public interface ApiService { 
  @Headers("Cache-Control: max-age=560000")
  @GET("/data")
  Call<List<Data>> getData();
} 

public interface ApiService { 
  @Headers({
    "Accept: application/vnd.yourapi.v1.full+json",
    "User-Agent: YourAppName"
  })
  @GET("/data/{user_id}")
  Call<Data> getData(@Path("user_id") long userId);
} 

public interface ApiService { 
  @GET("/data")
  Call<List<Data>> getData(@Header("Content-Range") String contentRange);
} 

OkHttpClient.Builder client = new OkHttpClient.Builder(); 
client.addInterceptor(new Interceptor() { 
  @Override
  public Response intercept(Interceptor.Chain chain) throws IOException {
    Request original = chain.request();
    Request request = original.newBuilder()
      .header("User-Agent", "YourAppName")
      .header("Accept", "application/vnd.yourapi.v1.full+json")
      .method(original.method(), original.body())
      .build();

    return chain.proceed(request);
  }
}

OkHttpClient httpClient = client.build(); 
Retrofit retrofit = new Retrofit.Builder() 
  .baseUrl(Constant.BASE_URL)
  .addConverterFactory(GsonConverterFactory.create())
  .client(httpClient)
  .build();