1.8 Is Rotation

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 (e.g.,"waterbottle"is a rotation
of "erbottlewat").

public static boolean isRotation(String s1, String s2) {
    if(s1 == null || s2 == null)    return false;
    if(s1.length() != s2.length())    return false;

    String s3 = s1 + s1;
    return isSubstring(s3, s2);
}