`NullPointerException` thrown while searching text file [closed]

问题

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?

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();
   }
}

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");
      }
   }
   }
}

回答1:

This code:

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

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

This code:

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

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

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

Edit

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

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.

回答2:

Change

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

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

directorySize is already Keeping track of the number of entries so any array entries above that will be null. Therefore trying to call equalsIgnoreCase() on them will get a NPE.

Actually this looks like a prime use for ArrayList rather than array. The list will expand as you need it and List.size() will give you the correct length.

来源：https://stackoverflow.com/questions/29324848/nullpointerexception-thrown-while-searching-text-file

标签

java

java.util.scanner