在HashMap中，在get和put（可能还有其他地方，没记那么多）的时候都对key的hashCode进行hash，返回一个hash然后进行添加或者get。具体是这样的：
int hash = hash(key.hashCode());然后hash函数的实现是这样：/**
* Applies a supplemental hash function to a given hashCode, which
* defends against poor quality hash functions. This is critical
* because HashMap uses power-of-two length hash tables, that
* otherwise encounter collisions for hashCodes that do not differ
* in lower bits. Note: Null keys always map to hash 0, thus index 0.
*/
static int hash(int h) {
// This function ensures that hashCodes that differ only by
// constant multiples at each bit position have a bounded
// number of collisions (approximately 8 at default load factor).
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}我把注释也贴出来了，因为就是看不懂注释，那些注释具体的是啥，英语实在太烂了。能举个例子说说最好了。
int hash = hash(key.hashCode());然后hash函数的实现是这样：/**
* Applies a supplemental hash function to a given hashCode, which
* defends against poor quality hash functions. This is critical
* because HashMap uses power-of-two length hash tables, that
* otherwise encounter collisions for hashCodes that do not differ
* in lower bits. Note: Null keys always map to hash 0, thus index 0.
*/
static int hash(int h) {
// This function ensures that hashCodes that differ only by
// constant multiples at each bit position have a bounded
// number of collisions (approximately 8 at default load factor).
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}我把注释也贴出来了，因为就是看不懂注释，那些注释具体的是啥，英语实在太烂了。能举个例子说说最好了。
for (Entry<K,V> e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
return e.value;
}
从这段代码我觉得这个是用来确定位置的，那个注释也是差不多确定位置的意思。因为HashMap里面有一个table（就是HashMap里面的Entry数组（Entry就是一个映射，里面有一个key和一个alue，HashMap的主要功能就是通过Entry来实现的）），在这个数组里面要确定你要取的是哪一个Entry就要调用indexFor()这个方法，这个方法的一个参数就是通过hash()函数来确定的。具体是怎么用的就不多说了，说起来要研究好久，不知道楼主明白了没？
0101 0000 0000 1111 = 20495
0111 0000 0000 1111 = 28687
假如hashmap的capacity是16，那么20495%16 = 15,28687%16=15,就冲突了