Deserializing Generic Types with GSON

Question

I have some problems with implementation of Json Deserialization in my Android application (with Gson library)

I've made class like this

public class MyJson<T>{
    public List<T> posts;
}

And Deserialization call is:

public class JsonDownloader<T> extends AsyncTask<Void, Void, MyJson<T>> {
...
protected MyJson<T> doInBackground(Void... params) {
  ...
    Reader reader = new InputStreamReader(content);
    GsonBuilder gson = new GsonBuilder();
    Type collectionType = new TypeToken<MyJson<T>>() {}.getType();
    result = gson.create().fromJson(reader, collectionType);
  ...
  }
}

Problem is that result.posts list after call holds one Array of LinkedTreeMap Objects(with correct values so problem is Deserialization) instead of MyJson Objects. When I use MyObject instead of T everything is running fine and MyObject is correct.

So is there any way to implement deserialization call without creating custom deserializer?

Answer 1

You have to specify the type of T at the time of deserialization. How would your List of posts get created if Gson didn't know what Type to instantiate? It can't stay T forever. So, you would provide the type T as a Class parameter.

Now assuming, the type of posts was String you would deserialize MyJson<String> as (I've also added a String json parameter for simplicity; you would read from your reader as before):

doInBackground(String.class, "{posts: [\"article 1\", \"article 2\"]}");

protected MyJson<T> doInBackground(Class<T> type, String json, Void... params) {

    GsonBuilder gson = new GsonBuilder();
    Type collectionType = new TypeToken<MyJson<T>>(){}.getType();

    MyJson<T> myJson = gson.create().fromJson(json, collectionType);

    System.out.println(myJson.getPosts()); // ["article 1", "article 2"]
    return myJson;
}

Similarly, to deserialize a MyJson of Boolean objects

doInBackground(Boolean.class, "{posts: [true, false]}");

protected MyJson<T> doInBackground(Class<T> type, String json, Void... params) {

    GsonBuilder gson = new GsonBuilder();
    Type collectionType = new TypeToken<MyJson<T>>(){}.getType();

    MyJson<T> myJson = gson.create().fromJson(json, collectionType);

    System.out.println(myJson.getPosts()); // [true, false]
    return myJson;
}

I've assumed MyJson<T> for my examples to be as

public class MyJson<T> {

    public List<T> posts;

    public List<T> getPosts() {
        return posts;
    }
}

So, if you were looking for to deserialize a List<MyObject> you would invoke the method as

// assuming no Void parameters were required
MyJson<MyObject> myJson = doInBackground(MyObject.class);