之前解析json一直用get的方法，但是如果抛异常会让解析中断很烦人。今天发现了JSONObject还提供了一个更好用的方法opt。看来以后文档还是要更加认真的读的，下面是文档中的原文。

A JSONObject constructor can be used to convert an external form JSON text into an internal form whose values can be retrieved with thegetandoptmethods,or to convert values into a JSON text using theputandtoStringmethods. Agetmethod returns a value if one can be found,and throws an exception if one cannot be found. Anoptmethod returns a default value instead of throwing an exception,and so is useful for obtaining optional values.

详见：http://www.json.org/javadoc/org/json/JSONObject.html