我是java编程新手。如果用户输入错误，我希望程序再次请求输入。我该怎么办？请帮帮我！如果你想避免混乱，就跳到“如果”部分。。。不要粗鲁，但如果你不能回答，请不要要求结束问题。

case 'A': case 'a':
            System.out.println("You selected NOS Tank.");
            int price;
            double quantity;
            double variant=0; 
            System.out.println("Select the variant: ");                                                    
            System.out.println("Enter 'D' without apostrophe for dry or 'W' for wet");
            variant=xss.next().charAt(0);                                                                  
            if (variant=='D' || variant=='d')
            {
                System.out.println("The price of dry nitrous oxide system is: $600");
            }
            else if (variant=='w' || variant=='W')
            {
                System.out.println("The price of wet nitrous oxide is: $740");
            }
            else 
            {
                System.out.println("Invalid input.");                                                      
            }