HashMap 键值对为null的情况

 public static void main(String[] args) {
        Map<String ,Object> map = new HashMap<>();
        map.put("","wang");
        map.put("f",null);
        map.put(null,"haha");
//        map.put(null,null);
        System.out.println(map);
    }

结果为：{=wang, null=haha, f=null}