如何解决Java中的NullPointerException? [英] How to work around a NullPointerException in Java?

问题描述

所以我有一个文本文件，其中包含导入到程序中的一串字符串，程序要做的是寻找第一个重复字符串的第一个索引:

So I have a text file that contains a bunch of strings that I import into the program and what my program does is look for the first index of the first duplicate string:

static final int NOT_FOUND = -1;
dupeIndex = indexOfFirstDupe( wordList, wordCount );
    if ( dupeIndex == NOT_FOUND )
        System.out.format("No duplicate values found in wordList\n");
    else
        System.out.format("First duplicate value in wordList found at index %d\n",dupeIndex);

和我用来查找重复项的第一个索引的方法如下:

and the method I use to find the first index of the duplicate is as follows:

static int indexOfFirstDupe( String[] arr, int count )
{       

    Arrays.sort(arr);
    int size = arr.length;
    int index = NOT_FOUND;

    for (int x = 0; x < size; x++) {
        for (int y = x + 1; y < size; y++) {
            if (arr[x].equals(arr[y])) {
                index = x;
                break;
            }
        }
    }
    return index;

问题是我收到此错误:

这是一个 NullPointerException ，据我了解，这意味着我的字符串数组中基本上有一个null值.有什么简单的解决方案，我错过了吗?可能改写我的方法?

It's a NullPointerException and from my understanding it means that there's basically a null value(s) in my array of strings(?). Is there any simple solution to this that I am missing? Possibly rewording my method?

推荐答案

假设您的诊断正确无误

...这意味着我的字符串数组中基本上有一个null值...

... it means that there's basically a null value(s) in my array of strings ...

...我可以想到两种解决方法.

... I can think of two workarounds.

1. 摆脱数组中的 null 引用.完全删除它们，或将其替换为(例如)" 或"null" 或其他无害的东西.

1. Get rid of the null references in the array. Remove them entirely, or replace them with (say) "" or "null" or something else harmless.

Arrays.sort 方法的重载带有第二个参数: Comparator .因此，您可以做的是实现一个 Comparator ，该处理器可以处理 null 而不会抛出NPE.(例如，它可以将 null 视为小于所有非null字符串.)

There is an overload of the Arrays.sort method that takes a second argument: a Comparator. So what you could do is to implement a Comparator that can handle null without throwing an NPE. (For example, it could treat null as smaller than all non-null strings.)

以下是处理 null 的示例比较器:

Here's an example comparator that deals with null:

    public class NullSafeStringComparator implements Comparator<String> {
        public int compare(String s1, String s2) {
            if (s1 == s2) {
                return 0;
            } else if (s1 == null) {
                return -1;
            } else if (s2 == null) {
                return 1;
            } else {
                return s1.compareTo(s2);
            }
        }
    }

或者，对于Java 8和更高版本，您可以按以下方式构建一个:

Alternatively, for Java 8 and later you can build one as follows:

    Comparator.nullsFirst(Comparator.naturalOrder())            

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