Java replace line in a text file

Question

I found this code from another question

private void updateLine(String toUpdate, String updated) throws IOException {
    BufferedReader file = new BufferedReader(new FileReader(data));
    String line;
    String input = "";

    while ((line = file.readLine()) != null)
        input += line + "\n";

    input = input.replace(toUpdate, updated);

    FileOutputStream os = new FileOutputStream(data);
    os.write(input.getBytes());

    file.close();
    os.close();
}

This is my file before I replace some lines

example1
example2
example3

But when I replace a line, the file now looks like this

example1example2example3

Which makes it impossible to read the file when there are a lot of lines in it.

How would I go about editing the code above to make my file look what it looked like at the start?

Answer 1

Use System.lineSeparator() instead of \n.

while ((line = file.readLine()) != null)
    input += line + System.lineSeparator();

The issue is that on Unix systems, the line separator is \n while on Windows systems, it's \r\n.

In Java versions older then Java 7, you would have to use System.getProperty("line.separator") instead.

As pointed out in the comments, if you have concerns about memory usage, it would be wise to not store the entire output in a variable, but write it out line-by-line in the loop that you're using to process the input.