A very simple opinionated subclass of a Retrofit.Callback

ResponseHandler.java

package com.example.gist.ResponseHandler

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

import java.io.IOException;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;

public abstract class ResponseHandler<T> implements Callback<T> {
    @Override
    public void onResponse(Call<T> call, Response<T> response) {
        if(response.isSuccessful()) {
            onSuccess(call, response);
        } else {
            Gson gson = new GsonBuilder().create();
            try {
                ErrorResponse error = gson.fromJson(response.errorBody().string(), ErrorResponse.class);
                onError(error.getMessage());
            } catch (IOException e) {
                onError("An unexpected error occurred");
            }
        }
    }

    @Override
    public void onFailure(Call<T> call, Throwable t) {
        String msg = t.getMessage();
        onError(msg);
    }

    public abstract void onSuccess(Call<T> call, Response<T> response);
    public abstract void onError(String message);

    public class ErrorResponse {
        private String message;
        public String getMessage() {
            return message;
        }
    }
}