public int addComputer(Computer com) {
        int n = 0;
        //create sql string
        String preSQL = "INSERT INTO Computer (cid, cname, quantity, price, functions) VALUES (?,?,?,?,?)";
        try {
            PreparedStatement pre = conn.prepareStatement(preSQL);
            pre.setInt(1, com.getCid());
            pre.setString(2, com.getCname());
            pre.setInt(3, com.getQuantity());
            pre.setDouble(4,com.getPrice());
            pre.setString(5, com.getFucntions());

            n = pre.executeUpdate();
        } catch (SQLException ex) {
            Logger.getLogger(DAO_Computer.class.getName()).log(Level.SEVERE, null, ex);
        }
        return n;
    }