Srednia

 public double getSrednia() {
        double srednia = 0;
        double counter = 0;
        String temp;
        for (int i = 0; i < ocena.length; i++) {
            temp = ocena[i].value();
            char[] chars = temp.toCharArray();
            for (char c : chars) {
                if (Character.isDigit(c)) {
                    counter++;
                    srednia = srednia + Character.getNumericValue(c);
                }
            }
        }
        return srednia / counter;
    }