替换文本文件的一行

Question

I'm trying to read the first line of a text file and then replacing that line with something different. The first line is an int value, so first I changed it to +1 of what is currently.

BufferedWriter writer = new BufferedWriter( new FileWriter("file.txt", true)); 

BufferedReader reader = new BufferedReader( new FileReader("file.txt"));

String amount;

amount = reader.readLine();

int amount1 = Integer.parseInt(amount);
amount1 = amount1 + 1;
String amount2 = String.valueOf(amount1);
amount = amount.replace(amount, amount2);

writer.write(amount);

Instead of replacing the original value with the new value, it just writes the new value next to the old one.

Answer 1

Well, you told the writer to append instead of replace via the second constructor argument of the FileWriter :

BufferedWriter writer = new BufferedWriter( new FileWriter("file.txt", true)); 

If you want to replace the file, you have to set append to false:

BufferedWriter writer = new BufferedWriter( new FileWriter("file.txt", false)); 

However, this replaces the entire file not just the first line.

Answer 2

If you are using a recent version of Java, you can do this:

try (BufferedReader reader = new BufferedReader( new FileReader("file.txt"));
        Writer writer = new FileWriter("newfile.txt")) {
    String firstLine = reader.readLine();
    firstLine = Integer.toString(Integer.parseInt(firstLine) + 1);
    writer.writeLine(firstLine);
    reader.transferTo(writer);
}

And then replace file.txt with newfile.txt

Answer 3

You must replace the entire file when changing a text file.

Path path = Paths.get("file.txt");
Charset charset = Charset.defaultCharset();
List<String> lines = Files.readAllLines(path, charset);
String firstLine = lines.get(0);
...
lines.set(0, firstLines);
Files.write(path, lines, charset);

However if you take care that the first line never changes in length, and has enough room, you could use a RandomAccessFile , and only change that piece of text.