HashMap的hash（）方法求祥解

昨天闲来无事看HashMap的源代码，看到这里hash（）方法，自己都搞不懂为什么是这个算法，有哪位大侠指点指点，谢谢了

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);
	}