HashMap简单小结

项目github地址：bitcarmanlee easy-algorithm-interview-and-practice
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1.HashMap的基础结构

在1.7中，HashMap 底层是基于数组 + 链表的结构组成。在1.8中，如果链表的长度大于一定的值，链表会转成红黑树。

2.HashMap的参数(1.8)

    /*** The default initial capacity - MUST be a power of two.*/static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16/*** 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;/*** The load factor used when none specified in constructor.*/static final float DEFAULT_LOAD_FACTOR = 0.75f;/*** 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;
.../*** The number of key-value mappings contained in this map.*/transient int size;.../*** 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;

DEFAULT_INITIAL_CAPACITY:16，数组的初始值，没太多好解释的。
MAXIMUM_CAPACITY：最大值，基本上不会达到这个值。
DEFAULT_LOAD_FACTOR：size / capacity(DEFAULT_INITIAL_CAPACITY)
threshold: capacity * load factor，超过这个阈值会rehash。

//一个桶的树化阈值
//当桶中元素个数超过这个值时，需要使用红黑树节点替换链表节点
//这个值必须为 8，要不然频繁转换效率也不高
static final int TREEIFY_THRESHOLD = 8;

//一个树的链表还原阈值
//当扩容时，桶中元素个数小于这个值，就会把树形的桶元素还原（切分）为链表结构
//这个值应该比上面那个小，至少为 6，避免频繁转换
static final int UNTREEIFY_THRESHOLD = 6;

//哈希表的最小树形化容量
//当哈希表中的容量大于这个值时，表中的桶才能进行树形化
//否则桶内元素太多时会扩容，而不是树形化
//为了避免进行扩容、树形化选择的冲突，这个值不能小于 4 * TREEIFY_THRESHOLD
static final int MIN_TREEIFY_CAPACITY = 64;

3.HashMap中DEFAULT_LOAD_FACTOR为什么是0.75

如果该值设为0.5，会在hashmap容量达到一半时候就扩容。比如从16扩大32，从32到64，64到128，浪费的空间会越来越大。
而如果该值设置为1，则每次空间使用完毕才会扩容，put时候操作耗时会增加。
所以0.75是时间与空间的一个平衡。

4.put时候操作

1.7插入元素到单链表中采用头插入法，1.8采用的是尾插入法

5.HashMap死循环

HashMap在并发执行put操作时会引起死循环，是因为多线程会导致HashMap的Entry链表形成环形数据结构。