非法参数异常:n 必须为正

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);
}

app() 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)

无法弄清楚这意味着什么以及 n 代表什么?

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