Different classes problems

Question

I posted a topic asking for feedback on a Vigenere Cipher program I made. It was said that I shouldn't have the entire program in one class, and should have different class... or methods?
I've been reading the tutorial for different classes, but I just can't seem to pick it up. I anticipate I'm gonna get a few replies with people saying 'you should read this smelly student' but I've tried, so I figure this may be a way to break me in :).
So, here's my code:
public class VigenereCipherComplete
     public static void main(String[] args)
     String key, message;
     String encrMessage = "";
     String[] options = new String[] {"Encrypter", "Decrypter", "Cancel"};
     int progChoice, keyLength, length, encrCharValue;
     int index;
     int keyIndex = 0;
     char keyChar;
     char messageChar;
     char encrChar;
     boolean shouldEncrypt = true;
     // Choice of whether to use the encrypter of decrypter
        progChoice = JOptionPane.showOptionDialog(null, "Which would you like to run?", "Program choice",
             JOptionPane.DEFAULT_OPTION, JOptionPane.QUESTION_MESSAGE, null, options, options[0]);
        if(progChoice == 0)
             shouldEncrypt = true;
        else if(progChoice == 1)
            shouldEncrypt = false;
    else
       System.exit(0);
     // Input of key and converting it to upper case
     key = JOptionPane.showInputDialog(null, "Please input the key you wish to use:",
          "Key input", JOptionPane.QUESTION_MESSAGE);
     key = key.toUpperCase();
     // Input of message and converting it to upper case
     message = JOptionPane.showInputDialog(null, "Please input the plaintext you wish to use:",
          "Plaintext input", JOptionPane.QUESTION_MESSAGE);
     message = message.toUpperCase();     
     keyLength = key.length();
     length = message.length();
     // Encryption/decryption
     for(index = 0; index < length; index++)
          keyChar = key.charAt(keyIndex);
          messageChar = message.charAt(index);
          if(messageChar >= 'A' && messageChar <= 'Z')
               // Encryption
               if(shouldEncrypt)     
                    encrCharValue = (keyChar - 'A') + messageChar;
               // Decryption
               else
                    if(messageChar >= keyChar)
                         encrCharValue = (messageChar + 'A') - keyChar;
                    else
                         encrCharValue = ((('Z' - keyChar) + messageChar) + 1);
               // Loops ascii char value larger than 90 so it stays within the 65-90 ascii char range
               if(encrCharValue > 'Z')
                    encrCharValue = (encrCharValue - 'Z') + 'A' - 1;
               // Converts the char value to a char and adds it to the converted string
               encrChar = (char)encrCharValue;
               encrMessage = encrMessage + encrChar;
               keyIndex++;     
          // If the char is not a letter, it leaves it as it is and adds it to the converted string     
          else
               encrCharValue = messageChar;
               encrChar = (char)encrCharValue;
               encrMessage = encrMessage + encrChar;
          // Resets key so it will loop for the full string wanting to be converted
          if (keyIndex >= keyLength)
               keyIndex = 0;
     // Output for encryption
     if(shouldEncrypt)
          System.out.println("Plaintext:\t" + message + "\nKey:\t\t" + key + "\nCiphertext:\t" + encrMessage);
     // Outfor for decryption
     else
          System.out.println("Ciphertext:\t" + message + "\nKey:\t\t" + key + "\nPlaintext:\t" + encrMessage);
}I just can't grasp how I'd do this... Sorry for the ambigious question.

Answer 1

I'm not going to read all your code. However, it's reasonable to put the code for one encryption algorithm into one class. If you were writing an entire encryption library, you might find that several of your algorithms (each in their own class) are all using the same code, and you're copying it from one class to another. In this case, you might extract that common code out to a utility class that various algorithms make use of.
Now, if we assume that your question is, "How can I break this code into smaller pieces?" here are a few suggestions, based on a cursory skimming of your code. Note that these are only guidelines, to give you an idea of the kinds of things that make sense to break out as pieces.
1. Keep the GUI separate from the encryption. You should have a VigenereEncryption or somesuch class that actually does the work of encrypting and decrypting, and this class should not know or care if there even is a GUI.
2. In the VE class, I would imagine you'd want an encrypt() method and a decrypt() method.
3. Anywhere you see multiple levels of nesting, one or more of those levels might be a candidate to pull out into its own method.
4. Anywhere you have repeated code that's more than a couple lines, put it in its own method.
5. Anywhere you have a comment like // perform such and such step above a block of code, that block is a good candidate to get pulled out into its own method, named similar to whatever that step is in your comments.
6. Similarly, when you learn what the high-level algorithm is that you're to be implementing, the steps of that algorithm are candidates to be their own methods.
Again, note that this is not carved in stone and is not meant to be taken literally as "do exactly all of these things." Rather, it is to give you ideas about the kinds of things that make for separation of classes and methods.