How to add an index in json data?

I have following data structure:

Class UserModel {
Long pkid;
String name;
public UserModel() {
this.pkid = new Long(1001);
this.name = "ABC";
}
}

Now I have converted this into json:

UserModel usrObj = new UserModel();
Gson gson = new Gson();
String json = gson.toJson(userObj);

So my json string is now like:

{  "pkid": 1001,
    "name": "ABC" }

But I need to create the json as

{"com.vlee.ejb.UserModel": [
{  "pkid": 1001,
    "name": "ABC" } ] }

I am not sure how I can add the key "com.vlee.ejb.UserModel"

.

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