String的hashCode分析

 /**
     * Returns a hash code for this string. The hash code for a
     * {@code String} object is computed as
     * <blockquote><pre>
     * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
     * </pre></blockquote>
     * using {@code int} arithmetic, where {@code s[i]} is the
     * <i>i</i>th character of the string, {@code n} is the length of
     * the string, and {@code ^} indicates exponentiation.
     * (The hash value of the empty string is zero.)
     *
     * @return  a hash code value for this object.
     */
　　　　private int hash;
　　　　private char[] value;

    public int hashCode() {
        int h = hash;
        if (h == 0 && value.length > 0) {
            char val[] = value;
            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];
            }
            hash = h;
        }
        return h;
    }

　　上面是hashCode源码，源码英文翻译就是,调用这个方法返回一个int值。空字符串的hashCode为0。hash值主要用来比较String是否equals,但不一定 ==。

　　至于为什么用31去构造算法，从数学上，31质数可以在JVM中很好的分配hashCode，如果数字相乘过大会导致溢出，从而导致数据的丢失。