自从JDK最初版本发布起，我们就可以使用java.util.Random类产生随机数了。在JDK1.2中，Random类有了一个名为nextInt()的方法：
public int nextInt(int n)

给定一个参数n，nextInt(n)将返回一个大于等于0小于n的随机数，即：0 <= nextInt(n) < n。 

    /*** Returns a pseudo-random uniformly distributed {@code int}* in the half-open range [0, n).*/public int nextInt(int n) {if (n <= 0) {throw new IllegalArgumentException("n <= 0: " + n);}if ((n & -n) == n) {return (int) ((n * (long) next(31)) >> 31);}int bits, val;do {bits = next(31);val = bits % n;} while (bits - val + (n - 1) < 0);return val;}