Java语法：JSONArray array =（JSONArray）obj

Question

I'm learning to Decode JSON files in Java and have come across some Syntax I do not understand. I am also new to Java. Here is the code snippet:

Object obj = JSONValue.parse(jsonResult);
JSONArray array = (JSONArray)obj;

In my best attempt at Programmer speak, I understand that "JSONArray" is a class. We are instantiating a new JSONArray and calling it "array". We are initializing "array" with the value on the right side of the equal sign.

My question is -- I don't understand what is happening on the right side of the equal sign. Why is "JSONArray" in parenthesis: (JSONArray)obj? I don't understand what is happening here.

Thanks!

Answer 1

That is an example of an (unchecked) type-cast. I suggest you use a checked cast using the type comparison operator instanceof first. Something like,

JSONArray array = null;
Object obj = JSONValue.parse(jsonResult);
if (obj instanceof JSONArray) {
    array = (JSONArray)obj;
}

If you're certain it will always be a JSONArray you might do it in one line like

JSONArray array = (JSONArray) JSONValue.parse(jsonResult);

The reason it's necessary to cast at all is because the result of parse() is an Object .

Answer 2

Received JSON may, as its outermost structure, be either an "object" (Map) or "array" (List). JSONValue.parse(jsonResult) produces either a JSONObject or JSONArray, depending on which sort of structure is outermost in the jsonResult string. So the parse method must be declared to return some common "ancestor" of those two classes. JSON-Simple is a rather crude JSON kit which has no common superclass for those two classes other than Object.

Presumably the programmer in this case knows that the received data will always have a JSON "array" as the outermost structure. He initially puts the result from parse in an Object reference (since Object is the formal type returned from parse ), then casts that value to JSONArray.

As Eliot suggests, it would have been better, formally, at least, to include an instanceof test, or, if one used a kit other than JSON-Simple, one could possibly make use of interfaces on a common superclass for the two classes that allowed querying their types.