public void deleteRec(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_transaction WHERE stockCode = '" + code.getSelectedItem() + "'";
            Statement stmt = con.createStatement();
            ResultSet rs = stmt.executeQuery(query);
            if(!rs.next())
            {
                PreparedStatement st = con.prepareStatement("DELETE FROM stock_transaction WHERE stockCode = ?");
                st.setString(1, (String)code.getSelectedItem());
                st.executeUpdate();
            }
            con.close();
        }
        catch(Exception e)
        {
            JOptionPane.showMessageDialog(null, "This is the error: " + e);
        }
        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);
            if(!rs.next())
            {
                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 is the error: " + e);
        }

    }

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