JDK1.8_HashMap源碼之resize

    final Node<K,V>[] resize() {
        //創建一個Node數組用於存放table中的元素，
        Node<K,V>[] oldTab = table;
        //獲取舊table的長度
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        //獲取舊的擴容閾值
        int oldThr = threshold;
        int newCap, newThr = 0;
        //如果舊的table中有元素
        if (oldCap > 0) {
            //如果舊table長度>=最大容量限制時不進行擴容，並將擴容閾值賦值為Integer.MAX_VALUE
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            // 將新table長度賦值為舊table的2倍，
            // 判斷舊table長度的二倍是否小於最大容量，且舊容量大於等於初始容量，
            // 以上判斷成立則將新的擴容閥值賦值為舊的擴容閾值的二倍
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold
        }
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            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"})
        //將舊table中的元素放到擴容后的newTable中
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    //如果數組對應下標位置只有一個元素，對hashCade取余並根據結果直接放到newTable相應的位置
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                        //如果數組對應下標位置的元素是一個紅黑樹,則拆分紅黑樹放到newTable中
                        // 如果拆分后的紅黑樹元素小於6，則轉化為鏈表
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                    //數組對應下標位置的元素是一個鏈表的情況
                    //根據(e.hash & oldCap)條件對鏈表進行拆分並放到newTable
                        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;
    }