Java Random.nextInt(int) return 设置不同种子时相同的值

Question

我写了一个演示来测试 java.util.Random 我想生成一个包含相同 5 个数字的重复列表，但是当设置不同 seeds.In 我的程序时我得到相同的值，种子范围从0到4。据我所知，不同的种子产生不同的值，相同的种子获得相同的值。所以我认为结果将是相同 5 个数字的重复列表。但是输出的实际值都是一样的。我的代码有什么问题？谁能告诉我吗？

import java.util.Random;

public class Main {
    public Main() {
    }

    public static void main(String[] args) {
        for (int i = 0 ; i <= 255; i++)
        {
            String hex = Integer.toHexString(randInt(0, 255, i % 5));
            System.out.println(hex);
        }
    }
    private static Random rand = new Random();
    public static int randInt(int min, int max, long seed) {
        rand.setSeed(seed);
        System.out.println("seed:" + seed);
        int randomNum = rand.nextInt((max - min) + 1) + min;
        return randomNum;
    }
}

结果是：

seed:0
bb
seed:1
bb
seed:2
bb
seed:3
bb
seed:4
bb
seed:0
bb
seed:1
bb
seed:2
bb
seed:3
bb
seed:4
bb
seed:0
bb
seed:1
bb
seed:2
bb
seed:3
bb
seed:4
bb
seed:0
bb
seed:1
...
...
...

Answer 1

As far as I know, different seeds produce different values

这是不正确的，不同的种子可能产生不同的值，它们也可以产生相同的值。

有 2^64 个可能的种子，rand.nextInt(256) 只能 return 256 个不同的值，因此许多种子必须 return 相同的值。

setSeed javadoc 也指出

The implementation of setSeed by class Random happens to use only 48 bits of the given seed

因此，如果您的种子仅在忽略的位上有所不同，则所有值都将相同。

Answer 2

我在 grepcode 上找到了这个实现，有一个 if 语句来检测 n 是否是 2 的幂。如果 n（即边界）是 2 的幂，则使用 (int)((n * (long)next(31)) >> 31); .

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;
}

我不知道您的 JDK 中是否使用了此实现，但这表明 2 次方边界的处理方式不同。

The algorithm treats the case where n is a power of two specially: it returns the correct number of high-order bits from the underlying pseudo-random number generator. In the absence of special treatment, the correct number of low-order bits would be returned. Linear congruential pseudo-random number generators such as the one implemented by this class are known to have short periods in the sequence of values of their low-order bits. Thus, this special case greatly increases the length of the sequence of values returned by successive calls to this method if n is a small power of two.

Java Random.nextInt(int) return 设置不同种子时相同的值

Java Random.nextInt(int) return the same value when set different seeds

java

random

seed