非法参数异常：n必须为正

问题描述

主类：

public class ECONAPP2 {
static Scanner input= new Scanner(System.in);
static int score = 0;
static ArrayList<Integer> usedArray = new ArrayList<Integer>();

public static void main(String[] args){
    app();
    arrayContents();
}

public static void arrayContents() {
    usedArray.add(2);
    usedArray.add(1);
}

应用（）方法：

public static void app() {
    Random generator = new Random ();
    int randomNumber = generator.nextInt(usedArray.size());
    System.out.println(randomNumber);
    if (randomNumber == 2) {
        score();
        question2();
        usedArray.remove(2);
        app();
    }
    if (randomNumber == 1) {
        score();
        question1();
        usedArray.remove(1);
        app();
    }

收到此错误：

Exception in thread "main" java.lang.IllegalArgumentException: n must be positive
at java.util.Random.nextInt(Random.java:250)
at ECONAPP2.app(ECONAPP2.java:65)
at ECONAPP2.main(ECONAPP2.java:10)

想不通这是什么意思，什么n为重的presentative？

can't work out what this means and what n is representative of ?

推荐答案

在此行

int randomNumber = generator.nextInt(usedArray.size());

您正试图生成随机数。

不过你有空usedArray，所以返回0。你不能在0范围内产生随机数为0 EXLUSIVE，所以它抛出异常。该值必须是1或更高。

However you have empty usedArray, so it returns 0. You cant generate random number in range 0 to 0 exlusive, so it throws exception. The value must be 1 or higher.

请注意文件：值介于0（含）和指定值（不），因此，例如所有呼叫generator.nextInt（1）返回0， generator.nextInt（2）返回0或1 ...

Note documentation : "value between 0 (inclusive) and the specified value (exclusive)", so for example generator.nextInt(1) return 0 on all calls, generator.nextInt(2) returns 0 or 1...

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