Android : Can't extrack information from JSON File

[Android] I have a JSON File that it is composed in this way:

 "5":{
        "15":{
            ...
        },
        "18":{
           ..
        },
        "7":{
            ...
        },
        "9":{
            ..
        }
    }
}

How I can extract the infomation at 15,18,7,9? this number 15, 18 ,7, 9 change from run to another.. My Code is:

JSONObject obj = new JSONObject(file);
            JSONArray uno = obj.getJSONArray("5");

and I obtain JSONException!! Anyone can help me?