Check if a new line exists at end of file

Question

I created a simple method to append text to a file:

void writeFile(String fileName, String... values) {
    File file = new File(fileName);
    file.getParentFile().mkdirs();
    try(BufferedWriter bw = new BufferedWriter(new FileWriter(file, true))) {
        for (String value: values) {
            bw.write(value);
            bw.newLine();
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}

However, I am at a loss on how to check if a new line exists at the end of the file before writing to ensure that it is stored properly.

Also, it would have to be scalable for when working with larger files.

possible duplicate of [Quickly read the last line of a text file?](http://stackoverflow.com/questions/686231/quickly-read-the-last-line-of-a-text-file) — SMA, Mar 01 '15 at 15:42
Also, don't forget to close your writer (`bw.close()`), which will flush the stream making sure everything is put to the file before the program ends. — jas, Mar 01 '15 at 15:44
@KSFT Before, because after it would have a new line from bw.newLine() — Jsilvermist, Mar 01 '15 at 15:48
@jas When using the try with resources syntax wouldn't be closed automatically? — Jsilvermist, Mar 01 '15 at 15:49
@Jsilvermist Yes it will. I think jas overlooked the try-with-resources statement — Erwin Bolwidt, Mar 01 '15 at 15:56
Indeed I did miss that, sorry!! No need for the explicit close. — jas, Mar 01 '15 at 15:59
Since you write a newline at the end of the file, why do you worry at all about it? — fge, Mar 01 '15 at 16:19
@fge Because the file it's writing to might not always have a new line, since it could be user editable. — Jsilvermist, Mar 01 '15 at 16:21

score 9 · Accepted Answer · edited Feb 17 '23 at 15:20

Thanks to SMA linking to Quickly read the last line of a text file?, I was able to create a method to check if a new line exists, and then create a new line if it doesn't.

static void writeFile(String fileName, String... values) {
    File file = new File(fileName);
    file.getParentFile().mkdirs();
    boolean fileExists = file.exists();
    try (BufferedWriter bw = new BufferedWriter(new FileWriter(file, true))) {
        if (fileExists && !newLineExists(file)) {
            bw.newLine();
        }
        for (String value : values) {
            bw.write(value);
            bw.newLine();
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}

static boolean newLineExists(File file) throws IOException {
  try (RandomAccessFile fileHandler = new RandomAccessFile(file, "r")) {
    long fileLength = fileHandler.length() - 1;
    if (fileLength < 0) {
      return true;
    }
    fileHandler.seek(fileLength);
    byte readByte = fileHandler.readByte();

    return readByte == 0xA || readByte == 0xD;
  }
}

Check if a new line exists at end of file

1 Answers1