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错误示范

public static void main(String[] args) {
        String studentJson= "{\"name\":\"张三\",\"age\":18,\"extraJson\":{\"height\":185,\"weight\":80}}";
        Map<String, Object> map = (Map<String, Object>) JSONObject.parse(studentJson);
        if (map != null) {
            System.out.println("map:"+map);
			System.out.println("map.get(\"extraJson\").getClass():"+map.get("extraJson").getClass());
            String extraJson= (String)map.get("extraJson");
            System.out.println("extraJson:"+extraJson);
        }else{
            System.out.println("map为null");
        }
    }

错误示范输出结果：

map.get("extraJson").getClass():class com.alibaba.fastjson.JSONObject
Exception in thread "main" java.lang.ClassCastException: com.alibaba.fastjson.JSONObject cannot be cast to java.lang.String

正确示范

public static void main(String[] args) {
        String studentJson= "{\"name\":\"张三\",\"age\":18,\"extraJson\":{\"height\":185,\"weight\":80}}";
        Map<String, Object> map = (Map<String, Object>) JSONObject.parse(studentJson);
        if (map != null) {
            System.out.println("map:"+map);
			System.out.println("map.get(\"extraJson\").getClass():"+map.get("extraJson").getClass());
            String extraJson= map.get("extraJson").toString();
            System.out.println("extraJson:"+extraJson);
        }else{
            System.out.println("map为null");
        }
    }

正确示范输出结果：

map.get("extraJson").getClass():class com.alibaba.fastjson.JSONObject
extraJson:{"height":185,"weight":80}

坑的原因:

Map<String, Object> map = (Map<String, Object>) JSONObject.parse(studentJson);

这里使用强转时，value值类型并没有转化为Object，而是JSONObject，所以再通过String强转就会报错，而通过JSONObject自带的toString却不会。