Java源码解析HashMap成员变量

网友投稿 277 2023-01-15


Java源码解析HashMap成员变量

本文基于jdk1.8进行分析

关于HashMap的简介,可以参考这篇文章https://jb51.net/article/154177.htm。

首先看一下HashMap的一些静态常量。第一个是DEFAULT_MfKcVxbINITIAL_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 uhttp://ntreeifying 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.

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