I implemented a Java client for my REST API using Retrofit. The client is a simple java app and not an Android Application. How do I write unit tests for my Java client ? Following is one of my service classes.
public interface CountryService {
    /**
     * Returns country list.
     * 
     * @param sortby
     *            Optional; Sorting order of list. Pass null to ignore.
     * 
     * @return Countries object with list of countries.
     * 
     * @throws SRSRetrofitException
     */
    @GET(SRSClientConstants.SLASH + SRSClientConstants.RESOURCE_COUNTRY)
    public Countries getCountries(@Query("sortby") String sortby)
            throws SRSRetrofitException;
}