hashmap to json

public class HashmapToJson {    public static String hashMapToJson(HashMap<String, String> map) {        String string = "{";        for (Iterator<Entry<String, String>> it = map.entrySet().iterator(); it.hasNext();) {            Entry<String, String> e = (Entry<String, String>) it.next();            string += "\"" + e.getKey() + "\":";            string += "\"" + e.getValue() + "\",";        }        string = string.substring(0, string.lastIndexOf(","));        string += "}";        return string;    }    public static void main(String[] args) {        HashMap<String, String> map = new HashMap<>();        map.put("access_token", "addd");        map.put("name", "Tom");        System.out.println(hashMapToJson(map));    }