java 随机数 nextInt(int n)的问题 - 未名空间MITBBS历史存档

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java 随机数 nextInt(int n)的问题

java 随机数 nextInt(int n)的问题# Java - 爪哇娇娃

b*y2009-03-17 07:03

1 楼

jdk里面的源程序如下：
public int nextInt(int n) {
if (n<=0)
throw new IllegalArgumentException("n must be positive");
if ((n & -n) == n) // i.e., n is a power of 2
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;
}
读不懂的地方是while(bits - val + (n-1) < 0);
为啥要这样判断呢？

c*t2009-03-17 07:03

2 楼

javadoc 里面写着呢：
*
* The loop at the bottom only accepts a value, if the random
* number was between 0 and the highest number less then 1<<31,
* which is divisible by n. The probability for this is high for small
* n, and the worst case is 1/2 (for n=(1<<30)+1).
*

【在 b******y 的大作中提到】

: jdk里面的源程序如下：
: public int nextInt(int n) {
: if (n<=0)
: throw new IllegalArgumentException("n must be positive");
: if ((n & -n) == n) // i.e., n is a power of 2
: return (int)((n * (long)next(31)) >> 31);
: int bits, val;
: do {
: bits = next(31);
: val = bits % n;