RxJavaRetrofitMethod.java

    public static Observable<User> getFromApi(String username) {
        Retrofit retrofit = new Retrofit.Builder()
                .addCallAdapterFactory(RxJava2CallAdapterFactory.createWithScheduler(Schedulers.io()))
                .addConverterFactory(GsonConverterFactory.create())
                .baseUrl(ENDPOINT)
                .build();

        UserService apiService = retrofit.create(UserService.class);
        return apiService.getUser(username);
    }