RetrofitApiService.kt

import retrofit2.Call
import retrofit2.Retrofit
import retrofit2.converter.scalars.ScalarsConverterFactory
import retrofit2.http.GET

private const val BASE_URL = "PUT_HERE_URL"

private val retrofit = Retrofit.Builder()
        .addConverterFactory(ScalarsConverterFactory.create())
        .baseUrl(BASE_URL)
        .build()

interface ApiService {
    @GET("PUT_HERE_GET_VALUE")
    fun get():
            Call<String>
}

object Api {
    val retrofitService: ApiService by lazy { retrofit.create(ApiService::class.java) }
}

Retrofit Sample

Created a retrofit object pointing to the desired BASE_URL. An interface that expose the get() method, where @get annotation indicates the endpoint will be requested with the GET HTTP method, it returns a retrofit callback with a String.

Usage

Api.retrofitService.get().enqueue( object: Callback<String> {
    override fun onResponse(call: Call<String>, response: Response<String>) {
        // Success
    }
    override fun onFailure(call: Call<String>, t: Throwable) {
        // Failure
    }
})