搜索文本文件时抛出`NullPointerException`[关闭]

Question

首先，我在下面 ** 的行中不断收到 NullPointerException。 其次，我的程序给出了错误的输出（我以某种方式让它工作，但后来又回到错误）。一定是逻辑错误。我有一个 11 行的文件 directory.txt，每行都有一个名称。当我运行我的程序以尝试查找某个名称时，它只在第一行找到第一个名称，而其他所有内容都找不到。如何修复这 2 个错误？

我有 2 节课。这是一级目录：

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();
   }
}

这是我尝试运行的第二个课程，但我不断收到异常主线程 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");
      }
   }
   }
}   

Answer 1

改变

for (int i = 0; i < directory.length; i++) {

到

for (int i = 0; i < directorySize; i++ ){

directorySize 已经在跟踪条目的数量，因此任何高于该数量的数组条目都将为空。因此，尝试对它们调用 equalsIgnoreCase() 将获得 NPE。

实际上，这看起来像是 ArrayList 而不是数组的主要用途。该列表将根据您的需要扩展，List.size() 将为您提供正确的长度。

Answer 2

这段代码：

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

只会填充与输入文件中的行一样多的directory（根据您的问题为 11 行）。

这段代码：

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

将遍历directory 中的每个条目，直至其长度 (1024)。

由于这些条目中有 1013 个是 null，因此尝试在它们上运行 equalsIgnoreCase() 将导致 NPE。

编辑

您可以通过以下几种方法之一来解决此问题。例如，你可以

• 跟踪您阅读的行数，并且只阅读到该点
• 在评估之前检查每个条目是否为null
• 使用动态大小的数据结构而不是数组，例如ArrayList
• 检查已知值（例如if (name.equalsIgnoreCase(directory[i]))）
• 等