import java.util.Set; import java.util.HashSet; class Main { public static void main(String[] args) { System.out.println(findDuplicateCharater("papper")); } public static String findDuplicateCharater(String s){ Set<Character> charSet = new HashSet<>(); Set<Character> res = new HashSet<>(); for(char a : s.toCharArray()){ if(charSet.contains(a)){ res.add(a); } else{ charSet.add(a); } } return res.toString(); } }