How can i get an empty string instead of null

[Hafeyang]

When I declare a map and put a null value like below:

        Map map =new HashMap();
        map.put("nullval",null);
        JSONObject o =JSONObject.fromObject(map);
        System.out.println(o.toString(0));

i get a json {"nullval":null}

But I want to get {"nullval":""}

I tried to registerJsonValueProcessor(Object.class,...) but none of use;

Thanks a lot!

[Hafeyang]

I forget to say I use jdk1.4