HashMap數據結構分析(jdk8)

看了下HashMap的源碼，做下記錄，首先還是先從流程圖開始

下面用代碼分析下方法

    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        //未初始化
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        //該索引處節點無值
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        //該節點處有值
        else {
            Node<K,V> e; K k;
            //和該處節點的 hash 相同並且 key相同
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            //是紅黑樹節點
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            //是鏈表
            else {
                for (int binCount = 0; ; ++binCount) {
                    //尾部節點
                    if ((e = p.next) == null) {
                        //添加到尾部
                        p.next = newNode(hash, key, value, null);
                        //大於等於7 轉為樹節點
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    //如果鏈表不是尾部節點，並且是否遇到了key和hash都相同的
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            // 找到了 key和value都相同的節點
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                // value不為null 並且 onlyIfAbsent為false 就重新賦下值
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                // 一個待實現的方法，hashMap無用
                afterNodeAccess(e);
                // 結束
                return oldValue;
            }
        }
        //結構更改的計數，為了在迭代中快速判斷是否被修改了 而拋出異常
        ++modCount;
        // 長度++，擴容閾值
        if (++size > threshold)
            resize();
        //一個待實現的方法，hashMap無用
        afterNodeInsertion(evict);
        return null;
    }


    //初始化和擴容
    final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        //舊容量
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        //舊閾值
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
            //舊容量已經是最大值了，就不擴容了，並且把閾值也調到最大
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            //新容量為舊容量*2 並且小於最大值，且舊容量大於最小的初始化容量
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                //新閾值也為舊閾值*2
                newThr = oldThr << 1; 
        }
        //舊閾值不為0，新的容量就是舊的閾值
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        //兩個值都是0，此時是初始化代碼
        else {            
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        //新閾值為0時 重新計算
        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"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        //數據從舊table復制到新table
        if (oldTab != null) {
            //從低到高復制
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    //單純一個Node對象結構，直接復制
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    //紅黑樹復制
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    //鏈表復制
                    else { // preserve order
                        //把原始的鏈表拆成兩個，判斷標准就是 hash& oldCap ==0 
                        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);
                        //拆完之后，新table的j位置放lohead
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        //拆完后，新table的j+oldCap的位置放hihead
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }