converting object to string error

can somebody please tell me if this is the right way to convert an object to string? Firstly the error below

---

       public String generateResponse(HashSet<String> words){
            Iterator it = words.iterator();
                while(it.hasNext()){
                   String word = it.next() // Object to string error
                   String input = responseMap.get(word);
                     if(input != null){
                       return input;
                     }
                }
       return pickDefaultResponse();
       }

---

Then i did this, and it worked.

       public String generateResponse(HashSet<String> words){
            Iterator it = words.iterator();
                while(it.hasNext()){   
                   String input = responseMap.get(it.next());// i put it here 
                     if(input != null){
                       return input;
                     }
                }
       return pickDefaultResponse();
       }

---

I was so curious about the error. I made a little research, since i'm just learning i don't know if this is right or wrong. it worked, but is it right?

        public String generateResponse(HashSet<String> words){
             Iterator it = words.iterator();
                 while(it.hasNext()){
                    String word = it.next().toString();// added
                    String input = responseMap.get(word);
                      if(input != null){
                        return input;
                      }
                 }
       return pickDefaultResponse();
       }