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

static final int hash(Object key) {
    int h;
    //為什么要右移16位？ 默認長度為2^5=16，與hash值&操作，容易獲得相同的值。
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

// 第四個參數(shù) onlyIfAbsent 如果是 true，那么只有在不存在該 key 時才會進行 put 操作
// 第五個參數(shù) evict 我們這里不關(guān)心
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)//初始判斷，初始數(shù)組為空時
        // resize()初始數(shù)組，需要進行擴容操作
        n = (tab = resize()).length;

    //這里就是上面的第三步，根據(jù)key的hash值找到數(shù)據(jù)在table中的位置
    if ((p = tab[i = (n - 1) & hash]) == null)
        //通過hash找到的數(shù)組下標，里面沒有內(nèi)容就直接賦值
        tab[i] = newNode(hash, key, value, null);
    else {//如果里面已經(jīng)有內(nèi)容了
        Node<K,V> e; K k;
        if (p.hash == hash &&
            //hash相同，key也相同，那就直接修改value值
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        else if (p instanceof TreeNode)
            //key不相同，且節(jié)點為紅黑樹，那就把節(jié)點放到紅黑樹里
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
        //表示節(jié)點是鏈表
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    //添加到鏈表尾部
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                 //如果滿足鏈表轉(zhuǎn)紅黑樹的條件，則轉(zhuǎn)紅黑樹
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        //傳入的K元素已經(jīng)存在，直接覆蓋value
        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)//檢查元素個數(shù)是否大于閾值，大于就擴容
        resize();
    afterNodeInsertion(evict);
    return null;
}

final void treeifyBin(Node<K,V>[] tab, int hash) {
    int n, index; Node<K,V> e;
    //檢查是否滿足轉(zhuǎn)換成紅黑樹的條件，如果數(shù)組大小還小于64，則先擴容
    if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
        resize();
    else if ((e = tab[index = (n - 1) & hash]) != null) {
        TreeNode<K,V> hd = null, tl = null;
     do {
           TreeNode<K,V> p = replacementTreeNode(e, null);
           if (tl == null)
                hd = p;
            else {
                p.prev = tl;
                tl.next = p;
            }
            tl = p;
       } while ((e = e.next) != null);
       if ((tab[index] = hd) != null)
           hd.treeify(tab);
    }
}

//table數(shù)組的擴容操作
final Node<K,V>[] resize() {
    //引用擴容前的node數(shù)組
    Node<K,V>[] oldTab = table;
    //舊的容量
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    //舊的閾值
    int oldThr = threshold;
    //新的容量、閾值初始化為0
    int newCap, newThr = 0;
    
    //計算新容量
    if (oldCap > 0) {
        //如果舊容量已經(jīng)超過最大容量，讓閾值也等于最大容量，以后不再擴容
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        //沒超過最大值，就令newcap為原來容量的兩倍
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            //如果舊容量翻倍沒有超過最大值，且舊容量不小于初始化容量16，則翻倍
            newThr = oldThr << 1; // double threshold
    }
    else if (oldThr > 0) // initial capacity was placed in threshold
        //舊容量oldCap = 0時，但是舊的閾值大于0，令初始化容量設(shè)置為閾值
        newCap = oldThr;
    else {               // zero initial threshold signifies using defaults
        //兩個值都為0的時候使用默認值初始化
        newCap = DEFAULT_INITIAL_CAPACITY;
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    
    
    if (newThr == 0) {
        //計算新閾值，如果新容量或新閾值大于等于最大容量，則直接使用最大值作為閾值，不再擴容
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                  (int)ft : Integer.MAX_VALUE);
    }
    //設(shè)置新閾值
    threshold = newThr;
    
    //創(chuàng)建新的數(shù)組，并引用
    @SuppressWarnings({"rawtypes","unchecked"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    
    //如果老的數(shù)組有數(shù)據(jù)，也就是是擴容而不是初始化，才執(zhí)行下面的代碼，否則初始化的到這里就可以結(jié)束了
    if (oldTab != null) {
        //輪詢老數(shù)組所有數(shù)據(jù)
        for (int j = 0; j < oldCap; ++j) {
            //以一個新的節(jié)點引用當(dāng)前節(jié)點，然后釋放原來的節(jié)點的引用
            Node<K,V> e;
            if ((e = oldTab[j]) != null) {//如果這個桶，不為空，說明桶中有數(shù)據(jù)
                oldTab[j] = null;
                //如果e沒有next節(jié)點，證明這個節(jié)點上沒有hash沖突，則直接把e的引用給到新的數(shù)組位置上
                if (e.next == null)
                    //確定元素在新的數(shù)組里的位置
                    newTab[e.hash & (newCap - 1)] = e;
                //如果是紅黑樹，則進行分裂
                else if (e instanceof TreeNode)
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                //說明是鏈表
                else { // preserve order
                    Node<K,V> loHead = null, loTail = null;
                    Node<K,V> hiHead = null, hiTail = null;
                    Node<K,V> next;
                    //從這條鏈表上第一個元素開始輪詢，如果當(dāng)前元素新增的bit是0，則放在當(dāng)前這條鏈表上
                    //如果是1，則放在"j+oldcap"這個位置上，生成“低位”和“高位”兩個鏈表
                    do {
                        next = e.next;
                        if ((e.hash & oldCap) == 0) {
                            if (loTail == null)
                                loHead = e;
                            else
                                //元素是不斷的加到尾部的
                                loTail.next = e;
                            //新增的元素永遠是尾元素
                            loTail = e;
                        }
                        else {
                            //高位的鏈表與低位的鏈表處理邏輯一樣，不斷的把元素加到鏈表尾部
                            if (hiTail == null)
                                hiHead = e;
                            else
                                hiTail.next = e;
                            hiTail = e;
                        }
                    } while ((e = next) != null);
                    //低位鏈表放到j(luò)這個索引的位置上
                    if (loTail != null) {
                        loTail.next = null;
                        newTab[j] = loHead;
                    }
                    //高位鏈表放到(j+oldCap)這個索引的位置上
                    if (hiTail != null) {
                        hiTail.next = null;
                        newTab[j + oldCap] = hiHead;
                    }
                }
            }
        }
    }
    return newTab;
}

