while循環，switch語句

Question

輸入“ p”大小寫時出現問題，完成后它再次重復我的while循環的頂部，最終用戶必須在其中輸入“ input”變量。 問題是用戶永遠不會被提示輸入任何內容作為“輸入”，然后再次循環，重新打印以前剛剛打印的所有信息。 圍繞它的第二次確實允許用戶放入一些東西作為“輸入”。 這僅在我運行“ p”情況后發生。

有人請幫助我！ 將不勝感激！

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

您應該解決的第一個問題是第44行：

while (input != x)

應該：

while (!input.equals(x)) 

比較兩個字符串時，使用==或！=不會檢查它們是否等於相同的字符/單詞，但不等於相同的字符串/含義意味着指向內存中的同一位置。

我沒有運行程序來測試它，因為我沒有您的軟件包“ PayandGoParking”。 而且您的代碼太長，無法通讀以查找更多問題。 解決此問題后，請嘗試使出現問題的代碼更簡潔。

Answer 2

這可能是因為您使用的是System.out.print而沒有刷新。 刷新意味着強制將System.out內部使用的輸出緩沖區發送到監視器。 僅保證println將您打印的內容發送到顯示器。 常規System.print不會，除非您的字符串中某處有換行符。 嘗試

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

實際上，嘗試在每次嘗試收集用戶輸入之前添加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");