HashMap簡單小結

1.HashMap的基礎結構

在1.7中，HashMap 底層是基於 數組 + 鏈表的結構組成。在1.8中，如果鏈表的長度大於一定的值，鏈表會轉成紅黑樹。

2.HashMap的參數(1.8)

    /**
     * The default initial capacity - MUST be a power of two.
     */
    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.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     */
    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.
     */
    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.
     */
    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.
     */
    static final int MIN_TREEIFY_CAPACITY = 64;
...

    /**
     * The number of key-value mappings contained in this map.
     */
    transient int size;
    ...
        /**
     * The next size value at which to resize (capacity * load factor).
     *
     * @serial
     */
    // (The javadoc description is true upon serialization.
    // Additionally, if the table array has not been allocated, this
    // field holds the initial array capacity, or zero signifying
    // DEFAULT_INITIAL_CAPACITY.)
    int threshold;

DEFAULT_INITIAL_CAPACITY:16，數組的初始值，沒太多好解釋的。
MAXIMUM_CAPACITY：最大值，基本上不會達到這個值。
DEFAULT_LOAD_FACTOR：size / capacity(DEFAULT_INITIAL_CAPACITY)
threshold: capacity * load factor，超過這個閾值會rehash。

//一個桶的樹化閾值
//當桶中元素個數超過這個值時，需要使用紅黑樹節點替換鏈表節點
//這個值必須爲 8，要不然頻繁轉換效率也不高
static final int TREEIFY_THRESHOLD = 8;

//一個樹的鏈表還原閾值
//當擴容時，桶中元素個數小於這個值，就會把樹形的桶元素 還原（切分）爲鏈表結構
//這個值應該比上面那個小，至少爲 6，避免頻繁轉換
static final int UNTREEIFY_THRESHOLD = 6;

//哈希表的最小樹形化容量
//當哈希表中的容量大於這個值時，表中的桶才能進行樹形化
//否則桶內元素太多時會擴容，而不是樹形化
//爲了避免進行擴容、樹形化選擇的衝突，這個值不能小於 4 * TREEIFY_THRESHOLD
static final int MIN_TREEIFY_CAPACITY = 64;

3.HashMap中DEFAULT_LOAD_FACTOR爲什麼是0.75

如果該值設爲0.5，會在hashmap容量達到一半時候就擴容。比如從16擴大32，從32到64，64到128，浪費的空間會越來越大。
而如果該值設置爲1，則每次空間使用完畢纔會擴容，put時候操作耗時會增加。
所以0.75是時間與空間的一個平衡。

4.put時候操作

1.7插入元素到單鏈表中採用頭插入法，1.8採用的是尾插入法

5.HashMap死循環

HashMap在併發執行put操作時會引起死循環，是因爲多線程會導致HashMap的Entry鏈表形成環形數據結構。