java string的hashcode

 

 

 

public int hashCode() {
	int h = hash;
	if (h == 0) {
	    int off = offset;
	    char val[] = value;
	    int len = count;

            for (int i = 0; i < len; i++) {
                h = 31*h + val[off++];
            }
            hash = h;
        }
        return h;
    }

 

实际上是使用霍纳法则计算

即

s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1] 的值

 

选择31是因为他为素数，且他等于左移5位-1的值，vm会自动做这种优化

 

http://book.douban.com/subject/1610337/  lafore的这本书写的很人性化，没有太多公式，适合入门，第11章讲到了这部分

 

http://en.wikipedia.org/wiki/Java_hashCode()

http://stackoverflow.com/questions/299304/why-does-javas-hashcode-in-string-use-31-as-a-multiplier

http://www.ibm.com/developerworks/java/library/j-jtp05273/index.html