无法在 Android Retrofit 中为我的 class 创建转换器

Question

I am trying to develop clent-server application with Retrofit. My application sends to server json with a string "image" and responses a json with a string with field "name".

My API:

public interface API {
    @FormUrlEncoded
    @POST("/api/classification/imagenet")
    Call<GestureJson> getName(@Body ImageJson json);
}

ImageJson:

public class ImageJson {
    public String imageString;
}

NameJson:

public class NameJson {
    public int gestureNumber;
}

When user pressed on button in Main Activity MyVoid is called (url is already known):

public void MyVoid() {
        String requestUrl = url;
        Retrofit retrofit = new Retrofit.Builder()
                .addCallAdapterFactory(GsonConverterFactory.create())
                .baseUrl(requestUrl)
                .build();
        API api = retrofit.create(API.class);
        ImageJson json = new ImageJson();
        json.imageString = image_uri.toString();
        Call<GestureJson> call = api.getName(json);
        call.enqueue(new Callback<GestureJson>() {
            @Override
            public void onResponse(Call<GestureJson> call, Response<GestureJson> response) {
                if (response.isSuccessful()) {
                    status = RESPONSE_SUCCESS;
                } else {
                    status = RESPONSE_FAIL;
                }
            }

            @Override
            public void onFailure(Call<GestureJson> call, Throwable t) {

            }
        });

I have three problems:

1) I don't know what's the difference between Retrofit and Retrofit2. What is better to use?

2).addCallAdapterFactory(GsonConverterFactory.create()) underlined how wrong (in Retorfit and Retrofit2).

3) I can compile application if i delete.addCallAdapterFactory(GsonConverterFactory.create()). But I have a problem:

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.opencvproject, PID: 17742
java.lang.IllegalArgumentException: Unable to create converter for class com.example.opencvproject.GestureJson
    for method API.getGesture

Answer 1

1. Retrofit2 is better because Retrofit outdated (last release was on 2014 https://github.com/square/retrofit/releases/tag/parent-1.6.0 ). Number 2 in name is just a library version.
2. GsonConverterFactory may be underlined because you did't add dependency com.squareup.retrofit2:converter-gson
3. If you delete addCallAdapterFactory(GsonConverterFactory.create()) then Retrofit would't know how to deserialize json to objects. GsonConverterFactory use Gson libarary ( https://github.com/google/gson ) under the hood to deserialize server json responses.