copying a file from assets folder to /sdcard and randomize lines in it

I have a text file to load from assets folder of an android app to "/sdcard/appname/" but i want the lines to be randomize and save while copied to sdcard, here is the code that i write to copy but i struck at the part of how to randomize

    data_path="/sdcard/<appname>/"
    AssetManager assetManager = getAssets();
            InputStream in = assetManager.open("file.txt");
            OutputStream out = new FileOutputStream(data_path + "file.txt");

            // Transfer bytes from in to out
            byte[] buf = new byte[1024];
            int len;
            //while ((lenf = gin.read(buff)) > 0) {
            while ((len = in.read(buf)) > 0) {
                out.write(buf, 0, len);
            }
            in.close();
            //gin.close();
            out.close();

How can i randomize the lines while copying (i'm not a java guy started working on it recently so this is kind of a noob question)