Remove duplicate char from a string value. Complexity = O (n^2)

gistfile1.java

public static String removeDuplicateChars(String s) {
   if (s == null)
      return s;
   int len = s.length();
   if (len < 2) {
      return s;
   }
   char[] chars = s.toCharArray();
   int index = 0;
   for (int i = 0; i < len; i++) {
      int j;
      for (j = 0; j < index; j++) {
         if (chars[j] == chars[i]) {
            break;
         }
      }
      if (j == index) {
         char c = chars[i];
         chars[i] = ' ';
         chars[index] = c;
         index++;
      }
   }
}