HashMap在JDK1.7是以数组加链表的形式组成，JDK1.8后新增了红黑树结构，当链表大于8并且容量大于64时，链表结构会转成红黑树结构。
JDK1.8 之所以会加入红黑树是因为当链表过长是会严重影响HashMap的性能，而红黑树具有快速增删改查的特点。
HashMap源码中包含了一下几个常量的定义：
/** * The default initial capacity - MUST be a power of two. * 初始化长度 */ 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. * 最大长度 */ static final int MAXIMUM_CAPACITY = 1 << 30; /** * The load factor used when none specified in constructor. * 默认扩容加载因子 */ 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. * 当链表长度大于此值且容量大于64时 */ 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. * 转换链表的临界值，当元素小于此值时，会将红黑树结构转换为链表结构 */ 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. * 最小树容量 */ static final int MIN_TREEIFY_CAPACITY = 64;
关于加载因子
加载因子也叫作扩容因子，用来判断什么时候进行扩容，假设加载因子为0.75，HashMap的初始容量为16，当HashMap中有16 * 0.75 = 12个容量时，HashMap就会进行扩容。
如果加载因子越大，扩容发生的频率就会比较低，占用空间比较小，但是发生hash冲突的几率会提升，对元素操作时间会增加，运行效率降低；
如果加载因子太小，那么表中的数据将过于稀疏（很多空间还没用，就开始扩容了），对空间造成严重浪费；
而且因为容量默认为2的次方，当加载因子为0.75时，容量和加载因子的乘积为整数。
所以系统默认加载因子取了0.5 -1 之间的0.75.