Using something else instead of String

I have a big file and I want to do some „operations” on it.(find some text, check if some text exists, get the offset of some text, maybe changing the file).

My current aproach is this:

public ResultSet getResultSet(String fileName) throws IOException {

    InputStream in = new FileInputStream(fileName);

    byte[] buffer = new byte[CAPACITY];
    byte[] doubleBuffer = new byte[2 * CAPACITY];


    long len = in.read(doubleBuffer);
    while (true) {
        String reconstitutedString = new String(doubleBuffer, 0 ,doubleBuffer.length);

        //...do stuff

        ByteArrayOutputStream os = new ByteArrayOutputStream();
        os.write(doubleBuffer, CAPACITY, CAPACITY);
        readUntilNow += len;
        len = in.read(buffer);
        if (len <= 0) {
            break;
        }
        os.write(buffer, 0, CAPACITY);
        doubleBuffer = os.toByteArray();
        os.close();
    }
    in.close();
    return makeResult();

}

I would like to change the String reconstitutedString into something else. What would be the best alternative considering I want to be able to get some information about the content of that data, information that I may get calling an IndexOf on a String