private String removeDuplicateCharacters(String s) {
		Set characters = new HashSet();
		StringBuffer sb = new StringBuffer();
		for (int i = 0; i < s.length(); i++) {
			Character c = new Character(s.charAt(i));
			if (characters.add(c))
				sb.append(c.charValue());
		}
		return sb.toString();
	}