Java为什么会出现此错误？ 线程“ main”中的异常java.lang.NullPointerException

Question

这是代码，它给我的错误是>>>>>>>我已经看过这个线程的异常了，但是我仍然不明白我需要改变什么:(我是一个初学者编程。

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class manyVowels {

  public static final String wordList = "words.txt";

  public static void main(String[] args) {
        Scanner fileIn = null;
        try {
            //locate and open file
            fileIn = new Scanner(new FileInputStream("words.txt"));
        } catch (FileNotFoundException e) {
            //if the file cannot be found, the program prints the message and quits
            System.out.println("File not found. ");
            System.exit(0);
        }
        String word = null;
        if (fileIn.hasNext()) //if there is another word continue
        {

            String finalWord = null; // defines the word with most consecutive vowels
            int maxVowels = 0;//sets initial value to 0
            while (fileIn.hasNext()) {
                // for each word in the file
                int vowels = 0;
/*error here-->*/ for (int i = 0; i < word.length() && i < word.length() - maxVowels + vowels; i++) {
                    // for each character in the word, and exit early if the word is not long enough to beat maxVowels
                    if (hasVowels(word.charAt(i))) {
                        // consonants reset this to 0
                        vowels++;
                    } else {
                        // reached the end of the word so check if the count is higher than maxVowels
                        if (vowels > maxVowels) {
                            maxVowels = vowels;
                            finalWord = word;
                        }
                        vowels = 0;
                    }
                }
                // comparing vowels to maxVowels
                if (vowels > maxVowels) {
                    maxVowels = vowels;
                    finalWord = word;
                }
            }

            //seek vowels
            word = fileIn.next();
            for (int i = 0; i < word.length(); i++) {
                if
                        ((word.charAt(i) == 'A')
                        || (word.charAt(i) == 'E')
                        || (word.charAt(i) == 'I')
                        || (word.charAt(i) == 'O')
                        || (word.charAt(i) == 'U')
                        || (word.charAt(i) == 'a')
                        || (word.charAt(i) == 'e')
                        || (word.charAt(i) == 'i')
                        || (word.charAt(i) == 'o')
                        || (word.charAt(i) == 'u')) {
                    //prints the final word with the most consecutive vowels
                    System.out.println("The word with the most consecutive vowels is: " + word);
                    System.exit(0);
                }

            }
        }
    }

    private static boolean hasVowels(char charAt) {
        throw new UnsupportedOperationException("Inserted by template."); //NetBeans generated method
    }
}

Answer 1

按照逻辑，将String word初始化为null ，然后继续调用word.length() 。

word为null ，没有length() 。 因此， NullPointerException 。

在尝试测量word长度之前，请给word分配一个字符串。

String word = "Hello!";

Answer 2

可变词似乎为空，我认为您跳过了一条指令，在该指令中，您使用了从fileIn读取的词来填充它。 您的代码应如下所示：

     while (fileIn.hasNext()) {
            // for each word in the file
            word = fileIn.next();
            int vowels = 0;
            for (int i = 0; i < word.length() && i < word.length() - maxVowels + vowels; i++) {
               ...

Answer 3

您将单词定义为空。 当您说word.length（）时，意味着您在说null.length（），这就是为什么您会得到null指针异常的原因。

在执行任何操作之前，您应该初始化String变量“ word”（使用'。'调用任何字符串方法）

如果您有任何预定义的值，请使用该值进行初始化，否则请使用空字符串进行初始化。

字串=“ xyz”; 字串=“”;