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扣丁學堂Java培訓之解析HashMap成員變量

2019-01-08 10:17:00 1236瀏覽

今天扣丁學堂Java培訓老師給大家分享一篇關于Java源碼解析HashMap成員變量，首先看一下HashMap的一些靜態(tài)常量。第一個是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(shù)。這里的16，采用的1左移4位實現(xiàn)。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(shù)的參數(shù)隱式指明時使用該值。必須是2的指數(shù)，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(shù)，即8。把紅黑樹轉化為鏈表的門限個數(shù)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(shù)。否則，如果太多節(jié)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(shù)據(jù)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(shù)。

/**
 * 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(shù)size，修改次數(shù)modCount，下一次進行resize的門限個數(shù)，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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