package com.spalmalo.yweather.backend;

import com.squareup.okhttp.OkHttpClient;

import retrofit.RestAdapter;
import retrofit.client.OkClient;

public class RestClient {

    private static Api REST_CLIENT;
    private static String ROOT =
            "http://api.openweathermap.org/data/2.5";

    static {
        setupRestClient();
    }

    private RestClient() {
    }

    public static Api get() {
        return REST_CLIENT;
    }

    private static void setupRestClient() {
        RestAdapter.Builder builder = new RestAdapter.Builder()
                .setEndpoint(ROOT)
                .setClient(new OkClient(new OkHttpClient()))
                .setLogLevel(RestAdapter.LogLevel.FULL);

        RestAdapter restAdapter = builder.build();
        REST_CLIENT = restAdapter.create(Api.class);
    }
}

package com.spalmalo.yweather.backend;

import com.squareup.okhttp.OkHttpClient;

import retrofit.RequestInterceptor;
import retrofit.RestAdapter;
import retrofit.client.OkClient;

public class RestClient {

    private static Api REST_CLIENT;
    private static String ROOT =
            "http://api.openweathermap.org/data/2.5";

    static {
        setupRestClient();
    }

    private RestClient() {
    }

    public static Api get() {
        return REST_CLIENT;
    }

    private static void setupRestClient() {
        RequestInterceptor requestInterceptor = new RequestInterceptor() {
            @Override
            public void intercept(RequestFacade request) {
                request.addPathParam("APPID", "089bf92069d4a25e7835d003635d0e1f");
            }
        };
        RestAdapter.Builder builder = new RestAdapter.Builder()
                .setEndpoint(ROOT)
                .setClient(new OkClient(new OkHttpClient()))
                .setRequestInterceptor(requestInterceptor)
                .setLogLevel(RestAdapter.LogLevel.FULL);

        RestAdapter restAdapter = builder.build();
        REST_CLIENT = restAdapter.create(Api.class);
    }
}

Add rest client interceptor