NullPointerExcetion 本机方法访问器...散列字问题 [英] NullPointerExcetion Native Method Accessor... Hashing Words Issue

问题描述

我正在编写一个读取文件并对单词"进行排序的项目.这段代码编译正确，但它给了我一个空指针异常.有什么想法吗?

I am writing a project that reads a file and sorts "Words". This code compiles correctly, yet then it gives me a null pointer exception. Any Ideas?

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();
      }
   }
}

异常的堆栈跟踪是:

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)

推荐答案

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() 将返回 a，然后 scanner.next() 将返回 b>，因为 nextLine 返回下一个结束行分隔的 String，而 next 返回输入文件中的下一个标记.这真的是你想要的吗?我建议试试这个:

Then scanner.nextLine() would be return a, then scanner.next() would return b, because nextLine returns the next end-line delimited String, and next returns the next token from the input file. Is this really what you want? I'd suggest trying this:

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

    linecount++;
}

请记住，这仅适用于每行只有一个单词的情况.如果你想每行处理多个单词，它会稍微长一点:

Keep in mind that this would only work if there's only a word per line. If you want to handle multiple words per line, it'd be slightly longer:

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

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

    linecount++;
}

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