retrofit2 interface in Android - https://android.jlelse.eu/handmade-backend-for-android-app-using-python-flask-framework-b173ba2bb3aa

APIs.kt

class APIKindaStuff {

    interface APIService {
        @GET("/users/{user}")
        fun greetUser(@Path("user") user: String): Call<ResponseBody>

        @Headers("Content-type: application/json")
        @POST("/api/post_some_data")
        fun getVectors(@Body body: JsonObject): Call<ResponseBody>
    }

    companion object {
        private val retrofit = Retrofit.Builder()
                .baseUrl("http://XXX.XXX.X.X:5000")
                .addConverterFactory(GsonConverterFactory.create(GsonBuilder().create()))
                .build()

        var service = retrofit.create(APIService::class.java)
    }
}