HashMap在Java8中的紅黑樹

 紅黑樹在HashMap中的應用

我們知道在jdk1.8之前，HashMap採用鏈表的方式解決衝突，不過在更新1.8版本之後，

HashMap採用了鏈表加紅黑樹的方式來優化了結構。

話不多說，我們來看源碼：

/**
     * The bin count threshold for using a tree rather than list for a
     * bin.  Bins are converted to trees when adding an element to a
     * bin with at least this many nodes. The value must be greater
     * than 2 and should be at least 8 to mesh with assumptions in
     * tree removal about conversion back to plain bins upon
     * shrinkage.
     */
    static final int TREEIFY_THRESHOLD = 8;

結點數量達到8個將鏈表轉爲紅黑樹

/**
     * The bin count threshold for untreeifying a (split) bin during a
     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
     * most 6 to mesh with shrinkage detection under removal.
     */
    static final int UNTREEIFY_THRESHOLD = 6;

resize時，如果結點數量小於6個，還是用鏈表

最常用的put（）方法

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);   //put（）方法轉putVal（）
    }
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,           
                   boolean evict) {  //evict爲false，the table is in creation mod 
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)   //初次put時會擴容
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)     //如果tab[index]處沒有元素，則直接將這對（key，value）放到這
            tab[i] = newNode(hash, key, value, null);
        else {    //如果已經有元素存在，則考慮時鏈表結構還是樹結構
            Node<K,V> e; K k;  
            if (p.hash == hash &&   
                    ((k = p.key) == key || (key != null && key.equals(k))))     //如果放入的key以前時第一個位置，則更新
                e = p;
            else if (p instanceof TreeNode)    //如果不是的話，判斷p是不是紅黑樹結構
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);  //時紅黑樹就調用紅黑樹put方法
            else {   //是鏈表結構
                for (int binCount = 0; ; ++binCount) {  //遍歷鏈表
                    if ((e = p.next) == null) {    判斷鏈表數量是否大於等於TREEIFY_THRESHOLD-1，決定是否要將鏈表轉換成紅黑樹
                        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))))  //鏈表形式的判斷
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key  將新的值更新
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)    //onlyIfAbsent爲true，則不需要更新value值
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }

這裏出現了兩個以前沒有的參數，onlyIfAbsent和evict。

onlyIfAbsent：onlyIfAbsent if true, don't change existing value。

在oldValue不爲null時，如果onlyIfAbsent爲true則不更新value值。

當然這個值默認爲false，是會正常更新的。

evict：evict if false, the table is in creation mode.標記狀態，默認爲true

當鏈表長度大於等於8時，則會將鏈表轉換爲紅黑樹存儲

static final int TREEIFY_THRESHOLD = 8