為什么此if語句在While循環中觸發？

Question

我是Java的新手，我正在創建一個while循環，其條件之一是：

if ((userChoice != 'p') || (userChoice != 'P') || (userChoice != 's') || (userChoice != 'S')) System.out.println("*** Use P or S, please. ***");

為什么當我輸入“ p”，“ P”，“ s”或“ S”時，程序仍輸出“” *請使用P或S。 * “ ??

這是整個程序：

import java.util.Scanner;

public class Foothill
{
   public static void main(String[] args)
   {
      // declare an object that can be used for console input
      Scanner inputStream = new Scanner(System.in);

      // declare variables
      String strUserInput;
      char userChoice, userCredits;
      int numYogurts, yogurtWallet = 0;
      // while loop for full transaction
      while (true)
      {
      // menu message
          System.out.println("Menu: \n P (process Purchase) \n S (Shut down)");
          strUserInput = inputStream.nextLine();
          userChoice = strUserInput.charAt(0);

        // condition that forces users to select only P or S
          if ((userChoice != 'p') || (userChoice != 'P') || (userChoice != 's') || (userChoice != 'S'))
              System.out.println("*** Use P or S, please. ***");

          System.out.println("Your choice:" + userChoice);
        // if condition that starts purchase part of transaction
          if ( (userChoice == 'p') || (userChoice == 'P') ) 
          {
              System.out.println("How many yogurts would you like to buy?");
              strUserInput = inputStream.nextLine();
              numYogurts = Integer.parseInt(strUserInput);

              yogurtWallet += numYogurts;

              System.out.println("You just earned " + numYogurts + " stamps and have a total of " + yogurtWallet + " to use");
              // if condition that tests number of purchased yogurts
              if (yogurtWallet >= 10)
              {
                  System.out.println("You qualify for a free yogurt. Would you like to use your credits? (Y or N)");
                  strUserInput = inputStream.nextLine();
                  userCredits = strUserInput.charAt(0);

                  if ((userCredits == 'Y') || (userCredits == 'y'))
                  {
                      yogurtWallet -= 10;
                      System.out.println("You have just used 10 credits and have " + yogurtWallet + " left. Enjoy your free yogurt.");
                  }
              }
          } 
          // if condition that stops the program 
          if ( (userChoice == 's') || (userChoice == 'S') )
          {
              System.out.println("Goodbye!");
              break;
          }

      }
   }
}

Answer 1

假設您輸入了p 。 由於短路評估 ，Java將繼續並檢查if語句內的所有條件，下一個檢查是!= 'P' ，這是true ！ 其他輸入也是如此：

userChoice |  != 'p' | != 'P' | != 's' | != 'S'
-----------+---------+--------+--------+-------
    p      |   Yes   |   --   |   --   |  --
    P      |   No    |   Yes  |   --   |  --
    s      |   No    |   No   |   Yes  |  --
    S      |   No    |   No   |   No   |  Yes

--表示由於短路評估而未評估。

因此，在任何情況下，你if是滿意！

Answer 2

無論您選擇哪個字母，它都將不等於其他選擇。 || 表示“或”。 因此，在A OR B中，如果A為true或B為true，則整個條件為true。 您需要做的是對＆使用&&。

Answer 3

使用if ((userChoice != 'p') && (userChoice != 'P') && (userChoice != 's') && (userChoice != 'S'))

這樣可以解決您的問題。

Answer 4

if ((userChoice != 'p') && (userChoice != 'P') && 
   userChoice != 's') && (userChoice != 'S')) 
       System.out.println("*** Use P or S, please. ***");