不同版本JDK的区别
- jdk1.7中hashmap先扩容再插入
- jdk1.8中hashmap为什么先插入再扩容
jdk7中hashmap为什么先扩容再插入
JDK1.7采用头插法，扩容后，计算hash，只需要插入链表头部就行
jdk8中hashmap为什么先插入再扩容
JDK1.8 Hashmap采用的是数组+链表+红黑树的数据结构
基本参数介绍
/**
* The default initial capacity - MUST be a power of two.
* * 桶的容量，默认16
*/
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.
* * 桶的最大容量
* */
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
* * 负载因子
*/
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. * 树化阀值
*/
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.
* * 树退化阀值
*/
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.
* * 最小树化容量阀值即容量必要大于64才开始树化
* * 避免树化和扩容冲突
*/
static final int MIN_TREEIFY_CAPACITY = 64;
JDK1.8采用尾插法，如果先扩容，扩容后需要遍历一遍，再找到尾部进行插入
