如何获取特定的JSON数据？

Question

Hi I am trying to read JSON from an ReST API but Im getting a nullpointer exception because mycode is not correct.

I have a JSON that I am reading from looking like this :

processJSON({
"LocationList":{
  "noNamespaceSchemaLocation":"http://api.vasttrafik.se/v1/hafasRestLocation.xsd",
  "servertime":"16:13",
  "serverdate":"2013-03-22",
  "StopLocation":[{
    "name":"Brunnsparken, Göteborg",
    "lon":"11.967824",
    "lat":"57.706944",
    "id":"9021014001760000",
    "idx":"1"
    },{
    "name":"Brunnsgatan, Göteborg",
    "lon":"11.959455",
    "lat":"57.693766",
    "id":"9021014001745000",
    "idx":"4"
    },{
    "name":"Brunnslyckan, Lerum",
    "lon":"12.410219",
    "lat":"57.812073",
    "id":"9021014017260000",
    "idx":"5"
    },

Now I want the name from the JSON document depending on what the user inputs.

how do I do this with code?

My code that is wrong is like this :

import org.json.simple.JSONObject;
import org.json.simple.JSONValue;

    public class JSONReader {

        private String jsonData = "";


        public String getJsonData(String location){


            try {

                    URL url = new     URL("http://api.vasttrafik.se/bin/rest.exe/v1/location.name?authKey=secret&format=json&jsonpCallback=processJSON&input=" + URLEncoder.encode(location, "UTF-8"));
                    URLConnection connection = url.openConnection();
                    BufferedReader readJsonFile = new BufferedReader(new InputStreamReader(connection.getInputStream(), "UTF-8"));
                    String temp = "";
                    while((temp = readJsonFile.readLine()) != null){
                            jsonData += temp;
                    }
                    readJsonFile.close();

                    System.out.println(jsonData);
                    return jsonData;

            }

            catch (IOException e) {

            }
            return null;
    }

       public void JSONParsing(){

                String location = Planner.getPlanner().getStartingLocation();
                JSONObject obj =(JSONObject)JSONValue.parse(getJsonData(location));

                //Set the text into the JList

                if (obj.containsValue(location));
                obj.get("name");
                }
    }

I want get the same name of the location out from the JSON as the user inputs. How do I do this with code?

Answer 1

I think that you are asking how to parse your JSONObject and get the corresponding values out of it that the user is interested in. Below is an example of how you can pull apart the JSONObject to create a Map whose key is the String id (since the name does not seem to be unique) and whose value is the whole JSONObject . You can use this map to lookup the input from the user and find the appropriate LLA if that's what you are interested in.

public Map<String, JSONObject> createLocationMap(JSONObject jsonObj){
    Map<String, JSONObject> nameToLocationMap = new HashMap<String, JSONObject>();
    JSONObject locationList = (JSONObject) jsonObj.get("LocationList");
    JSONArray array = (JSONArray) locationList.get("StopLocation");
    for (int i = 0; i < array.length(); i++) {
        String name = (String) ((JSONObject) array.get(i)).get("id");
        nameToLocationMap.put(name, ((JSONObject)array.get(i)));
    }
    return nameToLocationMap;
}

You can tailor this method as you see fit. For example if you are interested in the relationship between the id and the name then you can create a similar method that uses those values instead of id and the entire JSONObject' . I hope that this helps~