(shù)
    ++modCount;
    // 大于臨界值
    if (++size > threshold)
    	// 將數(shù)組大小設(shè)置為原來的2倍，并將原先的數(shù)組中的元素放到新數(shù)組中
        // 因?yàn)橛墟湵恚t黑樹之類，因此還要調(diào)整他們
        resize();
    // 回調(diào)以允許LinkedHashMap后置操作
    afterNodeInsertion(evict);
    return null;
}

// 初始化或者擴(kuò)容之后元素調(diào)整
final Node<K,V>[] resize() {
	// 獲取舊元素?cái)?shù)組的各種信息
    Node<K,V>[] oldTab = table;
    // 長(zhǎng)度
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    // 擴(kuò)容的臨界值
    int oldThr = threshold;
    // 定義新數(shù)組的長(zhǎng)度及擴(kuò)容的臨界值
    int newCap, newThr = 0;
    // 如果原table不為空
    if (oldCap > 0) {
    	// 如果數(shù)組長(zhǎng)度達(dá)到最大值，則修改臨界值為Integer.MAX_VALUE
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        // 下面就是擴(kuò)容操作（2倍）
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            // threshold也變?yōu)槎?
            newThr = oldThr << 1; // double threshold
    }
    else if (oldThr > 0) // initial capacity was placed in threshold
        newCap = oldThr;
    // threshold為0，則使用默認(rèn)值
    else {               // zero initial threshold signifies using defaults
        newCap = DEFAULT_INITIAL_CAPACITY;
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    // 如果臨界值還為0，則設(shè)置臨界值
    if (newThr == 0) {
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                  (int)ft : Integer.MAX_VALUE);
    }
    // 更新填充因子
    threshold = newThr;
    @SuppressWarnings({"rawtypes","unchecked"})
    Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    // 調(diào)整數(shù)組大小之后，需要調(diào)整紅黑樹或者鏈表的指向
    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;
                // 紅黑樹調(diào)整
                else if (e instanceof TreeNode)
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                else { // preserve order
                	// 鏈表調(diào)整
                    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;
}

oldCap = oldTab.length = 3
newcap = oldCap << 1 = 6
threshold = newThr = newCap * loadFactor = 6 * 0.75 = 4