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

我认为这段代码会奏效。请尝试一下:

import java.util.Random;
public final class RandomNumber {

    public static final void main(String... aArgs) {
        log("Generating 10 random integers in range 1..10.");
        int START = 1;
        int END = 10;
        Random randomGenerator = new Random();
        for (int idx=1; idx<=10; ++idx) {

            // int randomInt=randomGenerator.nextInt(100);
            // log("Generated : " + randomInt);
            showRandomInteger(START,END,randomGenerator);
        }
        log("Done");
    }

    private static void log(String aMessage) {
        System.out.println(aMessage);
    }

    private static void showRandomInteger(int aStart, int aEnd, Random aRandom) {
        if (aStart > aEnd) {
            throw new IllegalArgumentException("Start cannot exceed End.");
        }
        long range = (long)aEnd - (long)aStart + 1;
        long fraction = (long) (range * aRandom.nextDouble());
        int randomNumber = (int) (fraction + aStart);
        log("Generated" + randomNumber);
    }
}

其他回答

只需对第一个解决方案稍作修改即可。

Random rand = new Random();
randomNum = minimum + rand.nextInt((maximum - minimum) + 1);

有关Random的实现,请参阅此处的更多信息

import java.util.Random; 

public class RandomUtil {
    // Declare as class variable so that it is not re-seeded every call
    private static Random random = new Random();

    /**
     * Returns a psuedo-random number between min and max (both inclusive)
     * @param min Minimim value
     * @param max Maximim value. Must be greater than min.
     * @return Integer between min and max (both inclusive)
     * @see java.util.Random#nextInt(int)
     */
    public static int nextInt(int min, int max) {
        // nextInt is normally exclusive of the top value,
        // so add 1 to make it inclusive
        return random.nextInt((max - min) + 1) + min;
    }
}

请原谅我过于挑剔,但大多数人建议的解决方案,即min+rng.nextInt(max-min+1),似乎很危险,因为:

rng.nextInt(n)无法达到整数.MAX_VALUE。当min为负值时,(max-min)可能会导致溢出。

万无一失的解决方案将为[Integer.min_VALUE,Integer.max_VALUE]内的任何min<=max返回正确的结果。请考虑以下简单的实现:

int nextIntInRange(int min, int max, Random rng) {
   if (min > max) {
      throw new IllegalArgumentException("Cannot draw random int from invalid range [" + min + ", " + max + "].");
   }
   int diff = max - min;
   if (diff >= 0 && diff != Integer.MAX_VALUE) {
      return (min + rng.nextInt(diff + 1));
   }
   int i;
   do {
      i = rng.nextInt();
   } while (i < min || i > max);
   return i;
}

尽管效率低下,但请注意while循环中成功的概率始终为50%或更高。

你可以这样做:

import java.awt.*;
import java.io.*;
import java.util.*;
import java.math.*;

public class Test {

    public static void main(String[] args) {
        int first, second;

        Scanner myScanner = new Scanner(System.in);

        System.out.println("Enter first integer: ");
        int numOne;
        numOne = myScanner.nextInt();
        System.out.println("You have keyed in " + numOne);

        System.out.println("Enter second integer: ");
        int numTwo;
        numTwo = myScanner.nextInt();
        System.out.println("You have keyed in " + numTwo);

        Random generator = new Random();
        int num = (int)(Math.random()*numTwo);
        System.out.println("Random number: " + ((num>numOne)?num:numOne+num));
    }
}

最好使用SecureDrandom,而不仅仅是Random。

public static int generateRandomInteger(int min, int max) {
    SecureRandom rand = new SecureRandom();
    rand.setSeed(new Date().getTime());
    int randomNum = rand.nextInt((max - min) + 1) + min;
    return randomNum;
}