凯撒移位密码java

Question

I'm trying to implement a basic Caesar Shift Cipher for Java to shift all the letters by 13. Here's my code so far.

    public static String cipher(String sentence){
    String s = "";
    for(int i = 0; i < sentence.length(); i++){
        char c = (char)(sentence.charAt(i) + 13);
        if (c > 'z')
            s += (char)(sentence.charAt(i) - 13);
        else
            s += (char)(sentence.charAt(i) + 13);
    }
    return s;
}

However, the program also changes the values of numbers and special characters and I don't want that.

String sentence = "abc123";

returns "nop>?@"

Is there a simple way to avoid the special characters and only focus on letters?

Edit: I should mention I want to keep all the other bits. So "abc123" would return "nop123".

Answer 1

In the following example I encrypt just the letters (more precisely AZ and az) and added the possibility to use any offset:

public static String cipher(String sentence, int offset) {
  String s = "";
  for(int i = 0; i < sentence.length(); i++) {
    char c = (char)(sentence.charAt(i));
    if (c >= 'A' && c <= 'Z') {     
      s += (char)((c - 'A' + offset) % 26 + 'A');
    } else if (c >= 'a' && c <= 'z') {
      s += (char)((c - 'a' + offset) % 26 + 'a');
    } else {
      s += c;
    }
  }
  return s;
}

Here some examples:

cipher("abcABCxyzXYZ123", 1)   // output: "bcdBCDyzaYZA123"
cipher("abcABCxyzXYZ123", 2)   // output: "cdeCDEzabZAB123"
cipher("abcABCxyzXYZ123", 13)  // output: "nopNOPklmKLM123"

Note: Due to your code, I assumed that you just want to handle/encrypt the "ordinary" 26 letters. Which means letters like eg the german 'ü' ( Character.isLetter('ü') will return true) remain unencrypted.

Answer 2

Problem is that you add 13 as a fixed number, and that will for some letters (in second half of alphabet mostly and digits) produce characters that aren't letters.

You could solve this by using array of letters and shifting through those characters. (similar for digits) So something like this

List<Character> chars = ... // list all characters, separate lists for upper/lower case
char c = chars.get((chars.indexOf(sentence.charAt(i)) + 13)%chars.size());