JSONObject怎么将null的值替换为 "" 空字符串

在网上找了好多几个处理的方法，也都没成功，索性找别人写的一个，（非常感谢他，虽然不知道他是谁，但把代码贴出来共享）下面我自己改了下！

import net.sf.json.JSONNull;
import org.json.JSONArray;
import org.json.JSONObject;
import java.util.Iterator;


public class TestJSON extends JsonUtil {
    public static void main(String[] args) {
        String str = "{body:{title:[{id:100,name:lisan,age:null,index:[{title:穿衣,age:null,zs:较冷}]},{id:1001,age:null,name:lisan1,index:[{title:穿衣1,age:null,zs:较冷1}]}]}}}";
        JSONObject object = new JSONObject(str);
        System.out.println(object);
        JSONObject filterNull = filterNull(object);
        System.out.println(filterNull);
    }

    /**
     * 将json对象中包含的null和JSONNull属性修改成""
     *
     * @param jsonObj
     */
    public static JSONObject filterNull(JSONObject jsonObj) {
        Iterator it = jsonObj.keys();
        Object obj = null;
        String key = null;
        while (it.hasNext()) {
            key = it.next();
            obj = jsonObj.get(key);
            if (obj instanceof JSONObject) {
                filterNull((JSONObject) obj);
            }
            if (obj instanceof JSONArray) {
                JSONArray objArr = (JSONArray) obj;
                for (int i = 0; i < objArr.length(); i++) {
                    filterNull(objArr.getJSONObject(i));
                }
            }
            if (obj == null || obj instanceof JSONNull) {
                jsonObj.put(key, "");
            }
            if (obj.equals(null)) {
                jsonObj.put(key, "");
            }
        }
        return jsonObj;
    }
}打印：
{"body":{"title":[{"name":"lisan","index":[{"zs":"较冷","title":"穿衣","age":null}],"id":100,"age":null},{"name":"lisan1","index":[{"zs":"较冷1","title":"穿衣1","age":null}],"id":1001,"age":null}]}}
{"body":{"title":[{"name":"lisan","index":[{"zs":"较冷","title":"穿衣","age":""}],"id":100,"age":""},{"name":"lisan1","index":[{"zs":"较冷1","title":"穿衣1","age":""}],"id":1001,"age":""}]}}