while循環導致打印語句打印兩次

Question

我的程序會加密和解密Caesar Shift代碼，但現在我遇到的主要問題是我的用戶選擇無法正常工作。

import java.util.Scanner; 
public class CaesarShiftTester 
{ 
public static void main(String [] args) 
{ 
    Scanner in = new Scanner(System.in); 
    String message = ""; 
    String userChoice = ""; 
    int shift = 0; 

    System.out.println("1 - Encrypt\n2 - Decrypt\nQ - Quit Program"); 
    while(!userChoice.equalsIgnoreCase("Q")) 
    { 
     System.out.println("Make a selection: "); 
     userChoice = in.nextLine(); 
     if(userChoice.equalsIgnoreCase("1")) 
     { 
      System.out.print("Please enter a message to be encrypted: "); 
      message = in.nextLine(); 

      System.out.print("Please enter a shift value for the cipher(0 - 25): "); 
      shift = in.nextInt(); 

      CaesarShiftEncryption.genCipherAlphabet(shift); 
      System.out.println(CaesarShiftEncryption.encrypt(message)); 
     } 

     else if(userChoice.equalsIgnoreCase("2")) 
     { 
      System.out.print("Please enter a message to be decrypted: "); 
      message = in.nextLine(); 

      System.out.print("Please enter a shift value for the cipher(0 - 25): "); 
      shift = in.nextInt(); 
     } 

     else if(userChoice.equalsIgnoreCase("Q")) 
     { 
      System.out.println("Thanks for using the program!"); 
     } 
    } 
} 
} 

第一次通過該程序後會「打印選擇」兩次打印。這是類中的另一個文件，它不會在問題中扮演重要角色，但如果您想爲自己測試文件，則可以在這裏找到。請注意我還沒有實現解密，所以只有「1」和「Q」選擇實際上現在做任何事情。

public class CaesarShiftEncryption 
{ 
private static final String ALPHABET = "abcdefghijklmnopqrstuvwxyz"; 
private static String newAlphabet = ""; 

public CaesarShiftEncryption() 
{ 

} 

public static String genCipherAlphabet(int shift) 
{ 
    for(int i = 0; i < ALPHABET.length(); i++) 
    { 
     int alphabetShift = i + shift; 
     alphabetShift = alphabetShift % 26; 
     newAlphabet += ALPHABET.charAt(alphabetShift); 
    } 
    return newAlphabet; 
} 

public static String encrypt(String message) 
{ 
    int letter = 0; 
    String encoded = ""; 
    char[] messageLetters = new char[message.length()]; 

    for(int i = 0; i < message.length(); i++) 
    { 
     messageLetters[i] = message.charAt(i); 
    } 

    for(int a = 0; a < message.length(); a++) 
    { 
     if(Character.isWhitespace(messageLetters[a])) 
     { 
      encoded += " "; 
      a++; 
     } 

     if(Character.isUpperCase(messageLetters[a])) 
      messageLetters[a] = Character.toLowerCase(messageLetters[a]); 

     letter = ALPHABET.indexOf(messageLetters[a]); 
     encoded += newAlphabet.substring(letter, letter + 1).toUpperCase(); 
    } 
    return encoded; 
} 
} 

Answer 1

當你閱讀與

shift = in.nextInt(); 

移位值線的末端留在掃描儀。在下一次循環迭代中，讀取該行的剩餘行結束，但由於沒有找到有效輸入（1,2或Q），循環將再次運行。

爲了解決這個問題，看了你的移值是這樣的：

shift = in.nextInt(); 
in.nextLine();