HashMap 特性

1. key,value可以為空
2. 執行緒不安全
3. 支援快速失敗(在for迴圈中操作remove,put方法時會報ConcurrentModifyException)

HashMap 的幾個基本屬性值

1. loadFactor: 載入因子,loadFactor越大,hashmap的空間利用率越大,反之空間利用率越低.預設值為0.75.
2. threshold: 容量閾值,超過該值會觸發hashMap的擴容操作.
3. capacity: 容量值為2的n的次方值預設值為16
4. size: hashMap中包含的node個數

HashMap 的put過程

1. 計算index值 index=(n-1)&hash
2. 拿到index確定該位置下是否有元素存在,如果不存在則建立一個新的元素賦給該index下,方法結束.
3. 如果存在則判斷key與hash值與傳入的key,hash是否一致,如果一致則根據傳入的ifAbsent值是否修改value的值.方法結束
4. 如果key與hash不一致,則遍歷index對應的連結串列,如果連結串列中存在key和hash值與傳入的值一致則根據ifAsent值修改value值,如果不在這個的記錄,則新建一個新的記錄放在連結串列的最後
5. 如果連結串列的長度超過設定的值,這會將連結串列結構資料轉化為紅黑數結構
6. 如果size超過threshold值則觸發resize操作

 final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }
 

HashMap resize 過程

/**
     * Initializes or doubles table size.  If null, allocates in
     * accord with initial capacity target held in field threshold.
     * Otherwise, because we are using power-of-two expansion, the
     * elements from each bin must either stay at same index, or move
     * with a power of two offset in the new table.
     *
     * @return the table
     */
    final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        
        //計算出Threshold
        if (oldCap > 0) {
            // 如果老的容量大於等於最大容量則Threshold為整型最大值,並且直接返回hashTable,不進                                       行擴容
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            // 設定新容量為老容量的兩倍且小於最大容量值,並且老容量大於等於預設容量,則新Threshold為之前的兩倍
            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;
        // 新容量為預設值,threshold為容量的0.75
        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);
        }

        //設定threshold值
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
            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) {
                    // 將hashTable中j位置的值置空
                    oldTab[j] = null;
                    // 如果e的next為空,意味著該節點上只有一個元素,不是一條連結串列
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    // 如果e是一個TreeNode則走紅黑樹的流程
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    // 如果e的next不為空,意味這改節點上是一條連結串列結構
                    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;
                            // e.hash值得最高位為0無需變化
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            // e.hash值最高位為1,位置變為之前位置加上oldCap
                            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;
    }

問題1: 為什麼用(n-1)&hash來計算資料下標值?

這樣可以減少hash碰撞.因為n的值是2的n次方,所以除最高位是1外其他位都是0,在與操作的時候不參與運算(任何數與0都為0).所以用n&hash會產生許多相同的值.

問題2: 在多執行緒執行put方法情況下不會死迴圈,但會有資料丟失的情況?

在多執行緒執行的情況下,如果發生hash碰撞,會發生資料覆蓋,導致資料丟失.