Question

I am currently working on an android project that requires me to invoke a web service that will return me a json file. I have been using GSON library to parse all the json file and get a JSON object. It has been working well until I come across this json data which the key fields are dynamic. An example of this file is as below:

{ "0": { "count":"5"},
  "1": { "title":"...", "desc":"" },
  "2": { "title":"...", "desc":"" },
  "3": { "title":"...", "desc":"" },
  "4": { "title":"...", "desc":"" },
  "5": { "title":"...", "desc":"" },
  "routes": { "route1":"...", "route3":"" },
}

I am able to get the count based on the key id of “0”, but I am not sure how do I make use of this value to get the other key objects (key id 1-5), which consist of the data that I needed. Will really appreciate if anyone is able to help me in this matter. Thanks.

Answer 1

The most straightforward approach I can think of is to just treat the structure as a Map (of Map).

With Gson, this is relatively easy to do, as long as the Map structure is statically known, every branch from the root has the same depth, and everything is a String.

import java.io.FileReader;
import java.lang.reflect.Type;
import java.util.Map;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

public class GsonFoo
{
  public static void main(String[] args) throws Exception
  {
    Gson gson = new Gson();
    Type mapType = new TypeToken<Map<String,Map<String, String>>>() {}.getType();
    Map<String,Map<String, String>> map = gson.fromJson(new FileReader("input.json"), mapType);
    System.out.println(map);

    // Get the count...
    int count = Integer.parseInt(map.get("0").get("count"));

    // Get each numbered entry...
    for (int i = 1; i <= count; i++)
    {
      System.out.println("Entry " + i + ":");
      Map<String, String> numberedEntry = map.get(String.valueOf(i));
      for (String key : numberedEntry.keySet())
        System.out.printf("key=%s, value=%s\n", key, numberedEntry.get(key));
    }

    // Get the routes...
    Map<String, String> routes = map.get("routes");

    // Get each route...
    System.out.println("Routes:");
    for (String key : routes.keySet())
      System.out.printf("key=%s, value=%s\n", key, routes.get(key));
  }
}

For more dynamic Map structure handling, I strongly suggest switching to use Jackson, instead of Gson, as Jackson will deserialize any JSON object of any arbitrary complexity into a Java Map, with just one simple line of code, and it will automatically retain the types of primitive values.

import java.io.File;
import java.util.Map;

import org.codehaus.jackson.map.ObjectMapper;

public class JacksonFoo
{
  public static void main(String[] args) throws Exception
  {
    ObjectMapper mapper = new ObjectMapper();
    Map map = mapper.readValue(new File("input.json"), Map.class);
    System.out.println(map);
  }
}

The same can be achieved with Gson, but it requires dozens of lines of code. (Plus, Gson has other shortcomings that make switching to Jackson well worth it.)