Parse JSON file using GSON

Imo, the best way to parse your JSON response with GSON would be creating classes that "match" your response and then use Gson.fromJson() method.
For example:

class Response {
    Map<String, App> descriptor;
    // standard getters & setters...
}

class App {
  String name;
  int age;
  String[] messages;
  // standard getters & setters...
}

Then just use:

Gson gson = new Gson();
Response response = gson.fromJson(yourJson, Response.class);

Where yourJson can be a String, any Reader, a JsonReader or a JsonElement.

Finally, if you want to access any particular field, you just have to do:

String name = response.getDescriptor().get("app3").getName();

You can always parse the JSON manually as suggested in other answers, but personally I think this approach is clearer, more maintainable in long term and it fits better with the whole idea of JSON.

I'm using gson 2.2.3

public class Main {

/**
 * @param args
 * @throws IOException 
 */
public static void main(String[] args) throws IOException {

    JsonReader jsonReader = new JsonReader(new FileReader("jsonFile.json"));

    jsonReader.beginObject();

    while (jsonReader.hasNext()) {

    String name = jsonReader.nextName();
        if (name.equals("descriptor")) {
             readApp(jsonReader);

        }
    }

   jsonReader.endObject();
   jsonReader.close();

}

public static void readApp(JsonReader jsonReader) throws IOException{
    jsonReader.beginObject();
     while (jsonReader.hasNext()) {
         String name = jsonReader.nextName();
         System.out.println(name);
         if (name.contains("app")){
             jsonReader.beginObject();
             while (jsonReader.hasNext()) {
                 String n = jsonReader.nextName();
                 if (n.equals("name")){
                     System.out.println(jsonReader.nextString());
                 }
                 if (n.equals("age")){
                     System.out.println(jsonReader.nextInt());
                 }
                 if (n.equals("messages")){
                     jsonReader.beginArray();
                     while  (jsonReader.hasNext()) {
                          System.out.println(jsonReader.nextString());
                     }
                     jsonReader.endArray();
                 }
             }
             jsonReader.endObject();
         }

     }
     jsonReader.endObject();
}
}

One thing that to be remembered while solving such problems is that in JSON file, a { indicates a JSONObject and a [ indicates JSONArray. If one could manage them properly, it would be very easy to accomplish the task of parsing the JSON file. The above code was really very helpful for me and I hope this content adds some meaning to the above code.

The Gson JsonReader documentation explains how to handle parsing of JsonObjects and JsonArrays:

Within array handling methods, first call beginArray() to consume the array's opening bracket. Then create a while loop that accumulates values, terminating when hasNext() is false. Finally, read the array's closing bracket by calling endArray().

Within object handling methods, first call beginObject() to consume the object's opening brace. Then create a while loop that assigns values to local variables based on their name. This loop should terminate when hasNext() is false. Finally, read the object's closing brace by calling endObject().

Parse JSON file using GSON

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