@Override
    public String toString() {
        String res = "";

        res += this.inventory.toString();
        res += "\n\n";
        res += "--- USERS ---\n";

        res += "\nManagers\n";
        for (User u : this.managers) {
            res += "\t" + u.toString() + "\n";
        }

        res += "\nStudents\n";
        for (User u : this.students) {
            res += "\t" + u.toString() + "\n";
        }

        res += "\nTeachers\n";
        for (User u : this.teachers) {
            res += "\t" + u.toString() + "\n";
        }

        res += "\n--- VALIDATION LOANS ---\n\n";
        for (Loan l : this.ongoingValidationLoans) {
            res += "\t" + l.toString() + "\n";
        }

        return res;
    }