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　　好程序員Java培訓分享HashMap成員變量解析，，首先看一下HashMap的一些靜態常量。diyi個是DEFAULT_INITIAL_CAPACITY，默認初始大小，16。從注釋中可以了解到，大小必須為2的指數。這里的16，采用的1左移4位實現。而“aka”，是asknownas的縮寫。

 /**

 * The default initial capacity - MUST be a power of two.

 **/

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

接下來是最大容量，當通過任何一個構造函數的參數隱式指明時使用該值。必須是2的指數，且小于等于1<<30，即2的30次方。

/**

 * 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;

接下來是負載因子，默認值為0.75F。

/**

 * The load factor used when none specified in constructor.

 **/

static final float DEFAULT_LOAD_FACTOR = 0.75f;

接下來是和紅黑樹相關的幾個常量。在jdk1.8中，如果哈希表中的鏈表太長，就會轉化為一個紅黑樹。

TREEIFY_THRESHOLD，表示要轉為紅黑樹的最小元素個數，即8。把紅黑樹轉化為鏈表的門限個數是6.MIN_TREEIFY_CAPACITY為64，表示把鏈表轉化為紅黑樹的最小元素個數。否則，如果太多節點在一個鏈表中時，哈希表會擴容，而不會轉化為紅黑樹。

/**

 * 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;

接下來是table，它是保存HashMap的最主要的數據結構，如下圖。從注釋中也可以了解到，table的大小一定是2的指數。

/**

 * The table, initialized on first use, and resized as

 * necessary. When allocated, length is always a power of two.

 * (We also tolerate length zero in some operations to allow

 * bootstrapping mechanics that are currently not needed.)

 **/

transient Node<K,V>[] table;

接下來是entrySet，如下圖。它保存緩存的映射關系集合。注意，keySet()和values()使用的是父類AbstractMap的屬性。

/**

 * Holds cached entrySet(). Note that AbstractMap fields are used

 * for keySet() and values().

 **/

transient Set<Map.Entry<K,V>> entrySet;

最后是一些其他的屬性，包括HashMap中元素個數size，修改次數modCount，下一次進行resize的門限個數，以及負載因子loadFactor，如下圖。需要注意的是，loadFactor是final的，也就是說，它一旦被賦值，就不能再修改了。

/**

 * The number of key-value mappings contained in this map.

 **/

transient int size;

/**

 * The number of times this HashMap has been structurally modified

 * Structural modifications are those that change the number of mappings in

 * the HashMap or otherwise modify its internal structure (e.g.,

 * rehash). This field is used to make iterators on Collection-views of

 * the HashMap fail-fast. (See ConcurrentModificationException).

 **/

transient int modCount;

/**

 * 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;

/**

 * The load factor for the hash table.

 *

 * @serial

 **/

final float loadFactor;

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