public int compareTo(Osoba other)
    {
        Student otherStudent = (Student) other;
        if((super.compareTo(otherStudent)) == 0){
            if ((this.sredniaOcen == otherStudent.sredniaOcen)) {
                return 0;
            }
            else if(this.sredniaOcen < otherStudent.sredniaOcen) return -1;
            else return 1;
        } else return super.compareTo(otherStudent);
    }