while循环，switch语句

Question

输入“ p”大小写时出现问题，完成后它再次重复我的while循环的顶部，最终用户必须在其中输入“ input”变量。 问题是用户永远不会被提示输入任何内容作为“输入”，然后再次循环，重新打印以前刚刚打印的所有信息。 围绕它的第二次确实允许用户放入一些东西作为“输入”。 这仅在我运行“ p”情况后发生。

有人请帮助我！ 将不胜感激！

package PayandGoParking;

/**
 * Purpose: Input:
 *
 * Processing done:
 *
 * Output:
 *
 * @author (your name)
 * @version (a version number or a date)
 */
import java.util.Scanner;
import java.util.*;
import java.io.*;
import java.text.DecimalFormat;

public class Assignment2 {

 public static void main(String[] args) throws IOException {

  Calendar now = Calendar.getInstance();
  Scanner k = new Scanner(System.in);
  DecimalFormat dm = new DecimalFormat("00");

  String input = " ",
    cardNumber,
    x = "x";
  double anyKey;

  double currentTime,
    currentCharge = 0,
    securityCode,
    totalCharge = 0;

  Calendar paidTill;

  int min = 0;

  while (input != x) {
   System.out.println("Welcome to Pay and Go parking");
   System.out.println("+ to add 30 minutes ($1.00 charge)");
   System.out.println("p to print the ticket");
   System.out.println("r to restart");
   System.out.println("x to exit the program");
   System.out.print("Please enter your selection: ");
   input = k.nextLine();
   switch (input) {
    case "+":
     min += 30;
     currentCharge += 1;
     paidTill = (Calendar) now.clone();
     paidTill.add(Calendar.MINUTE, min);
     System.out.println("================================================");
     System.out.printf("%s%s%s%s%s%.2s%s%.2s\n", 
       "Current Time: ", now.get(Calendar.MONTH),
       "-", now.get(Calendar.DAY_OF_MONTH), " ", now.get(Calendar.HOUR_OF_DAY), ":", 
       dm.format(now.get(Calendar.MINUTE)));
     System.out.printf("%s%s%s%s%s%.2s%s%.2s\n", 
       "Paid Till: ", paidTill.get(Calendar.MONTH),
       "-", paidTill.get(Calendar.DAY_OF_MONTH),
       " ", paidTill.get(Calendar.HOUR_OF_DAY), ":",
       dm.format(paidTill.get(Calendar.MINUTE)));
     System.out.printf("%s%.2f\n", "Current Charge $", currentCharge);
     System.out.println("================================================");
     break;

    case "p":
     System.out.print("Enter Credit Card Number: ");
     cardNumber = k.nextLine();
     System.out.print("Enter 3 digit security code from back of Credit Card: ");
     securityCode = k.nextDouble();
     System.out.println("=================================================");
     System.out.println("Pay and Go Parking");
     System.out.println("Lot Location: 123 Main St,");
     System.out.println("Honolulu, Hawaii");
     System.out.println("-----------------------------");
     totalCharge += currentCharge;
     paidTill = (Calendar) now.clone();
     paidTill.add(Calendar.MINUTE, min);
     System.out.printf("%s%s%s%s%s%.2s%s%.2s\n", "Current Time: ", 
       now.get(Calendar.MONTH), "-",
       now.get(Calendar.DAY_OF_MONTH), " ", now.get(Calendar.HOUR_OF_DAY), 
       ":", dm.format(now.get(Calendar.MINUTE)));
     System.out.printf("%s%s%s%s%s%.2s%s%.2s\n", 
       "Paid Till: ", paidTill.get(Calendar.MONTH),
       "-", paidTill.get(Calendar.DAY_OF_MONTH),
       " ", paidTill.get(Calendar.HOUR_OF_DAY), ":",
       dm.format(paidTill.get(Calendar.MINUTE)));
     System.out.printf("%s%.2f\n", "Total charged to Credit Card $", currentCharge);
     System.out.println("==========================================");
     System.out.println("Tear off receipt and place on dash");
     System.out.print("Press any key to continue");
     anyKey = k.nextDouble();
     System.out.println("================================================");
     paidTill = (Calendar) now.clone();
     System.out.printf("%s%s%s%s%s%.2s%s%.2s\n", "Current Time: ", 
       now.get(Calendar.MONTH), 
       "-", now.get(Calendar.DAY_OF_MONTH), " ", 
       now.get(Calendar.HOUR_OF_DAY), ":", dm.format(now.get(Calendar.MINUTE)));
     System.out.printf("%s%s%s%s%s%.2s%s%.2s\n", 
       "Paid Till: ", paidTill.get(Calendar.MONTH),
       "-", paidTill.get(Calendar.DAY_OF_MONTH),
       " ", paidTill.get(Calendar.HOUR_OF_DAY), ":",
       dm.format(paidTill.get(Calendar.MINUTE)));
     currentCharge = 0;
     min = 0;
     System.out.printf("%s%.2f\n", "Current Charge $", currentCharge);
     System.out.println("================================================");
     break;

    case "r":
     currentCharge = 0;
     min = 0;
     paidTill = (Calendar) now.clone();
     System.out.println("================================================");
     System.out.printf("%s%s%s%s%s%.2s%s%.2s\n",
       "Current Time: ", now.get(Calendar.MONTH),
       "-", now.get(Calendar.DAY_OF_MONTH), " ",
       now.get(Calendar.HOUR_OF_DAY), ":",
       dm.format(now.get(Calendar.MINUTE)));
     System.out.printf("%s%s%s%s%s%.2s%s%.2s\n", 
       "Paid Till: ", paidTill.get(Calendar.MONTH),
       "-", paidTill.get(Calendar.DAY_OF_MONTH), 
       " ", paidTill.get(Calendar.HOUR_OF_DAY),
       ":", dm.format(paidTill.get(Calendar.MINUTE)));
     System.out.printf("%s%.2f\n", "Current Charge $", currentCharge);
     System.out.println("================================================");
     break;

    case "x":
     input = x;
     break;
   }
  }
  System.out.println("\nTotals for Pay and Go Parking");
  System.out.println("========================");
  System.out.printf("%s%12s%7.2f\n", "Totals", "$", totalCharge);

 }

}

Answer 1

您应该解决的第一个问题是第44行：

while (input != x)

应该：

while (!input.equals(x)) 

比较两个字符串时，使用==或！=不会检查它们是否等于相同的字符/单词，但不等于相同的字符串/含义意味着指向内存中的同一位置。

我没有运行程序来测试它，因为我没有您的软件包“ PayandGoParking”。 而且您的代码太长，无法通读以查找更多问题。 解决此问题后，请尝试使出现问题的代码更简洁。

Answer 2

这可能是因为您使用的是System.out.print而没有刷新。 刷新意味着强制将System.out内部使用的输出缓冲区发送到监视器。 仅保证println将您打印的内容发送到显示器。 常规System.print不会，除非您的字符串中某处有换行符。 尝试

 System.out.print("Please enter your selection: ");
 System.out.flush();

实际上，尝试在每次尝试收集用户输入之前添加System.out.flush（）。 或者在您要打印的字符串中添加新行

 System.out.print("Please enter your selection: \n");