Replace every third (3rd) duplicate character in string with dollor $- Java

 import java.util.HashMap;

import java.util.Iterator;

public class test1 {
    public static void main(String[] args) {

        StringBuilder abc = new StringBuilder("dbbababtradd");
        System.out.println("Input String :"+abc);

        HashMap<Character,Integer> hm  = new HashMap<>();
        HashMap<Character,Integer> hmIndex  = new HashMap<>();

        for(int i =0;i<abc.length();i++){
            if(!hm.containsKey(abc.charAt(i))){
                hm.put(abc.charAt(i),1);
            }else{
                hm.put(abc.charAt(i),hm.get(abc.charAt(i))+1);
                if(hm.get(abc.charAt(i))==3){
                    hmIndex.put(abc.charAt(i),i);
                }
            }
        }

        Iterator itr =  hmIndex.keySet().iterator();

        while (itr.hasNext()){
            abc.setCharAt(hmIndex.get(itr.next()),'$');
        }

        System.out.println("Output String :"+abc);
    }
}