groovy dependencies { // RxAndroid implementation 'io.reactivex.rxjava3:rxandroid:3.x.y' // Retrofit implementation 'com.squareup.retrofit2:retrofit:2.x.y' implementation 'com.squareup.retrofit2:converter-gson:2.x.y' // Gson Converter } public interface ApiService { @GET("users/{userId}") Observable<User> getUserInfo(@Path("userId") String userId); } OkHttpClient okHttpClient = new OkHttpClient.Builder() .build(); Retrofit retrofit = new Retrofit.Builder() .baseUrl("https://api.example.com/") .addConverterFactory(GsonConverterFactory.create()) .client(okHttpClient) .build(); ApiService apiService = retrofit.create(ApiService.class); apiService.getUserInfo("123") .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(new Observer<User>() { @Override public void onSubscribe(Disposable d) { } @Override public void onNext(User user) { } @Override public void onError(Throwable e) { } @Override public void onComplete() { } });