试图诊断NullPointerException

Question

我正在编写课程代码.我们的意思是扫描用户指定的文件中的行,在每行中搜索用户指定的一个或多个单词,并报告整个文件(逐行处理)存在这些单词的次数.我们给了三个班,并打算写第四个.我的代码在第12行抛出NullPointerException,我不知道为什么.据我所知,我已经声明并初始化了该行中涉及的变量.我的代码是:

import java.io.IOException;
import java.util.*;

public class WordFreq extends Echo{
  String[] searchWordsAsStrings;
  WordCount[] searchWords;

  public WordFreq(String f, String w) throws IOException{
    super(f);
    searchWordsAsStrings = w.split(" ");
    for(int a = 0; a < searchWordsAsStrings.length; a++){
      searchWords[a] = new WordCount("");
    }
    for(int a = 0; a < searchWordsAsStrings.length; a++){
      searchWords[a].setWord(searchWordsAsStrings[a]);
    }
  }

  public void processLine(String line){
    StringTokenizer st = new StringTokenizer(line);
    while(st.hasMoreTokens()){
      for(int a = 0; a < searchWords.length; a++){
        if(searchWords[a].getWord() == st.nextToken()){
          searchWords[a].incCount();
        }
      }  
    }
  }

  public void reportFrequencies(){
    System.out.println("Word counts:");
    for(int a = 0; a < searchWords.length; a++){
      System.out.println(searchWords[a].toString());
    }
  }
}

堆栈跟踪:

java.lang.NullPointerException
    at WordFreq.<init>(WordFreq.java:12)
    at FreqStudy.main(FreqStudy.java:20)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)

这个应用程序的主要类如下.我已经注释掉了给定的代码并提供了我自己的代码,以便诊断问题.

import java.util.*;
import java.io.*;

public class FreqStudy{
  public static void main(String[] args) throws IOException
  {
    /*
    Scanner scan = new Scanner(System.in);
    System.out.println("enter file name");
    String fileName = scan.next();
    */
    String fileName = "pg37997.txt";
    /*
    Scanner scan2 = new Scanner(System.in);
    System.out.println("enter words to search for");
    System.out.println("enter lower case, separated by spaces");
    String wordString = scan2.nextLine();
    */
    String wordString = "cow horse chicken goat pig";
    WordFreq f = new WordFreq(fileName,wordString);
    f.readLines();
    f.reportFrequencies();
  }
}

此应用程序还使用Echo和WordCount类,如下所示:

import java.util.Scanner;
import java.io.*;

public class Echo{
  String fileName; // external file name
  Scanner scan; // Scanner object for reading from external file

  public Echo(String f) throws IOException
  {
    fileName = f;
    scan = new Scanner(new FileReader(fileName));
  }

  public void readLines(){ // reads lines, hands each to processLine
    while(scan.hasNext()){
      processLine(scan.nextLine());
    }
    scan.close();
  }

  public void processLine(String line){ // does the real processing work
    System.out.println(line);
  }
}

public class WordCount{

  private String word;
  private int count;

  public WordCount(String w){
    word = w;
    count = 0;
  }

  public String getWord(){
    return word;}

  public void setWord(String w){
    word = w;
  }

   public int getCount(){
    return count;}

  public void incCount(){count++;}

  public String toString() {
    return(word +  " --- " + count);
  }

  public boolean equals(Object other){
    WordCount i = (WordCount)other;
    return (this.word.equals(i.word));
  }
}

我确实无法追踪异常的来源.我已经阅读了生成它的内容(本网站上的许多解释),但我无法诊断实际代码中的异常是什么.

非常感谢.

Answer 1

您的问题是因为阵列初始化不正确!

WordCount[] searchWords;

但你永远不会宣布 searchWords = new WordCount[dimension];

当您尝试引用此数组时,您尝试访问不存在的内容.因为WordCount[]它本身就是一个对象,所以它抛出一个NullPointerException因为它还没有引用一个WordCount[]对象.