Find a line in a file and remove it

Question

I\'m looking for a small code snippet that will find a line in file and remove that line (not content but line) but could not find. So for example I have in a file following

Answer 1

Here you go. This solution uses a DataInputStream to scan for the position of the string you want replaced and uses a FileChannel to replace the text at that exact position. It only replaces the first occurrence of the string that it finds. This solution doesn't store a copy of the entire file somewhere, (either the RAM or a temp file), it just edits the portion of the file that it finds.

public static long scanForString(String text, File file) throws IOException {
    if (text.isEmpty())
        return file.exists() ? 0 : -1;
    // First of all, get a byte array off of this string:
    byte[] bytes = text.getBytes(/* StandardCharsets.your_charset */);

    // Next, search the file for the byte array.
    try (DataInputStream dis = new DataInputStream(new FileInputStream(file))) {

        List matches = new LinkedList<>();

        for (long pos = 0; pos < file.length(); pos++) {
            byte bite = dis.readByte();

            for (int i = 0; i < matches.size(); i++) {
                Integer m = matches.get(i);
                if (bytes[m] != bite)
                    matches.remove(i--);
                else if (++m == bytes.length)
                    return pos - m + 1;
                else
                    matches.set(i, m);
            }

            if (bytes[0] == bite)
                matches.add(1);
        }
    }
    return -1;
}

public static void replaceText(String text, String replacement, File file) throws IOException {
    // Open a FileChannel with writing ability. You don't really need the read
    // ability for this specific case, but there it is in case you need it for
    // something else.
    try (FileChannel channel = FileChannel.open(file.toPath(), StandardOpenOption.WRITE, StandardOpenOption.READ)) {
        long scanForString = scanForString(text, file);
        if (scanForString == -1) {
            System.out.println("String not found.");
            return;
        }
        channel.position(scanForString);
        channel.write(ByteBuffer.wrap(replacement.getBytes(/* StandardCharsets.your_charset */)));
    }
}

Example

Input: ABCDEFGHIJKLMNOPQRSTUVWXYZ

Method Call:

replaceText("QRS", "000", new File("path/to/file");

Resulting File: ABCDEFGHIJKLMNOP000TUVWXYZ