Download a file from Server using Retrofit2

public interface ApiInterface {
    @GET("movie/now_playing")
    Call<MovieResponse> getNowPlayingMovies(@Query("api_key") String apiKey, @Query("page") int page);

    // option 1: a resource relative to your base URL
    @GET("resource/example.zip")
    Call<ResponseBody> downloadFileWithFixedUrl();

    // option 2: using a dynamic URL
    @GET
    Call<ResponseBody> downloadFileWithDynamicUrl(@Url String fileUrl);
}

public class ServiceGenerator {

    public static final String API_BASE_URL = "http://your.api-base.url/";

    private static OkHttpClient.Builder httpClient = new OkHttpClient.Builder();

    private static Retrofit.Builder builder =
            new Retrofit.Builder()
            .baseUrl(API_BASE_URL)
            .addConverterFactory(GsonConverterFactory.create());

    public static <S> S createService(Class<S> serviceClass){
        Retrofit retrofit = builder.client(httpClient.build()).build();
        return retrofit.create(serviceClass);
    }

}

private void downloadFile(){
        ApiInterface apiInterface = ServiceGenerator.createService(ApiInterface.class);

        Call<ResponseBody> call = apiInterface.downloadFileWithFixedUrl();

        call.enqueue(new Callback<ResponseBody>() {
            @Override
            public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {
                if (response.isSuccessful()){
                    boolean writtenToDisk = writeResponseBodyToDisk(response.body());

                    Log.d("File download was a success? ", String.valueOf(writtenToDisk));
                }
            }

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

            }
        });
    }

private boolean writeResponseBodyToDisk(ResponseBody body) {
        try {
            // todo change the file location/name according to your needs
            File futureStudioIconFile = new File(getExternalFilesDir(null) + File.separator + "Future Studio Icon.png");

            InputStream inputStream = null;
            OutputStream outputStream = null;

            try {
                byte[] fileReader = new byte[4096];

                long fileSize = body.contentLength();
                long fileSizeDownloaded = 0;

                inputStream = body.byteStream();
                outputStream = new FileOutputStream(futureStudioIconFile);

                while (true) {
                    int read = inputStream.read(fileReader);

                    if (read == -1) {
                        break;
                    }

                    outputStream.write(fileReader, 0, read);

                    fileSizeDownloaded += read;

                    Log.d("File Download: " , fileSizeDownloaded + " of " + fileSize);
                }

                outputStream.flush();

                return true;
            } catch (IOException e) {
                return false;
            } finally {
                if (inputStream != null) {
                    inputStream.close();
                }

                if (outputStream != null) {
                    outputStream.close();
                }
            }
        } catch (IOException e) {
            return false;
        }
    }