HashMap的hash（）方法求祥解

昨天閒來無事看HashMap的源代碼，看到這裏hash（）方法，自己都搞不懂爲什麼是這個算法，有哪位大俠指點指點，謝謝了

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);
	}