Using Gson to convert Json into Java Object

Question

I am currently working on a project where I am using the Steam Web API. The API returns data in JSON format and I wanted to make a Java program that uses this data. Here is the following JSON format from the API:

{
    "response": {
        "players": [
            {
                "steamid": "---",
                "communityvisibilitystate": 1,
                "profilestate": 1,
                "personaname": "---",
                "lastlogoff": 1429915502,
                "profileurl": "---",
                "avatar": "---",
                "avatarmedium": "---",
                "avatarfull": "---",
                "personastate": 0
            }
        ]

    }
}

I am using Google's JSON API called Gson and I am having trouble setting up the Java classes so that I can use the fromJson() method.

From the JSON data, I know that there is an array of players objects that contain all the data. The one thing that is confusing me is the outer tag called response. I know that I have to construct a class representing the players objects but do I also have to create a class that represents response since it encloses players?

As of right now, I have a file named Response.java that contains the following:

public class Response {

    private ArrayList<Player> playerSummaries = new ArrayList<Player>();

    public String toString() {
        return playerSummaries.get(0).toString();
    }

}


class Player {
    private String steamid;
    private String personaname;


    public String getSteamID() {
        return steamid;
    }

    public void setSteamID(String newSteamID) {
        steamid = newSteamID;
    }

    public String getPersonaName() {
        return personaname;
    }

    public void setPersonaName(String name) {
        personaname = name;
    }

    //Rest of getters and setters omitted.

    @Override
    public String toString() {
        return "<<" + "steamid=" + steamid + "\n" + "name=" + personaname + "\n" + ">>";
    }
}

I only included the variables that I plan to use. The JSON data above is contained in a String called jsonData and this is what I am testing in my main method:

Response response = gson.fromJson(jsonData, Response.class);

System.out.println(response.getPlayerAt(0));

However, running this gives me an IndexOutOfBoundsException. It seems as if Gson was unable to get the information and store it into an object? Could anyone shed some light on this problem I am having? I am mostly curious to know if I have set up my classes correctly.

Answer 1

Replace

private ArrayList<Player> playerSummaries = new ArrayList<Player>();

with

private ArrayList<Player> players = new ArrayList<Player>();

GSON uses reflection to look up which field it should populate. In your case it is looking up whether you have a field named players which you do not.

You also do not need to instantiate the field, GSON will do that for you.

EDIT:

You also need a wrapper class around your top-level object. So

class MyObject {
    public Response response;
}

MyObject myObject = gson.fromJson(jsonData, MyObject.class);

Answer 2

Why you are using

System.out.println(response.getPlayerAt(0));

You should use

response.getPlayerSummaries.get(0); 

where getPlayers is the getter of your

   private ArrayList<Player> playerSummaries = new ArrayList<Player>();

getPlayerSummaries will get the arraylist and then you will use get method of arraylist class over it.