public void connectDB(JComboBox code)
    {
        try
        {
        Class.forName("com.mysql.jdbc.Driver");
        Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/stock","root","");
        String query = "SELECT stockCode FROM stock_master";
        Statement stmt = con.createStatement();
        ResultSet rs = stmt.executeQuery(query);
            while(rs.next())
            {
                int stockCode = rs.getInt(1);
                //String description = rs.getString(2);
                //int unitSize = rs.getInt(3);
                //String unitOfMeasure = rs.getString(4);
                //double price = rs.getDouble(5);
                //int quantityInStock = rs.getInt(6);
                code.setSelectedItem(Integer.toString(stockCode));

            }
            con.close();
        }
        catch(Exception e)
        {
            JOptionPane.showMessageDialog(null, "This is the error: " + e);
        }
    }

buttonConnectToDB.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e)
            {
                connectDB(comboBoxStockCode);
            }
        });