關於HashMap的加載因子相關理解

HashMap在JDK1.7是以數組加鏈表的形式組成，JDK1.8后新增了紅黑樹結構，當鏈表大於8並且容量大於64時，鏈表結構會轉成紅黑樹結構。

JDK1.8 之所以會加入紅黑樹是因為當鏈表過長是會嚴重影響HashMap的性能，而紅黑樹具有快速增刪改查的特點。

HashMap源碼中包含了一下幾個常量的定義：

/**
     * The default initial capacity - MUST be a power of two.
     * 初始化長度
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 
/**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     * 最大長度
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     *  默認擴容加載因子
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * 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.
     * 當鏈表長度大於此值且容量大於64時
     */
    static final int TREEIFY_THRESHOLD = 8;

    /**
     * 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.
     * 轉換鏈表的臨界值，當元素小於此值時，會將紅黑樹結構轉換為鏈表結構
     */
    static final int UNTREEIFY_THRESHOLD = 6;

    /**
     * The smallest table capacity for which bins may be treeified.
     * (Otherwise the table is resized if too many nodes in a bin.)
     * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
     * between resizing and treeification thresholds.
     *  最小樹容量
     */
    static final int MIN_TREEIFY_CAPACITY = 64;

關於加載因子

加載因子也叫作擴容因子，用來判斷什么時候進行擴容，假設加載因子為0.75，HashMap的初始容量為16，當HashMap中有16 * 0.75 = 12個容量時，HashMap就會進行擴容。

如果加載因子越大，擴容發生的頻率就會比較低，占用空間比較小，但是發生hash沖突的幾率會提升，對元素操作時間會增加，運行效率降低；

如果加載因子太小，那么表中的數據將過於稀疏（很多空間還沒用，就開始擴容了），對空間造成嚴重浪費；

而且因為容量默認為2的次方，當加載因子為0.75時，容量和加載因子的乘積為整數。

所以系統默認加載因子取了0.5 -1 之間的0.75.