而 （！scannerObject.hasNext（patternObject）） 只适用于一次迭代

问：

问题

我只从由条件控制的循环中获得一次成功的迭代。我尝试“清除”输入流 （？），但没有成功。whilehasNext(Pattern pattern).next()

行为

如果我第一次输入无效的输入，看似有效的后续输入会继续执行该行。但是，只有当没有有效输入时，此行才应执行。System.out.print("You did not enter a valid size. Try again: ")Scanner getPizzaSize

相反，如果我第一次输入有效输入，它会起作用并且程序会前进，但是当进行第二次迭代时，所有输入（有效与否）都会执行行。while (orderingPizza)getPizzaSizeSystem.out.print("You did not enter a valid size. Try again: ")

我只从我的循环中得到一个成功的迭代：!getPizzaSize.hasNext(validPizzaSizes)

Please place the order for your pizza.
Size (s = small, l = large, f = family): s
Please place the order for your pizza.
Size (s = small, l = large, f = family): s
You did not enter a valid size. Try again: 

我应该得到：

Please place the order for your pizza.
Size (s = small, l = large, f = family): s
Please place the order for your pizza.
Size (s = small, l = large, f = family): s
Please place the order for your pizza.
Size (s = small, l = large, f = family):

（我使用的正则表达式应该匹配：s、S、l、L、f、F https://regexr.com/5gdla。

解决方案

如果我在循环中重新声明，循环的行为符合预期：Scanner getPizzaSize.hasNext(Pattern pattern)

1. 无效输入后跟 ;和You did not enter valid input . . . try again
2. 有效的输入通过程序前进

我想了解的内容

我如何从 .但我不知道问题出在哪里。这是我的正则表达式吗？关于与 ？对于整数输入，并且可以完美地工作。getPizzaSize.next().next().hasNext(Pattern pattern).hasNextInt().next()

import java.util.Scanner;
import java.util.regex.Pattern;

public class PizzaOrder{

    public static void main(String[] args) {
        // Initialise Scanner object.
        Scanner getPizzaSize = new Scanner(System.in);

        // Initialise Pattern object for detecting valid size input.
        Pattern validPizzaSizes = Pattern.compile("^[slf]$", Pattern.CASE_INSENSITIVE);

        while (true) {
            // Ask for pizza size.
            System.out.print("Please place the order for your pizza.\n" +
                    "Size (s = small, l = large, f = family): ");
            // Validate size input.
            while (!getPizzaSize.hasNext(validPizzaSizes)) {
                System.out.print("You did not enter a valid size. Try again: ");
                getPizzaSize.next();
            }
            // Set pizza size.
            String pizzaSize = getPizzaSize.next();
        }
    }
}