Java源码解析HashMap成员变量

本文基于jdk1.8进行分析

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

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.

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