We can use the config variables from build.gradle in your java code

use_build_config.java

// java code

...
  //init the service
  return new ExpensiveServiceThatMyAppUses.Builder()
                    .initWithKey(BuildConfig.SERVICE_THAT_I_NEED); // use the value here
                    .someConfig(true)
                    .build();

...
  //init retrofit so that i can make http calls to my backend
  return new Retrofit.Builder()
                    .addConverterFactory(GsonConverterFactory.create(gson))
                    .baseUrl(BuildConfig.MY_BACKEND_URL) //use the value here
                    .client(okHttpClient)
                    .build();
...