title: serialize json object which extends map
body: <p>Why this does not work?</p>
  
  <pre><code>public static class MyBean extends HashMap&lt;String, String&gt; {
      public String city;
  }
  
  /**
   * @param args
   */
  public static void main(String[] args) {
      MyBean bean = new MyBean(); 
      bean.city = "some city";
  
      Gson gson = new Gson();
  
      String json = gson.toJson(bean);
      System.out.println(json);
      }
  </code></pre>
  
  <p>Why I dont see city value in json?</p>