java将list对象转换为json字符串数据显示为空的信息

1、首先是一个main 方法

 public static void main(String[] args){
    List l=new ArrayList();
    ImageInfo a=new ImageInfo("images/gonggao_01.jpg","测试1");
    l.add(a);
    l.add(new ImageInfo("slideImages/1.jpg","测试2"));
    l.add(new ImageInfo("images/gonggao_01.jpg","测试3"));
    l.add(new ImageInfo("slideImages/2.jpg","测试4"));
    l.add(new ImageInfo("images/gonggao_01.jpg","测试5"));
    System.out.println(JSONObject.fromObject(a).toString());
    System.out.println(JSONArray.fromObject(l).toString());
    
    List l2=new ArrayList();
    l2.add("a");l2.add("b");l2.add("c");l2.add("d");
    System.out.println(JSONArray.fromObject(l2).toString());
    System.out.println("-------------------------------");
    try {
     System.out.println(JSONUtil.serialize(l).toString());
    } catch (JSONException e) {
     // TODO Auto-generated catch block
     e.printStackTrace();
    }
 }

 public static void main(Strin