private void add_btnActionPerformed(java.awt.event.ActionEvent evt) {

            String temp = (String) IDcombo.getSelectedItem();
            String temp2 = (String) Namecombo.getSelectedItem();

            String sql = "select ItemID,ItemName,CostPrice from druginfo where ItemID=?";
        try {
            pst=conn.prepareStatement(sql);
            pst.setString(1, temp);


            rs=pst.executeQuery();
            addDataToTable(tableSale,DbUtils.resultSetToTableModel(rs));

           //here is the table model listener code i added,
            tableSale.getModel().addTableModelListener( new TableModelListener() {

            @Override
            public void tableChanged(TableModelEvent e) {
                int row = e.getFirstRow();
                int column = e.getColumn();
                if (column == 6 ) {
                    TableModel model = tableSale.getModel();

                    double cost = ( model.getValueAt(row, 3)==null)?0:((double)model.getValueAt(row, 3));
                    double quantity = ( model.getValueAt(row, 4)==null)?0:((double) model.getValueAt(row, 4));
                    double discount = ( model.getValueAt(row, 5)==null)?0:((double) model.getValueAt(row, 5));


                    double total = new Double ((quantity * cost)-discount);
                    System.out.println("pppppppp"+total);
                    model.setValueAt(total, row, 6);

                }
            }

        });


            IDcombo.setSelectedItem(null);
            Namecombo.setSelectedItem(null);
            exptxt.setText(null);
            instock.setText(null);




        } catch (Exception ex) {
           JOptionPane.showMessageDialog(null, "errorrr"+ex);
        }




    }