java - 为什么我收到 "bound must be positive"错误？

Question

我正在尝试编写模拟拼字游戏的代码。我设计了一个应该模拟拼字游戏袋的类，我试图通过在选择随机瓷砖后在 main 中打印 tileID 来测试它。每当我运行代码时，我都会收到以下错误:
Exception in thread "main" java.lang.IllegalArgumentException: bound must be positive
at java.util.Random.nextInt(Random.java:388)
at hw3.RandomBag.randomPick(RandomBag.java:39)
at hw3.RandomBag.main(RandomBag.java:59
有人能告诉我为什么我会收到那个错误吗？

import java.util.*;

public class RandomBag<E> implements Iterable<E> {

    // instance varibles
    private List<E> bag; // arraylist as the container
    private Random rand; // random number generator

    // constructors
    public RandomBag() { 
        bag = new ArrayList<E>();
        rand = new Random();    
    }

    public RandomBag(int seed) { 
        bag = new ArrayList<E>();
        rand = new Random(seed);

    }

    // returns the size of the bag
    public int size() { return this.bag.size(); }  

    public boolean isEmpty() { // returns true/false if the bag is/is not empty
        if (this.bag.isEmpty())
            return true;
        else
            return false;
    }  

    // adds the parameter element in the bag
    public void add (E element) {this.bag.add(element);} 

    // randomly selects an element using the random number generator 'rand' and removes that element from the bag, and returns the element
    public E randomPick() {

        int index = rand.nextInt(this.bag.size());
        E tileID = bag.remove(index);
        return tileID;
    } 

    // obtains an iterator for the bag, and returns it
    public Iterator<E> iterator() {
        // traverse bag using an iterator
        Iterator it = bag.iterator();
        return it;
    }

      //**
      //** main() for testing RandomBag<E>
      //**
    public static void main(String[] args) {

        RandomBag bag = new RandomBag();   

        Object tileID = bag.randomPick(); 
        System.out.println(tileID);

    }
}

Answer 1

如 this.bag.size()是 0，你传递了一个无效的参数给 nextInt() .

这在 Javadoc 中有明确说明:

n the bound on the random number to be returned. Must be positive.

nextInt(n)返回一个介于 0 和 n-1 之间的数字。你有什么期望 nextInt(0)返回？

在您的主要方法中，您试图选择一个空袋子的元素。它不能工作。您应该在调用 randomPick() 之前检查袋子的尺寸.和 randomPick()当包为空时，应该可能会抛出异常。

public static void main(String[] args) {

    RandomBag bag = new RandomBag();   

    Object tileID = null;
    if (bag.size() > 0)
        tileID = bag.randomPick(); 
    System.out.println(tileID);

}