我通过httpPost获得一字符串，格式如下：
{
"errorCode":null,
"success":true,
"sign":"50031DA7FB7796A6FB0D6A65653D78C7",
"errorMessage":null,
"requestId":"20130516183049",
"projects":[
{
"chargeAreas":[
{"areaName":"四川","cities":[{"areaName":"成都","areaId":"4000000357059","companies":[]},
{"areaName":"北京","cities":[{"areaName":"北京","areaId":"113271345596","companies":[]}],"areaId":"113271345594"}
],"areaId":"4000000357058"}],"projectName":"水费","projectId":"1001","projectAlias":"water"
},
{
"chargeAreas":[
{"areaName":"浙江","cities":[{"areaName":"宁波","areaId":"3601333306670","companies":[{"chargeWaies":[{"fields":[{"fieldName":"billkey","fieldType":"VARCHAR","required":false,"fieldLabel":"用户号码","minLen":5,"maxLen":10}],"wayName":"用户号码","wayId":"3601454127868"}],"companyId":"3601454127867","companyName":"宁波市自来水公司","companyAlias":"ningboshizilaishuigongsi"}]},{"areaName":"杭州","areaId":"360103359926","companies":[]}],"areaId":"360103359925"},
{"areaName":"北京","cities":[{"areaName":"北京","areaId":"113271345596","companies":[]}],"areaId":"113271345594"}
],"projectName":"水费","projectId":"1001","projectAlias":"water"
}
]
}
我使用 JSONObject jsonObject1 = JSONObject.fromObject(result); 这种方法解析该字符串，当我获取其中某个字段值（如：
String retCode1 = (String) jsonObject1.get("errorCode");）时报错：
Exception in thread "main" java.lang.ClassCastException: net.sf.json.JSONNull cannot be cast to java.lang.String
该怎么解决这个问题，如何解析这个字符串，能够方便的获取 errorCode、success、errorMessage、projects的值，也能够获取projects中每个 chargeAreas 的值，以及chargeAreas中各字段值。 希望看到的朋友能帮忙指点一下，谢谢哈！
------解决方案--------------------
会不会是null的原因？先判断下null != jsonObject1.get("errorCode") 试试呗
------解决方案--------------------
看错误提示就是null的原因，像一楼说的判断为null就行了