从另一个类中定义的方法返回字符串不返回任何内容

Question

我已经阅读并试图找到答案，但一直无法找到。我创建了一个在其自己的类中工作的方法，具有两个字符串参数，然后使用它们返回一个字符串值。然后我试图在另一个类中调用一个类中定义的这个方法，但我得到一个空指针异常。我已经尝试了几件事，测试了不同的方法来做到这一点，但一直无法让它发挥作用。请原谅我糟糕的编程约定，我才刚刚开始。这是返回字符串的方法：

public class Cipher {   

public  static String encrypt(String Tobeencrypted , String key) 
{                   
   row row[] = new row[26];


   for (int counter = 0; counter < 26 ; counter++ )
   {
      row[counter] = new row();

      for (int counter2 = 0; counter2 < 26 ; counter2++){
          int charnumber = counter + 97 + counter2;
              if (charnumber > 122){
                  charnumber = charnumber - 26;
              }
          row[counter].alpha[counter2] = (char)(charnumber);
      }
   }

   int numberofcharacters = Tobeencrypted.length();
   int keylength = key.length();
   char keychar[] = new char[numberofcharacters];

   int keyindexvalue = 0;
   for (int counter = 0 ; counter < numberofcharacters ; counter++){
   if (keyindexvalue == keylength) 
           keyindexvalue = keyindexvalue - keylength;
       keychar[counter] = key.charAt(keyindexvalue++); 
   }

   char textchars[] = new char[numberofcharacters];
     for (int counter = 0 ; counter < numberofcharacters ; counter++){
     textchars[counter] = Tobeencrypted.charAt(counter);
   }
   String encryptedstring = "";
     for (int counter = 0 ; counter < numberofcharacters ; counter++){
     textchars[counter] = row[(int(keychar[counter])-97].alpha[(int)                       
         (textchars[counter])-97];
     encryptedstring = encryptedstring + textchars[counter];    
     }
     return encryptedstring;
 }

当我在 Cipher 类中有一个 main 方法使用它时，此方法非常有效。但在以下代码中使用该方法时，该方法不返回任何内容。这些文件都在同一个源文件夹中：

 public class testingmethod {

public static void main(String[] args)  {
    try {
    Scanner diskScanner = new Scanner(new         File("C:/Users/Isaac/Desktop/test.txt"));

    int counter = 0;
    String random ="";
    while ( diskScanner.hasNext()){

        random = diskScanner.next();
        counter++;      
    }
    //counter tells you the exact amount of strings there are if there are 10   strings counter = 10
    String strings[] = new String[counter];
    int counter2 = 5;
    while ( diskScanner.hasNext()){
        strings[counter2] = diskScanner.next();
        counter2++;     
    }
    String encrypted[] = new String[counter];
    Cipher cipher = new Cipher();
    PrintStream diskWriter = new PrintStream(new File("C:/Users/Isaac/Desktop/test.txt"));
    String password = JOptionPane.showInputDialog("What password would you like to use? (please make this one word)");
    for ( int counter3 = 0 ; counter3 < counter ; counter3++){
        encrypted[counter3] = cipher.encrypt(strings[counter3], password);
        diskWriter.print(encrypted[counter3] + "    ");
        if(counter3 % 11 == 0) diskWriter.println("");
    }


    } catch (FileNotFoundException e1) {

        e1.printStackTrace();}

}

}

我已根据您所有可爱的帮助进行了更改，并且打印语句有效，但现在我有一个空指针异常，这是我使用 tobeencrypted = String() 解决的问题。

错误是：

    Exception in thread "main" java.lang.NullPointerException
at introtojava.Cipher.encrypt(Cipher.java:43)
at introtojava.testingmethod.main(testingmethod.java:34)

非常感谢，

艾萨克

Answer 1

您故意丢失了参数对要使用此行加密的字符串的引用：

Tobeencrypted = new String();

然后你得到那个字符串的长度，现在是 0：

int numberofcharacters = Tobeencrypted.length();

所以，这个for 循环中没有迭代：

for (int counter = 0 ; counter < numberofcharacters ; counter++){

这是您构建返回字符串的位置，encryptedstring。

看起来你可以删除这一行：

Tobeencrypted = new String();

Answer 2

从删除线开始

Tobeencrypted = new String();