本文基于jdk1.8进行分析

首先看一下HashMap的一些静态常量。第一个是DEFAULT_INITIAL_CAPACITY，默认初始大小，16。从注释中可以了解到，大小必须为2的指数。这里的16，采用的1左移4位实现。而“aka”，是as known as的缩写。

/**

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

**/

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

接下来是最大容量，当通过任何一个构造函数的参数隐式指明时使用该值。必须是2的指数，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;

接下来是负载因子，默认值为0.75F。

/**

* The load factor used when none specified in constructor.

**/

static final float DEFAULT_LOAD_FACTOR = 0.75f;

接下来是和红黑树相关的几个常量。在jdk1.8中，如果哈希表中的链表太长，就会转化为一个红黑树。

TREEIFY_THRESHOLD，表示要转为红黑树的最小元素个数，即8。把红黑树转化为链表的门限个数是6. MIN_TREEIFY_CAPACITY为64，表示把链表转化为红黑树的最小元素个数。否则，如果太多节点在一个链表中时，哈希表会扩容，而不会转化为红黑树。

/**

* 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;

接下来是table，它是保存HashMap的最主要的数据结构，如下图。从注释中也可以了解到，table的大小一定是2的指数。

/**

* 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[] table;

接下来是entrySet，如下图。它保存缓存的映射关系集合。注意，keySet()和values()使用的是父类AbstractMap的属性。

/**

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

* for keySet() and values().

**/

transient Set> entrySet;

最后是一些其他的属性，包括HashMap中元素个数size，修改次数modCount，下一次进行resize的门限个数，以及负载因子loadFactor，如下图。需要注意的是，loadFactor是final的，也就是说，它一旦被赋值，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;

This is the end.

总结

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