try{
     String url="jdbc:sqlserver://LENOVO-PC\SQL2012:1433;databaseName=qseal" ;
     String user="sa";
     String pass="123";
     Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");
     Connection con=DriverManager.getConnection(url,user,pass);
     String sql="update inventorytable set quntity=(quantity-?) where product_code=?";
     PreparedStatement pst=con.prepareStatement(sql);
     pst.setInt(1,Integer.parseInt(Quantity.getText()));
     pst.setString(2,ProductCode.getText());
     Quantity.setText("");
     ProductCode.setText("");
     close();
     GenerateBill gb=new GenerateBill();
     gb.setVisible(true);
}
catch(Exception e){
     JOptionPane.showMessageDialog(this, e.getMessage());
}

"update inventorytable set quntity=(quantity-" + UserData + ") where product_code=?"