扣丁学堂Java培训之解析HashMap成员变量

2019-01-08 10:17:00 407浏览

今天扣丁学堂Java培训老师给大家分享一篇关于Java源码解析HashMap成员变量，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asknownas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<K,V>[] 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<Map.Entry<K,V>> 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;

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