JAVA implements caesar Caesar encryption algorithm

public class Caesar {
 public static final String SOURCE = "abcdefghijklmnopqrstuvwxyz";
 public static final int LEN = SOURCE.length();
 /**
  * @param args
  */
 public static void main(String[] args) {
     String result = caesarEncryption("newyork");
     System.out.println("encryption result:" + result);
     System.out.println("decryption result:" + caesarDecryption(result));
 }
 //Encryption
 public static String caesarEncryption(String s) {
     StringBuilder sb = new StringBuilder();
     if (s == null || s.length() < 1) {
         System.out.println("you Input nothing.");
         return null;
     }
     if (!isAlp(s)) {
         System.out.println("input ABC... only");
         return null;
     }
     s = s.toLowerCase();
     int len = s.length();
     for (int j = 0; j < len; j++) {
         char c = s.charAt(j);
         int a = SOURCE.indexOf(c);
         if (a == LEN -1) a = -1;
         if (a == LEN -2) a = -2;
         if (a == LEN - 3) a = -3;
         sb.append(SOURCE.charAt(a + 3));
     }
     return sb.toString();
 }
 //Decryption
 public static String caesarDecryption(String s) {
     StringBuilder sb = new StringBuilder();
     if (s == null || s.length() < 1) {
         System.out.println("you Input nothing.");
         return null;
     }
     if (!isAlp(s)) {
         System.out.println("input ABC... only");
         return null;
     }
     s = s.toLowerCase();
     for (int i = 0; i < s.length(); i++) {
         char c = s.charAt(i);
         int a = SOURCE.indexOf(c);
         if (a == 2) a = LEN + 2;
         if (a == 1) a = LEN + 1;
         if (a == 0) a = LEN;
         sb.append(SOURCE.charAt(a - 3));
     }
     return sb.toString();
 }
 public static boolean isAlp(String s) {
     String p = "^[A-Za-z]+$";
     Pattern pattern = Pattern.compile(p);
     Matcher matcher = pattern.matcher(s);
     if (matcher.find()) {
         return true;
     }
     return false;
 }
}