我不明白为什么用数组覆盖文件的内容会导致文件空

Question

所以我修复了覆盖问题，但现在的问题是它占用了文本文件的最后一行。或者我想它只是没有把它写回文件。有什么想法吗？第一种方法应该将密码更改为随机密码，第二种方法只用于计算文本文件中的行数。

/**
 *  Method for resetting a password
 * Replaces current password with randomly generated one.
 * 
 *  @param testUserName is the username used for finding the password
 *       to replace
 */
public String resetPassword(String testUserName) throws IOException  {
    int fileLength = countLines();
    FileReader fr = new FileReader("Password.txt");
    BufferedReader br = new BufferedReader(fr);
    String[] storeData = new String[fileLength];
    Random rand = new Random();
    int pwNumber = (int)(rand.nextDouble() * 10000);
    String pwReset = "Password" + pwNumber;
    for (int i=0; i < fileLength; i ++) {
        storeData[i] = br.readLine();
    }
    fr.close();     
    for (int i=0; i < (fileLength-1); i += 4) {
        if (testUserName.equals(storeData[i])) {
            storeData[i+1] = pwReset;
        }   
    }
    PrintWriter reset = new PrintWriter("Password.txt");
    for (int i=0; i < fileLength; i ++) {
        reset.println(storeData[i]);
    }
    reset.close();
    return pwReset;
}
/**
 *  Method for counting number of lines in a file
 *  Used in conjuction with other methods for reading
 * specific lines of files.
 */
public int countLines() throws IOException {
   InputStream is = new BufferedInputStream(new FileInputStream("Password.txt"));
   try {
      byte[] c = new byte[1024];
      int count = 0;
      int readChars = 0;
      boolean empty = true;
      while ((readChars = is.read(c)) != -1) {
         empty = false;
         for (int i = 0; i < readChars; ++i) {
            if (c[i] == 'n')
               ++count;
        }       
      }
      return (count == 0 && !empty) ? 1 : count;
    } finally {
      is.close();
   }
}

Answer 1

我怀疑countLines（）方法正在重新读取文件，以确定此时的行数。这不仅效率很低，而且也是您的bug的来源。

PrintWriter reset = new PrintWriter("Password.txt");
// file is now truncated.
countLine() == 0

我建议你数一次行数。最好是读一遍。

---

你可以通过进行改进

• 读取该文件一次
• 不将文件存储在内存中，所以不管文件有多大
• 如果未能写入文件，则原始文件将不受影响

public String resetPassword(String testUserName) throws IOException {
    File passwdFile = new File("Password.txt");
    BufferedReader br = new BufferedReader(new FileReader(passwdFile));
    File tmpPasswdFile = new File("Password.txt.tmp");
    PrintWriter reset = new PrintWriter(tmpPasswdFile);
    String pwReset = null;
    try {
        boolean resetNextLine = false;
        for (String line; (line = br.readLine()) != null; ) {
            if (resetNextLine) {
                Random rand = new Random();
                int pwNumber = (int) (rand.nextDouble() * 10000);
                pwReset = "Password" + pwNumber;
                reset.println(pwReset);
            } else {
                reset.println(line);
            }
            resetNextLine = testUserName.equals(line);
        }
    } finally {
        reset.close();
        br.close();
    }
    passwdFile.delete();
    tmpPasswdFile.renameTo(passwdFile);
    return pwReset;
}

Answer 2

countLines()是如何工作的？

我对这种方法有点怀疑，因为您正在使用这种方法的结果来迭代您的数组。我认为使用的长度迭代数组可能更直观（正确吗？）。