I guess that's stupid question, but why are you using

    if (current.keySet().stream().filter(x -> !x.equals(END)).findAny().isPresent()) {
        System.out.printf(BAD_SET_FORMAT, word);
        return;
    }

    if (current.containsKey(END)) {
        System.out.printf(BAD_SET_FORMAT, word
        return;
    }

instead of

    if (!current.isEmpty()){
        System.out.printf(BAD_SET_FORMAT, word
        return;
    }