IsEmailInUseMethod

 public static boolean isEmailInUse(String aEmail) throws IOException {

        boolean found = false;
        File aFile = new File("users.txt");
        if (!aFile.exists())
            return false;
        else {
            Scanner in = new Scanner(aFile);
            String fileElements[];
            while (in.hasNext()) {
                fileElements = (in.nextLine()).split(",");
                if ((fileElements[0]).equals(aEmail))
                    found = true;
                JOptionPane.showMessageDialog(null, "Email already in use");
            }
        }
        if (!found)
            return true;
        else
            return false;
    }