Lexicographically smallest and largest

    public static String getSmallestAndLargest(String s, int k) {
        String smallest = "";
        String largest = "";
        
        //String a[] = new String[1000];
        int start = 0;
        int end = k;
        //List<String> l = new List<String>();
        String curr = s.substring(start,end);
        smallest = curr;
        largest = curr;

        for(int i=k;i<s.length();i++)
        {
            curr = curr.substring(1,k)+s.charAt(i);
            if(smallest.compareTo(curr)>0)
            {
                smallest = curr;
            }
            if(largest.compareTo(curr)<0)
            {
                largest = curr;
            }
        }
        // Complete the function
        // 'smallest' must be the lexicographically smallest substring of length 'k'
        // 'largest' must be the lexicographically largest substring of length 'k'
        
        return smallest + "\n" + largest;
    }

Input (stdin)
    welcometojava
    3

Expected Output
    ava
    wel