搜索文本文件时抛出`NullPointerException` [英] `NullPointerException` thrown while searching text file

问题描述

首先，我在下面的**行中不断获取NullPointerException. 其次，我的程序输出错误(我以某种方式使它工作，但随后又返回错误).它必须是逻辑错误.我有11行的文件directory.txt，每行上都有一个名称.当我运行程序尝试查找某个名称时，它只会在第一行中找到名字，其他都找不到.如何解决这两个错误?

First, I keep getting a NullPointerException on the line I put in ** below. Second, my program is giving the wrong output (I somehow got it to work but then it went back to error). It must be a logic error. I have a file directory.txt of 11 lines, each with a name on it. When I run my program to try to find a certain name, it only finds the first name on the first line and everything else, it can't find. How can I fix these 2 errors?

我有2节课.这是一流的目录:

I have 2 classes. This is the first class Directory:

import java.util.*;
import java.io.*;
public class Directory {
   //public static void main(String[] args) {
   final int maxDirectorySize = 1024;
   String directory[] = new String[maxDirectorySize];
   int directorySize = 0;
   File directoryFile = null;
   Scanner directoryDataIn = null;

   public Directory(String directoryFileName) {
      directoryFile = new File(directoryFileName);
      try {
         directoryDataIn = new Scanner(directoryFile);
      }
      catch (FileNotFoundException e) {
         System.out.println("File is not found, exiting!" + directoryFileName);
         System.exit(0);
      }
      while (directoryDataIn.hasNext()) {
         directory[directorySize++] = directoryDataIn.nextLine();
      }
   }
   public boolean inDirectory(String name) {
      boolean inDir = true;
      for (int i = 0; i < directory.length; i++) {
         **if (directory[i].equalsIgnoreCase(name))** 
            inDir = true;
         else 
            inDir = false;
      }
      return inDir;
   }
   public boolean add(String name) {
      if (directory.length == 1024)
         return false;
      for (int i = 0; i < directory.length; i++) {
         if (directory[i].equalsIgnoreCase(name))
            return false;
         else
            directory[directorySize++] = name;
            return true;
      }
      return false;
   }          

   public boolean delete(String name) {
      for (int i = 0; i < directory.length; i++) {
         if (directory[i].equalsIgnoreCase(name)) {
            directory[i] = null;
            return true;
         }   
         else
            return false;
      }
      return false;
   }

   public void closeDirectory() {
      directoryDataIn.close();
      PrintStream directoryDataOut = null;
      try {
          directoryDataOut = new PrintStream(directoryFile);
      }
      catch (FileNotFoundException e) {
         System.out.printf("File %s not found, exiting!", directoryFile);
         System.exit(0);
      }
      String originalDirectory[] = {"Mike","Jim","Barry","Cristian","Vincent","Chengjun","susan","ng","serena"};
      if (originalDirectory == directory)
         System.exit(0);
      else
         for (int i = 0; i < directorySize; i++)
            directoryDataOut.println(directory[i]);
         directoryDataOut.close();
   }
}

这是我试图运行的第二个类，但我一直在获取异常主线程NullPointerException.

AND this is my second class which I'm trying to run but I keep getting exception main thread NullPointerException.

import java.io.*;
import java.util.*;
public class DirectoryWithObjectDesign {
   public static void main(String[] args) throws IOException {
   String directoryDataFile  = "Directory.txt";
   Directory d = new Directory(directoryDataFile);
   Scanner stdin = new Scanner(System.in);
   System.out.println("Directory Server is Ready!");
   System.out.println("Format: command name");
   System.out.println("Enter ^Z to end");
   while (stdin.hasNext()) {
      String command = stdin.next();
      String name = stdin.next();
      if (command.equalsIgnoreCase("find")) {
         if (d.inDirectory(name))
            System.out.println(name + " is in the directory");
         else 
            System.out.println(name + " is NOT in the directory");
      }
      else if (command.equalsIgnoreCase("add")) {
         if (d.add(name))
            System.out.println(name + " added");
         else 
            System.out.println(name + " cannot add! " + "no more space or already in directory");
      }
      else if (command.equalsIgnoreCase("delete")) {
         if (d.delete(name))
            System.out.println(name + " deleted");
         else
            System.out.println(name + " NOT in directory");
      }
      else {
         System.out.println("bad command, try again");
      }
   }
   }
}   

推荐答案

此代码:

  while (directoryDataIn.hasNext()) {
     directory[directorySize++] = directoryDataIn.nextLine();
  }

只会填充输入文件中的行(根据您的问题，为11行)中的directory条.

will only fill up as much of directory as there are lines in the input file (11 according to your question).

此代码:

  for (int i = 0; i < directory.length; i++) {
     **if (directory[i].equalsIgnoreCase(name))** 

将循环遍历directory中的每个条目，直到其长度(1024).

will loop over every entry in directory, up to its length (1024).

由于这些条目中的1013是null，因此尝试在它们上运行equalsIgnoreCase()会导致NPE.

Since 1013 of those entries are null, trying to run equalsIgnoreCase() on them will result in a NPE.

您可以通过以下几种方法之一解决此问题.例如，您可以

You can solve this one of several ways. For instance, you could

• 跟踪读取的行数，并且只读取到该点为止
• 在评估每个条目之前检查每个条目是否为null
• 使用动态大小的数据结构代替数组，例如ArrayList
• 对已知值(例如if (name.equalsIgnoreCase(directory[i])))执行检查
• 等

• keep track of the number of lines you read, and only read up to that point
• check each entry to see if it is null before evaluating it
• use a dynamically sized data structure instead of an array, such as ArrayList
• perform the check on the known value (e.g. if (name.equalsIgnoreCase(directory[i])))
• etc.

这篇关于搜索文本文件时抛出`NullPointerException`的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！