protected function validateSentence(string $sentence): void
    {
        if (empty($sentence)) {
            throw new EmptySentenceException("Sentence can not be empty");
        }

        $sentenceChars = str_split($sentence);
        if (count(array_diff($sentenceChars, $this->availableChars))) {
            throw new InvalidCharactersException("Sentence contains invalid symbols");
        }
    }