public void deleteRec(JComboBox code)
    {
        boolean flag1 = false;
        boolean flag2 = false;
        try
        {
            Class.forName("com.mysql.jdbc.Driver");
            Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/stock","root","");
            String query = "SELECT stockCode FROM stock_transaction WHERE stockCode = '" + code.getSelectedItem() + "'";
            Statement stmt = con.createStatement();
            ResultSet rs = stmt.executeQuery(query);
            while(rs.next())
            {

            }
            con.close();
        }
        catch(Exception e)
        {
            flag1 = true;
        }
        try
        {
            Class.forName("com.mysql.jdbc.Driver");
            Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/stock","root","");
            String query = "SELECT stockCode FROM stock_master WHERE stockCode = '" + code.getSelectedItem() + "'";
            Statement stmt = con.createStatement();
            ResultSet rs = stmt.executeQuery(query);
            while(rs.next())
            {

            }
            con.close();
        }
        catch(Exception e)
        {
            flag2 = true;
        }
        if(flag1 == true && flag2 == true)
        {
            try
            {
            Class.forName("com.mysql.jdbc.Driver");
            Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/stock","root","");
            PreparedStatement st = con.prepareStatement("DELETE FROM stock_master WHERE stockCode = ?");
            st.setString(1, (String)code.getSelectedItem());
            st.executeUpdate();
            con.close();
            }
            catch(Exception e)
            {
                JOptionPane.showMessageDialog(null, "This code is already in the transaction table and cannot be deleted ");
            }
        }
    }

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