//初始化容量
public HashMap(int initialCapacity) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

public HashMap(int initialCapacity, float loadFactor) {
    //保證initialCapacity在合理范圍內(nèi)，大于0小于最大容量
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity);
    if (initialCapacity > MAXIMUM_CAPACITY)
          initialCapacity = MAXIMUM_CAPACITY;
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
           throw new IllegalArgumentException("Illegal load factor: " + loadFactor);
    this.loadFactor = loadFactor;
    this.threshold = tableSizeFor(initialCapacity);
}

static final int tableSizeFor(int cap) {
    int n = cap - 1;
    //|=計算方式：兩個二進制對應(yīng)位都為0時,結(jié)果等于0,否則結(jié)果等于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;
}

/**
 * 紅黑樹分裂方法
 */
final void split(HashMap<K,V> map, Node<K,V>[] tab, int index, int bit) {
     //當(dāng)前這個節(jié)點的引用，即這個索引上的樹的根節(jié)點
     TreeNode<K,V> b = this;
     // Relink into lo and hi lists, preserving order
     TreeNode<K,V> loHead = null, loTail = null;
     TreeNode<K,V> hiHead = null, hiTail = null;
     //高位低位的初始樹節(jié)點個數(shù)都設(shè)成0
     int lc = 0, hc = 0;
     for (TreeNode<K,V> e = b, next; e != null; e = next) {
            next = (TreeNode<K,V>)e.next;
           e.next = null;
           //bit=oldcap,這里判斷新bit位是0還是1，如果是0就放在低位樹上，如果是1就放在高位樹上，這里先是一個雙向鏈表
           if ((e.hash & bit) == 0) {
               if ((e.prev = loTail) == null)
                    loHead = e;
               else
                    loTail.next = e;
               loTail = e;
               ++lc;
            }
            else {
                if ((e.prev = hiTail) == null)
                    hiHead = e;
                else
                    hiTail.next = e;
                hiTail = e;
                ++hc;
            }
       }
 
       if (loHead != null) {
            if (lc <= UNTREEIFY_THRESHOLD)
                //！！！如果低位的鏈表長度小于閾值6，則把樹變成鏈表，并放到新數(shù)組中j索引位置
                tab[index] = loHead.untreeify(map);
            else {
                tab[index] = loHead;
                //高位不為空，進行紅黑樹轉(zhuǎn)換
                if (hiHead != null) // (else is already treeified)
                    loHead.treeify(tab);
            }
        }
        if (hiHead != null) {
            if (hc <= UNTREEIFY_THRESHOLD)
                tab[index + bit] = hiHead.untreeify(map);
            else {
                tab[index + bit] = hiHead;
                if (loHead != null)
                    hiHead.treeify(tab);
            }
        }
}

/**
 * 將樹轉(zhuǎn)變?yōu)閱蜗蜴湵? */
final Node<K,V> untreeify(HashMap<K,V> map) {
     Node<K,V> hd = null, tl = null;
     for (Node<K,V> q = this; q != null; q = q.next) {
          Node<K,V> p = map.replacementNode(q, null);
          if (tl == null)
                hd = p;
          else
               tl.next = p;
          tl = p;
      }
     return hd;
}

/**
 * 鏈表轉(zhuǎn)換為紅黑樹，會根據(jù)紅黑樹特性進行顏色轉(zhuǎn)換、左旋、右旋等
 */
final void treeify(Node<K,V>[] tab) {
      TreeNode<K,V> root = null;
      for (TreeNode<K,V> x = this, next; x != null; x = next) {
             next = (TreeNode<K,V>)x.next;
           x.left = x.right = null;
           if (root == null) {
                x.parent = null;
                x.red = false;
                root = x;
           }
           else {
                K k = x.key;
                int h = x.hash;
                Class<?> kc = null;
                for (TreeNode<K,V> p = root;;) {
                    int dir, ph;
                    K pk = p.key;
                    if ((ph = p.hash) > h)
                         dir = -1;
                    else if (ph < h)
                         dir = 1;
                    else if ((kc == null &&
                              (kc = comparableClassFor(k)) == null) ||
                             (dir = compareComparables(kc, k, pk)) == 0)
                          dir = tieBreakOrder(k, pk);
 
                    TreeNode<K,V> xp = p;
                    if ((p = (dir <= 0) ? p.left : p.right) == null) {
                        x.parent = xp;
                        if (dir <= 0)
                            xp.left = x;
                        else
                            xp.right = x;
                        //進行左旋、右旋調(diào)整
                        root = balanceInsertion(root, x);
                        break;
                    }
                 }
            }
        }
        moveRootToFront(tab, root);
}

int threshold;             // 容納鍵值對的最大值
final float loadFactor;    // 負載因子
int modCount;  
int size;