Find Duplicate Characters

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();
    }
}