为什么我收到“必须是正面的”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包的类，我试图在选择一个随机拼图后，通过在main中打印tileID来测试它。每当我虽然运行的代码，我不断收到以下错误：

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

有人可以告诉我为什么我收到这个错误？

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

如果this.bag.size()为0，则传递了无效的参数nextInt()。

这在Javadoc明确规定：

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

nextInt(n)返回0和n-1之间的数。你认为nextInt(0)返回什么？

在你的主要方法中，你试图挑选一个空袋子的元素。它不能工作。在致电randomPick()之前，您应该检查包的大小。当袋子空了时，randomPick()应该会抛出异常。

public static void main(String[] args) { 

    RandomBag bag = new RandomBag(); 

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

} 

Answer 2

错误说明了一切：

Exception in thread "main" java.lang.IllegalArgumentException: bound must be positive 
    at java.util.Random.nextInt(Random.java:388) 

每当必然是zero或negative，它是一个非法参数rand.nextInt()。确保this.bag.size()从不zero。

Answer 3

该错误是因为线62在类的情况发生：

int index = rand.nextInt(this.bag.size()); 

如果空着袋子调用此代码，它将始终返回0

下一个步骤是检查

if (bound <= 0) 
     throw new IllegalArgumentException(BadBound); 

由于我们：线387类java.util.Random的总是从this.bag.size()得到0，它总是会抛出异常。

要让它工作，确保元素添加到包，如：

public static void main(String[] args) { 

    RandomBag bag = new RandomBag(); 

    for (int i = 0; i < 10; i++) { 
     bag.add(i); // adding elements 
    } 

    Object tileID = bag.randomPick(); 

    System.out.println(tileID); 

}