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。
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<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;
this is the end.
总结
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