循环测试结果为空，程序崩溃[重复]

Question

我一直在为一个项目筛选我的代码，但无法让这种方法按照设计的方式工作。应该发生的是系统搜索作为参数传递的数组以查找用户输入的值。以下是所写的整个方法（有一些随机文本我已经抛出用于调试）：

    public static void updatePC(int ARRAY_SIZE, int count, String[] customerName, String[] customerID, String[] os, String[] typeOfProblem, int[] turnAroundTime) {
    Scanner keyboard = new Scanner(System.in);
    String toUpdate;
    String[] customerIDClone = new String[count];
    int intToUpdate = -1, loop = count, n;
    char correct = 'n';

    customerIDClone = Arrays.copyOf(customerID, count);
    while (correct == 'n') {
        System.out.println("Please enter the customer ID of the work order you wish to modify");
        toUpdate = keyboard.nextLine();
        if (Arrays.asList(customerIDClone).contains(toUpdate) == true) {
            intToUpdate = Integer.parseInt(toUpdate);
        }
        while (Arrays.asList(customerIDClone).contains(toUpdate) == false) {
            System.out.println("The customer ID you entered was not found. Please try again.");
            toUpdate = keyboard.nextLine();
            intToUpdate = Integer.parseInt(toUpdate);
        }
        System.out.println("Beginning search loop. The value to find is " + toUpdate);
        for (n = 0; n < loop; n++);
        {
            System.out.println("Loop loop looooooooop");
            System.out.println(customerID[n]);
            System.out.println(customerID[0]);
            if (customerID[count].equals(toUpdate)) {
                System.out.println("Found it!");
                count = n;
            }
        }

        System.out.println("Testing run 1.");
    }
    System.out.println("");
    System.out.println("--");
    System.out.println("                    Name:  " + customerName[count]);
    System.out.println("             Customer ID:  " + customerID[count]);
    System.out.println("                      OS:  " + os[count]);
    System.out.println("         Type of Problem:  " + typeOfProblem[count]);
    System.out.println("Expected Turnaround Time:  " + turnAroundTime[count]);
    System.out.println("--");
    System.out.println("Is this new record correct? Please enter y or n.  ");
    correct = keyboard.next().charAt(0);
    keyboard.nextLine();
}

我一直在整理网站和其他地方的其他问题，但我无法弄清楚问题所在。该方法的输出是

Please enter the customer ID of the work order you wish to modify
3
Beginning search loop. The value to find is 3
Loop loop looooooooop
null
1
Exception in thread "main" java.lang.NullPointerException
    at mckelvey_project3.McKelvey_Project3.updatePC(McKelvey_Project3.java:183)
    at mckelvey_project3.McKelvey_Project3.main(McKelvey_Project3.java:45)
Java Result: 1
BUILD SUCCESSFUL (total time: 7 seconds)

我写了一行

System.out.println(customerID[0]);

进入我的代码只是为了演示并确保数组已定义和初始化。此外，

System.out.println(customerID[1]);

也被定义。我的循环测试中似乎存在问题；运行

System.out.println(loop + count + n)

显示它们都等于 3。n 在哪里设置为 3？

编辑 添加了@hfontanez建议的以下代码

        customerIDClone = Arrays.copyOf(customerID, count);
    for (int i = 0; i < count; i++) {
        if (customerIDClone[i] == null) {
            System.out.println("Element " + i + " is null. Creating new String.");
            customerIDClone[i] = new String();
        }

输出如下：

Please enter the customer ID of the work order you wish to modify
3
Beginning search loop. The value to find is 3
Loop loop looooooooop
null
2
Exception in thread "main" java.lang.NullPointerException
    at mckelvey_project3.McKelvey_Project3.updatePC(McKelvey_Project3.java:190)
    at mckelvey_project3.McKelvey_Project3.main(McKelvey_Project3.java:45)
Java Result: 1
BUILD SUCCESSFUL (total time: 14 seconds)

编辑 2 我发现了问题......我有一个流浪者;这里：

for (n = 0; n < count; n++);

这个问题现在已经解决了。

Answer 1

Java 中的数组是一个对象。当您创建对象数组时，例如字符串数组，您必须初始化数组和每个元素。行：

String[] customerIDClone = new String[count];

初始化一个包含 'COUNT' 个 String 对象的数组，但每个元素仍然为空。这就是你得到空指针异常的原因。

在调用 customerID[count].equals(toUpdate) 之前，customerID[count] 处的 (String) 元素必须正确实例化。

这种情况下的问题是新数组不是原始数组的精确副本。这一行：

customerIDClone = Arrays.copyOf(customerID, count);

创建一个“COUNT”长度的新数组。如果 'COUNT' 大于原始长度，则所有新创建的元素都用 NULL 填充。这解释了为什么 customerID[0] 有效，而 customerID[n] 无效。要解决此问题，您可以遍历新数组并将new String() 添加到每个 NULL 元素。

更新：

试试这个：

customerIDClone = Arrays.copyOf(customerID, count);
for (int i = 0; i < count; i++)
{
    if(customerIDClone[i] == null)
    {
        System.out.println("Element " + i + " is null. Creating new String.");
        customerIDClone[i] = new String();
    }
}