有没有办法修改文件并从具有新内容的新文件中读取？

Question

When I delete a line from a text file, add it to a temporary text file, then replace the temporary file with the original, the scanner isn't loading the correct text. It is loading the content from the original file, not the updated file. I want it to load the updated file with the new content.

try
{
    File inputFile = new File("department.txt");
    File tempFile = new File("departmentTemp.txt");

    BufferedReader reader = new BufferedReader(new FileReader(inputFile));
    BufferedWriter writer = new BufferedWriter(new FileWriter(tempFile));

    String lineToRemove = departResIDField.getText();
    String currentLine;

    while((currentLine = reader.readLine()) != null) 
    {
         String trimmedLine = currentLine.trim();
         if(trimmedLine.startsWith(lineToRemove)) continue;
         writer.write(currentLine + System.getProperty("line.separator"));
    }

    writer.close(); 
    reader.close(); 
    inputFile.delete();
    tempFile.renameTo(inputFile);

    fWriter = new FileWriter(inputFile.getAbsoluteFile(), true);
    printOut = new PrintWriter(fWriter);
    writer = new BufferedWriter(fWriter);
    writer.write(departResIDField.getText() + "\t" + 
    departResNameField.getText() + "\r\n");
    writer.close();

    int result = Integer.parseInt(departResIDField.getText());
    depart.add(new Department(result, departResNameField.getText()));

    insResDepartModel.addElement(departResNameField.getText());

    depart.remove();

    //This is where I am having problem with the code
    //The scanner is scanning old data not the updated data             
    Scanner scan = new Scanner(inputFile);

    while(scan.hasNext())
    {
        depart.add(new Department(scan.nextInt(), scan.next()));
    }
    scan.close();

    for(int i = 0; i < depart.getSize(); i++)
    {
        Department d1 = depart.get(i);
        System.out.println(d1.departID + d1.departName);
    }

    menuLayout.show(cardLayout, "15");
    departResIDField.setText(null);
    departResNameField.setText(null);
    departUpdateButton.setVisible(false);
    departCancelButton.setVisible(false);
} catch (IOException f) 
    {
        f.printStackTrace();
    }

Output: 1 Biology 2 Chemistry 4 Statistics 5 Physics 3 Math

Expected Output: 1 Biology 2 Chemistry 4 Statistics 5 Physics 3 Calculus

Code removes "3 Math" and adds "3 Calculus

Answer 1

Of course there is a way. But I would recommend you to save with something like a LinkedList , on an text file if you want only save subjects, and try to read and write only one File . You can save complex data structures with ObjectOutputStream for example. LinkedList java utility has an instruction named replace to modify certain index. So if you want to modify the File you load the LinkedList saved then you modify the list and save it in the same file.

Answer 2

From my understanding, you want to replace the line containing "3 Math" with "3 Calculus". If that's the case you can do something like this:

    Path wiki_path = Paths.get("data/sample.txt");
    Charset charset = Charset.forName("UTF-8");

    FileInputStream fis = null;
    FileOutputStream fos = null;
    try {
      List<String> lines = Files.readAllLines(wiki_path, charset);
      int targetToReplaceIndex = -1;

      // You can modify this part to match your logic
      for (int i = 0; i < lines.size(); i++) {
          if (lines.get(i).startsWith("3 Math")) {
              targetToReplaceIndex = i;
              break;
          }
      }
      lines.set(targetToReplaceIndex, "3 Calculus");

      File temp = File.createTempFile("temp", ".tmp");
      temp.deleteOnExit();

      BufferedWriter out = new BufferedWriter(new FileWriter(temp));

      out.write(convertListToContent(lines));
      out.close();

      File orig = new File("data/sample.txt");

      fis = new FileInputStream(temp);
      fos = new FileOutputStream(orig);

      FileChannel src = fis.getChannel();
      FileChannel dest = fos.getChannel();
      dest.transferFrom(src, 0, src.size());

    } catch (IOException e) {
      e.printStackTrace();
    } finally {
        try {
            fis.close();
            fos.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

The method to convert a list of Strings into a single String is pretty straight forward using Java 8:

private static String convertListToContent(List<String> lines) {
    StringBuilder contentBuilder = new StringBuilder();

    lines.forEach(l -> contentBuilder.append(l).append("\n"));

    return contentBuilder.toString();
}

Hope it helps.