解密時出錯while循環

Question

應該發生什麼是當輸入-2被要求'輸入數字到decrpyt'將結束while循環，並在此之後打印result2（解密字）。相反-2產生錯誤 'java.lang.StringIndexOutOfBoundsException：字符串索引超出範圍：-2'

如何繞過此？

import java.util.Scanner; 

public class Decrypt { 

    public static void main(String[] args) { 

     Scanner sc; 
     int result, input2, num, end = -2; 
     String word, sourcetext, answer, encrypt = "1", decrypt="2"; 

     char input, result2; 
     sc = new Scanner(System.in); 

     System.out.println("please enter sourcetext"); 
     sourcetext = sc.nextLine(); 

     System.out.print("Would you like to 1: encrypt, or 2: decrypt?"); 
     answer = sc.next(); 

     System.out.println("please enter numbers to decrypt"); 
     while (answer.equals(decrypt)) { 

      num = sc.nextInt();     // number to decrypt 
      result2 = sourcetext.charAt(num);  // num decrypted 

      if (num <= end) {      // int end = -2 

       System.out.print(result2); 

      } 

     } 
    } 
} 

Answer 1

當輸入-2，它傳遞一個到charAt會拋出異常，因爲沒有-2指數的字符串中。您應該將if (num <= end)塊移到while循環的開始位置，並將其餘部分放到else塊中。

像這樣：

if (num <= end){      
    System.out.print(result2); 
} else { 
    num = sc.nextInt();     // number to decrypt 
    result2 = sourcetext.charAt(num);  // num decrypted 
}