longest substring without any repetition of characters

I'm not using java very often. This program serves as a good practice for playing with String and Hashtable in java.

/* Write a code to find out longest substring without any repetition of characters with O(n) complexity. */

import java.util.*;

public class longestSubstring {

     public static String longestsubstr(String s) {

         Hashtable<Character, Integer> visited = new Hashtable<Character, Integer> ();

         String sub = new String();

         int start = 0;

         for (int i = 0; i < s.length(); ++i) {

              if (visited.containsKey(s.charAt(i))) {

                   if (i-start > sub.length()) {

                       sub = s.substring(start, i);                      

                   }

                   for (int j = start; j < visited.get(s.charAt(i)); ++j) {

                       visited.remove(s.charAt(j));

                   }

                   start = visited.get(s.charAt(i)) + 1;

                   visited.put(s.charAt(i), i);

              } else {

                   visited.put(new Character(s.charAt(i)), i);                                     

              }

         }

         if (s.length() - start > sub.length()) {

              sub = s.substring(start, s.length());

         }

         return sub;

     }

    

     public static void main(String args[]) {

         String str = "aaa1234abcde";

         System.out.println(longestsubstr(str));

         String str2 = "0123456756777890123456";

         System.out.println(longestsubstr(str2));        

     }

}