How to call DB Function with select query

    public String getFaAcUnitandMainAcCombValid(String compCode, String unitCode,String mainAcCode) {
        String valid = null;
        String sqlproc = "select Fa_chk_valid_acunit_comb(?,?,?) from dual";
        PreparedStatement preparedStatement = this.getDBTransaction().createPreparedStatement(sqlproc, 0);
        try {
            preparedStatement.setString(1, compCode);
            preparedStatement.setString(2, unitCode);
            preparedStatement.setString(3, mainAcCode);
            ResultSet rs = preparedStatement.executeQuery();
            rs.next();
            valid = rs.getString(1);
            preparedStatement.close();
        } catch (SQLException sqle) {
            sqle.printStackTrace();
        }
        return valid;
    }