爲什麼我收到「必須是正面的」錯誤？

Question

我正在嘗試編寫模擬拼字遊戲的代碼。我設計了一個應該模擬拼字遊戲包的類，我試圖在選擇一個隨機拼圖後，通過在main中打印tileID來測試它。每當我雖然運行的代碼，我不斷收到以下錯誤：

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

有人可以告訴我爲什麼我收到這個錯誤？

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()爲0，則傳遞了無效的參數nextInt()。

這在Javadoc明確規定：

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

nextInt(n)返回0和n-1之間的數。你認爲nextInt(0)返回什麼？

在你的主要方法中，你試圖挑選一個空袋子的元素。它不能工作。在致電randomPick()之前，您應該檢查包的大小。當袋子空了時，randomPick()應該會拋出異常。

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

錯誤說明了一切：

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

每當必然是zero或negative，它是一個非法參數rand.nextInt()。確保this.bag.size()從不zero。

Answer 3

該錯誤是因爲線62在類的情況發生：

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

如果空着袋子調用此代碼，它將始終返回0

下一個步驟是檢查

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

由於我們：線387類java.util.Random的總是從this.bag.size()得到0，它總是會拋出異常。

要讓它工作，確保元素添加到包，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); 

}