UniqueCharacterInString

public String uniqueChar(String str) {
  Set<Character> showed = new HashSet<Character>();
  StringBuilder sb = new StringBuilder();
  
  for (int i = 0; i < str.length(); ++i) {
    final char c = str.charAt(i);
    if (!showed.contains(c)) {
      showed.add(c);
      sb.append(c);
    }
  }
  
  return sb.toString();
}