从Java中的文本文件删除特定行

Question

I am setting up a rank system where each member has a username and a rank. The program reads the username and rank from a text file and assigns it to the user. One username and rank per line, such as:

user1 1
user2 2
user3 3

I have set up a program to add usernames and ranks to the text file, however I cannot seem to figure out how to delete a specific user from the list, such as if I wanted to only delete user 2 and his/her rank and leave the other two, however it is important that afterwards there isn't a blank line left behind.

Just for reference here is the code for how I write it to the file in the first place:

             try {
                 BufferedWriter out = new BufferedWriter(new FileWriter("stafflist.txt", true));
                 for (int i = 0; i < 1; i++) {
                 out.newLine();
                 out.write(target.getUsername() + " " + target.getRights());
                 }
             out.close();
             SerializableFilesManager.savePlayer(target);
                    if (loggedIn) {
                    target.getPackets().sendGameMessage(modString + Utils.formatPlayerNameForDisplay(member.getUsername()) + "!", true);}
             member.getPackets().sendGameMessage(successString + Utils.formatMemberNameForDisplay(target.getUsername()) + " to a Moderator.",true);
             loggedIn = false;
                } catch (IOException e) {
                System.out.println("GiveMod - Can't find stafflist.txt");
                }
             return true;

Answer 1

You cannot delete data from the middle of a file (without leaving nulls ). You need to rewrite at least what underneath it. A Simple solution would be loading everything in memory, remove that line and dump the collection again.

An alternative solution would be to:

1. Open a FileChannel from a RandomAccessFile
2. read the file line by line and keep the file-pointer of the line head. fileChannel.position();file.readLine(); load what comes after that into a collection. truncate the file from that position file.setLength(linePosition); and then dump the collection at the end of the file.

If your data doesn't fit in memory then you can use a temp file instead of a collection. Create a temp-file File.createTempFile(...) , read the remaining data line by line and write to temp, truncate the original file ,read temp line by and write to original.

OR, guess what, use a database.

Answer 2

There seems to be an issue in your for loop. It is looping between 0 and 1, so I think the output you posted is incorrect. Anyway, if you want to only print out certain lines you can filter it as follows:

 for (int i = 0; i < 1; i++) {
        if(!target.getUsername().equals("user2")){
             out.newLine();
             out.write(target.getUsername() + " " + target.getRights());
        }
 }

Answer 3

将文件读入某个Collection中，删除所需的用户，然后使用修改后的Collection重写该文件。