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final Node<K,V>[] resize() {
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    Node<K,V>[] oldTab = table;
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    int oldCap = (oldTab == null) ? 0 : oldTab.length;
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    int oldThr = threshold;
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    int newCap, newThr = 0;
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    if (oldCap > 0) {
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        // 超过最大值就不再扩充了，就只好随你碰撞去吧
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        if (oldCap >= MAXIMUM_CAPACITY) {
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            threshold = Integer.MAX_VALUE;
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            return oldTab;
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        }
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        // 没超过最大值，就扩充为原来的2倍
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        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
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                 oldCap >= DEFAULT_INITIAL_CAPACITY)
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            newThr = oldThr << 1; // double threshold
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    }
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    else if (oldThr > 0) // initial capacity was placed in threshold
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        newCap = oldThr;
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    else {               // zero initial threshold signifies using defaults
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        newCap = DEFAULT_INITIAL_CAPACITY;
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        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
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    }
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    // 计算新的resize上限
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    if (newThr == 0) {
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        float ft = (float)newCap * loadFactor;
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        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
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                  (int)ft : Integer.MAX_VALUE);
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    }
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    threshold = newThr;
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    @SuppressWarnings({"rawtypes","unchecked"})
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        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
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    table = newTab;
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}