Example Singleton RetrofitClient with Authorization

RetrofitClient.java

import android.content.Context;
import android.content.SharedPreferences;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.IOException;

import okhttp3.Interceptor;
import okhttp3.MediaType;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;
import okhttp3.logging.HttpLoggingInterceptor;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class RetrofitClient {

    private final String baseUrl = "www.google.com";
    private static RetrofitClient instance;
    private static Retrofit retrofit;
    private Context context;

    public RetrofitClient(Context context) {
        this.context = context;
        HttpLoggingInterceptor httpLoggingInterceptor = new HttpLoggingInterceptor();
        httpLoggingInterceptor.setLevel(HttpLoggingInterceptor.Level.BODY);
        CachingControlInterceptor cachingControlInterceptor = new CachingControlInterceptor();
        OkHttpClient okHttpClient = new OkHttpClient.Builder()
                .addInterceptor(cachingControlInterceptor)
                .addNetworkInterceptor(httpLoggingInterceptor)
                .build();
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.setLenient();
        Gson gson = gsonBuilder.create();
        retrofit = new Retrofit.Builder()
                .baseUrl(baseUrl)
                .client(okHttpClient)
                .addConverterFactory(GsonConverterFactory.create(gson))
                .build();
    }

    public static synchronized RetrofitClient getInstance(Context context) {
        if (instance == null) {
            instance = new RetrofitClient(context);
        }
        return instance;
    }

    class CachingControlInterceptor implements Interceptor {

        @Override
        public Response intercept(Chain chain) throws IOException {
            SharedPreferences sharedPreferences = context.getSharedPreferences("PREF_TOKEN", Context.MODE_PRIVATE);
            String token = sharedPreferences.getString("key_token", null);
            Request request = chain.request();
            if (token != null) {
                try {
                    request.newBuilder()
                            .addHeader("Accept", "application/json")
                            .addHeader("Content-Type", "application/json")
                            .post(RequestBody.create(MediaType.parse("application/json;charset=utf-8"), new JSONObject().put("token", token).toString()));
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            }
            return chain.proceed(request).newBuilder().build();
        }
    }

}