Test a particular method with rest-assured in Java

I want to test a specific method in a class named AdRestService that has some @GET, @POST, etc. I want to test it from another testing class named AdRestServiceTest. I want to know how can I call a method in AdRestService from AdRestServiceTest and how can I test if the return is ok or not.

public class AdRestService {

    @GET
    @Path("{id}")
    @Produces("application/json")
    public Ad get(@PathParam("id") Long adId) {
        return adService.get(adId); // This points to the DB
    }

}

Now the rest-assured testing :

public class AdRestServiceTest {

    @InjectMocks
    private AdRestService adRestService;

    @Test
    public void getAdTest() {
        given().
        when().
            get("website.com/ad").
        then().
            assertThat().
            statusCode(200).
        and().
            contentType(ContentType.JSON).
        and().
            // ???
                // How can I call the method get(@PathParam("id") Long adId) from AdRestService and test if the return is correct ?
    }




}