如何在特定范围内生成随机int值?

以下方法存在与整数溢出相关的错误:

randomNum = minimum + (int)(Math.random() * maximum);
// Bug: `randomNum` can be bigger than `maximum`.
Random rn = new Random();
int n = maximum - minimum + 1;
int i = rn.nextInt() % n;
randomNum =  minimum + i;
// Bug: `randomNum` can be smaller than `minimum`.

当前回答

Use:

minValue + rn.nextInt(maxValue - minValue + 1)

其他回答

以前的大多数建议都不考虑“溢出”。例如:min=整数.min_VALUE,max=100。到目前为止,我采用的正确方法之一是:

final long mod = max- min + 1L;
final int next = (int) (Math.abs(rand.nextLong() % mod) + min);

Use:

minValue + rn.nextInt(maxValue - minValue + 1)

在Java 1.7或更高版本中,执行此操作的标准方法如下:

import java.util.concurrent.ThreadLocalRandom;

// nextInt is normally exclusive of the top value,
// so add 1 to make it inclusive
int randomNum = ThreadLocalRandom.current().nextInt(min, max + 1);

请参阅相关的JavaDoc。这种方法的优点是不需要显式初始化java.util.Random实例,如果使用不当,可能会导致混淆和错误。

然而,相反,没有办法明确设置种子,因此在测试或保存游戏状态等有用的情况下,很难再现结果。在这些情况下,可以使用下面所示的Java 1.7之前的技术。

在Java 1.7之前,执行此操作的标准方法如下:

import java.util.Random;

/**
 * Returns a pseudo-random number between min and max, inclusive.
 * The difference between min and max can be at most
 * <code>Integer.MAX_VALUE - 1</code>.
 *
 * @param min Minimum value
 * @param max Maximum value.  Must be greater than min.
 * @return Integer between min and max, inclusive.
 * @see java.util.Random#nextInt(int)
 */
public static int randInt(int min, int max) {

    // NOTE: This will (intentionally) not run as written so that folks
    // copy-pasting have to think about how to initialize their
    // Random instance.  Initialization of the Random instance is outside
    // the main scope of the question, but some decent options are to have
    // a field that is initialized once and then re-used as needed or to
    // use ThreadLocalRandom (if using at least Java 1.7).
    // 
    // In particular, do NOT do 'Random rand = new Random()' here or you
    // will get not very good / not very random results.
    Random rand;

    // nextInt is normally exclusive of the top value,
    // so add 1 to make it inclusive
    int randomNum = rand.nextInt((max - min) + 1) + min;

    return randomNum;
}

请参阅相关的JavaDoc。实际上,java.util.Random类通常比java.lang.Math.Random()更好。

特别是,当标准库中有一个简单的API来完成任务时,无需重新发明随机整数生成轮。

假设您希望范围在0-9之间,0是最小值,9是最大值。以下函数将打印0到9之间的任何内容。所有范围都一样。

public static void main(String[] args) {
    int b = randomNumberRange(0, 9);
    int d = randomNumberRange (100, 200);
    System.out.println("value of b is " + b);
    System.out.println("value of d is " + d);
}

public static int randomNumberRange(int min, int max) {
    int n = (max + 1 - min) + min;
    return (int) (Math.random() * n);
}

您可以使用以下方法来执行此操作:

int range = 10;
int min = 5
Random r = new Random();
int = r.nextInt(range) + min;