Spark Retrofit REST interface

RetrofitSetup.java

RequestInterceptor requestInterceptor = new RequestInterceptor() {
    @Override
    public void intercept(RequestFacade request) {
        String authenticationToken = ...;

        if (authenticationToken != "")
            request.addHeader("Authorization", String.format("Bearer %s", authenticationToken));
    }
};
RestAdapter restAdapter = new RestAdapter.Builder()
        .setLogLevel(RestAdapter.LogLevel.FULL)
        .setEndpoint("https://api.spark.io")
        .setRequestInterceptor(requestInterceptor)
        .build();
this.sparkService = restAdapter.create(SparkService.class);

SparkService.java

import java.util.List;

import retrofit.Callback;
import retrofit.client.Response;
import retrofit.http.Field;
import retrofit.http.FormUrlEncoded;
import retrofit.http.GET;
import retrofit.http.Header;
import retrofit.http.Multipart;
import retrofit.http.POST;
import retrofit.http.PUT;
import retrofit.http.Part;
import retrofit.http.Path;
import retrofit.mime.TypedOutput;

public interface SparkService {
    @GET("/v1/devices")
    void getDevices(Callback<List<SparkDevice>> callback);

    @GET("/v1/devices/{deviceId}/{variable}")
    void getVariable(@Path("variable") String variable, @Path("deviceId") String deviceId, Callback<SparkVariable> callback);

    @FormUrlEncoded
    @POST("/v1/devices/{deviceId}/{function}")
    void invokeFunction(@Path("deviceId") String deviceId, @Path("function") String function, @Field("args") String args, Callback<Response> callback);

    @Multipart
    @PUT("/v1/devices/{deviceId}")
    void flashFirmware(@Part("file") TypedOutput firmware, @Path("deviceId") String deviceId, Callback<UploadSparkFirmwareResponse> callback);
}