HashMap_resize

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) {
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            } // 1.1 容量大于等于1<<30,则修改阈值为Integer.MAX_VALUE
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY){
                newThr = oldThr << 1; // double threshold
            }  //  1.2 容量扩大一倍后 小于 1<<30 && 旧的容量大于等于16，把阈值扩大一倍

        } // 1 旧数组的容量大于0  说明数组已经初始化
        else if (oldThr > 0){
            newCap = oldThr;
        } // 2  数组没有初始化且阈值大于0  说明没用空参构造      initial capacity was placed in threshold
        else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY; // 16
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);  // 0.75 * 16
        } // 3  数组没有初始化并且阈值等于0 说明第一次初始化数组用了空参构造
        if (newThr == 0) {
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        } // 4 新的阈值为空
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];  // 创建node数组
        table = newTab; // hashMap的使用场景是单线程场景，所以不用考虑在put的时候，get会怎么样
        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;
                    }  // 5.1.1.1    当前槽位上只有一个节点，直接迁移到新的数组
                    else if (e instanceof TreeNode){
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    }  // 5.1.1.2  当前槽位上是红黑树，把当前树分成两棵树，前一棵在当前槽位，后一棵在原来槽位加上原来的容量
                    else {
                        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;
                        }
                    } // 5.1.1.3 当前槽位上是链表     把链表分成两段，前一段原来槽位，后一段在原来槽位加上原来的容量
                }   // 5.1.1  当前槽位不为空
            } // 5.1 遍历旧数组
        } // 5 迁移数据
        return newTab;
    }