Service (endpoint) interfaces are the core things when working with Retrofit library. Basically, they let you define the structure and parameters of HTTP requests by putting metadata on a simple Java Class interface.

Let's take a look at this example that we will use to query financial stocks:

import io.reactivex.Single;
import retrofit2.http.GET;
import retrofit2.http.Query;

public interface YahooService {

@GET("yql?format=json")
    Single<YahooStockResult> yqlQuery(
            @Query("q") String query,
            @Query("env") String env
    );
}

Just by taking a quick look at this HTTP interface definition, we can already tell a few things:

• There will be just a single object as a response of the YahooStockResult type
• The first query parameter is format, and it's set to json

• The part of the path that will be queried is yql
• Two parameters are specified by the caller: q--the query and env--data source table

If we want to make the same request as mentioned, we would need to set q (query) to the following:

String query = "select * from yahoo.finance.quote where symbol in ('YHOO','AAPL','GOOG','MSFT')";

Also, env is to be set to the following:

String env = "store://datatables.org/alltableswithkeys";

All the encoding that's required for special symbols such as / and others will be handled by Retrofit during the execution.