JAVA实现caesar凯撒加密算法

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;
 }
}