java - 如何在Java中生成特定范围内的随机整数？

Question

如何生成特定范围内的随机 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`.

Answer 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);

参见the relevant 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，可能会造成困惑和错误。

但是，相反，无法显式设置种子，因此在有用的情况下(例如测试或保存游戏状态或类似情况)可能很难重现结果。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;
}

参见the relevant JavaDoc 。在实践中，java.util.Random class 通常优于 java.lang.Math.random() .

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。