Simple Encryption and Simple Question

Question

Anyway, I hope it is simple!
This is from an example in my text book, I have coded it up and it works great. I think I understand everything except for the subtraction of 'A' in the encryptMessage method. Any ideas? I thought it may be subtracting the ASCII value of 'A', but isn't that just 1?
Any help is appreciated.
public class CeasarCryptography
     public static final int ALPHASIZE = 26; //Standard English alphabet size, ALL uppercase
     public static char[] alpha = {'A','C','B','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
     protected char[] encryptionKey = new char[ALPHASIZE];
     protected char[] decryptionKey = new char[ALPHASIZE];
     public CeasarCryptography()
               //Initializes encryptionArray
               for (int x = 0; x < encryptionKey.length; x++)
                         encryptionKey[x] = alpha[ (x + 3) % ALPHASIZE ]; //Rotate alphabet by three places
               //Initializes encryptionArray and decryptionArray
               for (int x = 0; x < decryptionKey.length; x++)
                         decryptionKey[encryptionKey[x] - 'A'] = alpha[x]; //Decypt is revers of encrypt
     public String encryptMessage(String secret)
               char[] mess = secret.toCharArray(); //First, place the string in an array
               for (int x = 0; x < mess.length; x++)
                         if (Character.isUpperCase(mess[x]))
                                   mess[x] = encryptionKey[mess[x] - 'A'];
               return new String(mess);
     public String decryptMessage(String secret)
               char[] mess = secret.toCharArray();
               for (int x = 0; x < mess.length; x++)
                         if (Character.isUpperCase(mess[x]))
                                   mess[x] = decryptionKey[mess[x] - 'A'];
               return new String(mess);
     public static void main(String[] args)
               CeasarCryptography cipherObject = new CeasarCryptography();
               System.out.println("Welcome to the oldest known enryption program.");
               System.out.println("\tEncryption Key: " + new String(cipherObject.encryptionKey));
               System.out.println("\tDecryption Key: " + new String(cipherObject.decryptionKey));               
               String testString = "JAVA IS NOT EASY";
               System.out.println("\nEncrypt the following: " + testString);
               System.out.println("\tOnce encrypted: " + cipherObject.decryptMessage(testString));
}

Answer 1

If the value of 'A' is not important, how is 'D' -
'A' 3? Because D is 3 later than A. So whether 'A' is -123 or 0 or 1 or 65, 'A' + 3 is always 'D'.
Or is this D (which is the 4th letter) minus A
(which is the 1st letter) for 4-1 = 3. Right. It's just that in ASCII, 'A' is 65, instead of 1 or 0 that we usually think of it. The whole minus 'A' part is just to say "how far away from A are we, and therfore how much are we shifting?"
And 3 is the
element in the Array that D is in?I was winging it here but that's what it comes down to. Whether it's an index into an array where 'A' is in 0, 'B' is in '1', etc., or just an on-the-fly shifting, either way, "minus 'A'" ultimately means "How far away from A is this letter?"