NullPointerExcetion本机方法访问器…散列单词问题

Question

我正在编写一个读取文件并排序“单词”的项目。这段代码可以正确编译，但是却给了我一个空指针异常。有任何想法吗？

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
import java.util.Hashtable;

public class Lab {
   Hashtable<String, Word> words = new Hashtable<String, Word>();

   public void addWord(String s, int i) {
      if (words.containsKey(s)) {
         words.get(s).addOne();
         words.get(s).addLine(i);
      } else {
         words.put(s, new Word(s));
         words.get(s).addLine(i);
      }
   }

   public void main(String[] args) {
      System.out.println("HI");
      File file = new File("s.txt");
      int linecount = 1;
      try {
         Scanner scanner = new Scanner(file);
         System.out.println("HUH");

         while (scanner.hasNextLine()) {
            String line = scanner.nextLine();
            while (line != null) {
               String word = scanner.next();
               addWord(word, linecount);
            }
            linecount++;

         }
      } catch (FileNotFoundException e) {
         e.printStackTrace();
      }
   }
}

异常的堆栈跟踪为：

java.lang.NullPointerException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:27‌​1)

Answer 1

这个while循环很奇怪：

while (scanner.hasNextLine()) {
    String line = scanner.nextLine();
    while (line != null) {
       String word = scanner.next();
       addWord(word, linecount);
    }
    linecount++;
}

如果您的输入文件是：

a
b

然后scanner.nextLine()将是return a，然后scanner.next()将是return
b，因为nextLine将返回下一个以行尾分隔的String，并next返回输入文件中的下一个标记。这真的是您想要的吗？我建议尝试一下：

while (scanner.hasNextLine()) {{
    String word = scanner.nextLine();
    addWord(word, linecount);

    linecount++;
}

请记住，这仅在每行只有一个单词的情况下有效。如果您想每行处理多个单词，则需要更长的时间：

while (scanner.hasNextLine()) {{
    String line = scanner.nextLine();

    Scanner lineScanner = new Scanner(line);
    while(lineScanner.hasNext()) {
        addWord(lineScanner.next(), linecount);
    }

    linecount++;
}