add "rotation" when we encrypt an uppercase letter by rotating 13 - encoding

Question

package edu.secretcode;
import java.util.Scanner;
/**
* Creates the secret code class.
*
*
*
*/
public class SecretCode {
/**
* Perform the ROT13 operation
*
* #param plainText
* the text to encode
* #return the rot13'd encoding of plainText
*/
public static String rotate13(String plainText) {
StringBuffer cryptText = new StringBuffer("");
for (int i = 0; i < plainText.length() - 1; i++) {
int currentChar = plainText.charAt(i);
String cS = currentChar+"";
currentChar = (char) ((char) (currentChar - (int) 'A' + 13) % 255 + (int)'A');
if ((currentChar >= 'A') && (currentChar <= 'Z')) {
currentChar = (((currentChar - 'A')+13) % 26) + 'A' - 1;
}
else {
cryptText.append(currentChar);
}
}
return cryptText.toString();
}
/**
* Main method of the SecretCode class
*
* #param args
*/
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
while (1 > 0) {
System.out.println("Enter plain text to encode, or QUIT to end");
Scanner keyboard = new Scanner(System.in);
String plainText = keyboard.nextLine();
if (plainText.equals("QUIT")) {
break;
}
String cryptText = SecretCode.rotate13(plainText);
String encodedText = SecretCode.rotate13(plainText);
System.out.println("Encoded Text: " + encodedText);
}
}
}
I need to make this rotation work by adding-13 to a character if the resulting character is greater-than 'Z' I am suppose to subtract 'Z' then add 'A' then subtract 1 (the number 1, not the letter '1') and do this only for capital letters. I did this in the if statement and when I typed in "HELLO WORLD!" I got 303923011009295302 and I was suppose to get "URYYB JBEYQ!" and the program is not encoding correctly. Any help would be appreciated. Thanks in advance.

Answer 1

You're appending an int rather than a char to cryptText. Use:
cryptText.append ((char)currentChar);
Update:
Wouldn't bother with the character value manipulation stuff. You're making all sorts of character set assumptions as it is (try running on an IBM i, which uses EBCDIC rather than ASCII, and watch it all break).
Use a lookup table instead:
private static final String in = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
private static final String out = "NOPQRSTUVWXYZABCDEFGHIJKLM";
...
final int idx = in.indexOf (ch);
cryptText.append ((-1 == idx) ? ch : out.charAt (idx));