HashMap詳解(二). resize方法詳解

/**
 * java8 hashMap resize
 */
final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length; //原來的容量,未初始化過則爲0
        int oldThr = threshold;                            //threshold,既 容量*加載因子
        int newCap, newThr = 0;
        if (oldCap > 0) {                                  //原來的容量是否大於0,既map裏面是否有元素
            if (oldCap >= MAXIMUM_CAPACITY) {              //若原來容量已經大於最大容量值，則只改變 threshold的大小，改成整型的最大最大值爲Integer.MAX_VALUE既2^31
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&  //newCap = oldCap << 1 擴容爲原來的兩倍
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold 非初始化時，threshol翻倍
        }
        else if (oldThr > 0) // 有初始化容量時,指定初始化容量,不然默認容量16
            newCap = oldThr;
        else {              
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        if (newThr == 0) {                                        //初始化的彷佛很，這裏會計算出新的threshold的值,最大值爲Integer.MAX_VALUE既2^31
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];  //分配table新的內存
        table = newTab;
        //將舊的table中的元素轉移到新的table中
        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;
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        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;
    }

resize的基本思路:

1.第一次put，初始化分配內存, threshold初始化.

2.不是初始化時,判斷如今容量,小於最大值時,直接double,不然tableSize爲 1<<30

3.分配新的內存空間

4.將舊的table轉移到新的table,這裏會有線程安全問題.多個線程同時進行resize的時候，在這裏會致使死循環產生。具體可見：http://www.cnblogs.com/FirstClass/p/6501290.html

經過分析這裏的代碼,不難發現,hashMap的table.size爲 1<<30時，將再也不擴容。