Append text to existing line in a file

Question

I'm working on a school project right now, and I amseriously struggling trying to append text to an existing line in my .txt file. What I would like to see is that if a "show name" matches with one in the .txt file the next line which contains [HOST] a username will be added. I've so far created this method:

    public void joinAShow(String username, String showName) throws IOException{
    keepGoing = true;
    keepGoing2 = true;
    while((line = bufferedReader2.readLine()) != null && keepGoing){
        if(line.equals(showTitle)){
            while((line = bufferedReader2.readLine()) != null && keepGoing2){
                if(line.contains("[HOST]")){
                    line += ", " + username;
                }
            }
        }
        keepGoing = false;
        keepGoing2 = false;
    }
}

At this point I'm quite certain I can't do it because I'm using a reader of some sort, and not a writer, but I wouldn't know how to come around this. I'm quite stuck and has been for a time at this point. I hope some clever minds can tell an quite easy way to come around this. My data set looks like the following:

Stream1
Review
Stream
2020-10-10 10:00
90
0.0
admin[HOST]
Stream2
Review
Stream
2020-10-10 10:00
90
0.0
admin[HOST]
Stream3
Review
Stream
2020-10-10 10:00
90
0.0
admin[HOST]

I really hope some one can help. Thanks in advance!

Answer 1

Try this:

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.List;

public class Demo {

    public static void main(String[] args) throws IOException {

        // Update line number 7 and append Test to it
        updateLine(7, "Test");
    }

    public static void updateLine(int lineNumber, String data) throws IOException {
        // Path of file
        Path path = Paths.get("etc/demo.txt");
        // Read all lines
        List<String> lines = Files.readAllLines(path, StandardCharsets.UTF_8);
        // Update line 7 - Replace admin HOST with admin HOST Test
        lines.set(lineNumber - 1, lines.get(lineNumber - 1) + " " + data);
        // Write back to the file
        Files.write(path, lines, StandardCharsets.UTF_8);
    }
}

Before demo.txt file:

After demo.txt file:

Explanation:

1. Reading data line by line from file demo.txt file.
2. Updated Line 7 and replace admin HOST with admin HOST Test.
3. Write data back to the file.

Note: This is an example that will help you to update code as per your requirement.