public class Test {

  static String ceasar(String text, int shift) {
    StringBuilder result = new StringBuilder(); // String is immutable
    for (int pos = 0; pos < text.length(); pos++) { // for each caharcetr position
      char c = text.charAt(pos); // get character
      if (Character.isAlphabetic(c)) { // if its a letter
        char d = Character.toUpperCase((char) (c + shift)); // shift and make upper case
        result.append(d); // add it on

      } else { //add character unchanged
        result.append(c);
      }
    }
    String str = new String(result); // make a String
    return str;
  }

  public static void main(final String[] args) {
    System.out.println(ceasar("abc", 0)); // ABC
    System.out.println(ceasar("hello, world!", 1)); // IFMMP, XPSME!
  }
}