public static boolean isOk(String s, ArrayList<String> guessed) {
        for (String i : guessed) { if (i.equals(s)) { return false; } }
        return (s.length() == 1 && isAlpha(s));
    }

//

                while (!isOk(userGuess, guessed)) {
                    if (!userGuess.equals("--")) {
                        System.out.println("Guess invalid.");
                    }
                    System.out.println("Guess a letter: ");
                    userGuess = s.nextLine().toLowerCase();
                }