CC150 : Check String Rotation

Given isSubstring() which checks if one word is a substring of another, write code to check if s2 is a rotation of s1.  

class Solution {
    public boolean isRotation(String s1, String s2) {
        if(s1.length() > 0 && s1.length(）== s2.length()) {
            String s1s1 = s1 + s1;
            return isSubstring(s1s1, s2);
        }
        return false;
    }
}