Converting List<Object> to String returns empty results

Disclaimer : I'm using this Post, as reference for List<Object> to List<String> and this Post for Java List<String> of strings to a JavaScript array.

I've List<Seat> and I want to get all values of it in a comma separated String, I tried in this way

import java.util.*;
import java.lang.*;

class Rextester
{  

    public Rextester(){
        Seat seat1 = new Seat();
        seat1.setSeatNumber(1);

        Seat seat2 = new Seat();
        seat2.setSeatNumber(2);

        Seat seat3 = new Seat();
        seat3.setSeatNumber(3);


        List<Seat> seatList = new ArrayList<Seat>();
        seatList.add(seat1);
        seatList.add(seat2);
        seatList.add(seat3);
        Utility util = new Utility();
        String stringSeats = util.toJavascriptArray(seatList);
        System.out.println("JavaScriptArray is " + stringSeats);
    }

    public static void main(String args[])
    {
        new Rextester();

    }



    private class Seat {

        private Integer seatNumber;        

        public Integer getSeatNumber() {
            return this.seatNumber;
        }

        public void setSeatNumber(Integer seatNumber) {
            this.seatNumber = seatNumber;
        }   
        public String toString() {
            return ""+ seatNumber;
        }
    }

    private class Utility {

        public String toJavascriptArray(List<Seat> listSeats){
            List<String> strings = new ArrayList<String>();
            for (Seat object : listSeats) {
                strings.add(object != null ? object.toString() : null);
            }
            String[] arr = new String[strings.size()];
            arr = strings.toArray(arr);
            StringBuffer sb = new StringBuffer();
            sb.append("[");
            for(int i=0; i<arr.length; i++){

                if(i+1 < arr.length){
                    sb.append(",");
                }
            }
            sb.append("]");
            return sb.toString();
        }
    }
}

but this gives me

JavaScriptArray is [,,]

on console, am I making some mistakes? an online working code is http://rextester.com/NDUGT61105