Java: get quantity of a product from the Database

I am learning on my own to do a management program for the sale of beverages.

I have a table called VENTAS_HOY that stores what products were sold, as well as their quantity.

It works for me, until it reaches the point of entering the table some product that is already. For example, I want to enter 5 Coca Colas, and in the table there are 2 for example. What I do not achieve is to do by some method, that I return the number 2 that is already in the table, to operate with it, and then update.

The table has 3 fields: Product, Quantity, Price. (The Beverages class has a constructor, among others, that accepts these 3 parameters for instantiation).

I currently have something like that ... But it does not work, or I can not understand it, it always returns 0. (Since I know that the product is already in the table and has a certain amount).

Can someone help me how to get this method to return an INT with the quantity of a product in the SALES_HOY table?

public int cantidadVenta (Bebidas vo){
 int cantidadVenta = vo.getCantidadCarrito();

 DBCon conec = new DBCon();
    String sql = "SELECT COUNT(cantidad) FROM VENTAS_HOY WHERE producto = ? ";
    PreparedStatement ps = null;
     try {
            ps = conec.Connect().prepareStatement(sql);
            ps.setInt(1, vo.getCantidadCarrito());
            ResultSet rs = ps.executeQuery();
            if (rs.next()){
            cantidadVenta=rs.getInt(1);
            }
            /*
            if (rs!= null ){                
        Object[] fields = (Object[]) rs.getObject(0); 
        cantidadVenta = Integer.parseInt((String) fields[0]);
        System.out.println("DAO variable CantidadVenta en VENTAS HOY es: "+cantidadVenta);
    }
            */


        } catch(SQLException e) {
            Logger.getLogger(DAO.class.getName())
                    .log(Level.SEVERE, "Error en consulta", e);
        }

 System.out.println("DAO variable CantidadVenta en VENTAS HOY es: "+cantidadVenta);
 return cantidadVenta;
}