Algorithm for checking substrings with only one call to isSubstring

Assume you have a method isSubstring which checks if one word is a substring of another. Given two strings, s1 and s2, write code to check if s2 is a rotation of s1 using only one call to isSubstring (i.e., “waterbottle” is a rotation of “erbottlewat”).


Just do the following checks
1. Check if length(s1) == length(s2). If not, return false.
2. Else, concatenate s1 with itself and see whether s2 is substring of the result.
      input: s1 = apple, s2 = pleap ==> apple is a substring of pleappleap
      input: s1 = apple, s2 = ppale ==> apple is not a substring of ppaleppale

public static boolean isRotation(String s1, String s2) {
     int len = s1.length();
   /* check that s1 and s2 are equal length and not empty */
      if (len == s2.length() && len > 0) {
   /* concatenate s1 and s1 within new buffer */
              String s1s1 = s1 + s1;
              return isSubstring(s1s1, s2);
     }
   return false;
}