L Question : Find Repeated String

public Set<String> findRepeatedSubString(String s,int length){
        if(s==null||s.length()==0) return null;
        Set<String> checkSet = new HashSet<String>();
        Set<String> result = new TreeSet<String>();
        for(int i=0;i<s.length()-length+1;i++){
            if(!checkSet.contains(s.substring(i,i+length)))
                checkSet.add(s.substring(i,i+length));
            else
                result.add(s.substring(i,i+length));
        }
        if(result.size()==0) return null;
        return result;